a level biology transition pack · 2020. 4. 28. · groups are carbohydrates, lipids and proteins....

A Level Biology

Transition Pack

This pack contains worksheets for you to practise the

key skills and knowledge you will need in your A Level

Biology studies.

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1Vocabulary

It is very important that you are able to use scientifi c vocabulary accurately. There are many

biological terms that you will be familiar with from your GCSE science course and it is now

essential that you can understand and use them appropriately. Complete the following task and

questions, and then compare your results with the Answers. Revise any areas where you have

made mistakes.

TaskUse the biological terms below to complete the defi nitions in the table. Some terms have not been

included to provide an extra challenge.

tissuephotosynthesis

cytoplasmliving organisms

highhomeostasis

internalconcentrated

enzymeactive transport

DNAdiffusionprotein

identicalsimilar

bacterianucleus

active sitelow

dilute waterchain

Scientifi c word Defi nition

Activation energy Energy needed to make a reaction take place

…………….. Place on the enzyme molecule where the substrate fi ts

A……………..

t……………..

Movement of substance against a concentration gradient requiring

……………..

…………….. A single-celled micro-organism with no nucleus

Cell Fundamental building block of …………….. ……………..

Chromosome Made up from …………….., found in the nucleus

C…………….. Found in all living cells where chemical reactions take place

Denatured When the shape of an enzyme molecule changes so it is not able to

function

D…………….. Net movement of molecules from an area of …………….. concentration

to one of …………….. concentration

E…………….. Biological catalyst that …………….. the rate of reaction

Food …………….. Feeding relationship between different organisms in an ecosystem

Gene A part of DNA that codes for a ……………..

H…………….. Maintaining a constant ……………... environment

Mitosis Cell division in which two …………….. daughter cells are produced

N…………….. An organelle that contains the genetic material and controls cell activity

(Continued)

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1 VocabularyG

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Task

s Scientifi c word Defi nition

Osmosis Diffusion of …………….. from a …………….. to a more ……………..

solution

P…………….. Process carried out by …………….. in which light …………….. is used

to produce glucose

Respiration Process where g…………….. is broken down to provide energy in all

cells

T…………….. A group of cells that have a …………….. structure and function

Questions

1 Where in the cell do the chemical reactions take place?

2 In which process is light energy used to produce glucose?

3 Defi ne the term ‘respiration’.

4 What is a gene?

5 What is the term used to describe the loss of function by enzymes?

6 What is tissue made up of?


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Cells are the basic building blocks of all living things. There are many similarities and differences

between plant and animal cells that you would have studied in your GCSE science course.

Complete the following tasks and questions.

Task 1Complete the table below, stating the function of each feature. Tick (✔) which cell type the feature

is present in and place a cross (✘) where it does not exist.

Feature Function Plant Animal

Cellulose cell wall

Cell (plasma) membrane

Nucleus

Cytoplasm

Large permanent vacuole

Task 2Label the plant and animal cells below.

Plant and Animal Cell Structure 2

A A

B

B

C

C

D

E

F

Questions

1 What structures are usually present in all cells, whether plant or animal?

2 Which cell structure is responsible for controlling the entry and exit of substances into

and out of the cell?

3 What structures are only present in palisade cells?

4 Which process occurs in the chloroplast?

5 State the function of the nucleus.

6 Where in the plant cell would you fi nd cell sap?

7 What is the function of the cellulose cell wall?

8 Where in the cell do most of the chemical reactions take place?


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Task

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Different types of food are needed in correct amounts to maintain a healthy body. The main food

groups are carbohydrates, lipids and proteins.

Complete the following task and questions.

TaskComplete the table below by placing a tick (✔) if the statement is correct for each food group or a

cross (✘) if incorrect.

Statement Carbohydrates Lipids Proteins

Major component found in the plant cell

wall – cellulose

Provides thermal insulation

Can be either found as fats (animals) or

oils (plants)

Needed to build up muscles in animals

Main compound used in respiration

Amino acids are the building blocks

Made up of fatty acids and glycerol

Examples include enzymes, hormones and

haemoglobin

Includes glucose, sucrose and starch

Denature/break down at high temperature

Biological Molecules3

Questions

1 Name the compound that is the source of energy in respiration.

2 What are built up from amino acids?

3 Which compound serves as a reserve source of energy in plants and animals?

4 What has a structural role in the plant cell wall?

5 List two functions of lipids.

6 What compound is made up of glycerol and fatty acids?

7 This forms compounds that carry oxygen in the blood.

8 Name the storage molecule found in plant cells.


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All living cells can carry out a process called respiration during which energy is released. Plants are

also able to make their own energy by carrying out a process called photosynthesis. Check your

understanding of photosynthesis and respiration by completing the following task and questions.

TaskComplete the table below by writing either photosynthesis or respiration to identify the process

each statement describes.

StatementPhotosynthesis or

respiration?

This reaction takes place in the mitochondria

Carbon dioxide is absorbed and used in this process

Oxygen is released as the waste product

Water is one of the end products

Light provides the energy needed for this process to take place

Glucose is broken down to release energy in the form of ATP

This process occurs in the palisade cells

Occurs in both plant and animal cells

Cannot take place in the dark

4Photosynthesis and Respiration

Questions

1 What is the main photosynthetic structure in plants?

2 Which process converts glucose into ATP?

3 Where in the leaf cell does photosynthesis occur?

4 What are the products of aerobic respiration?

5 List the three raw materials required for photosynthesis.

6 Write the balanced chemical equation for photosynthesis.

7 How can the rate of photosynthesis be measured?

8 State three uses of the ATP produced in respiration in cells.

9 Which cell organelle would you expect to produce most ATP?

10 Write the word equation for aerobic respiration.

11 What other compound can be used for respiration?

12 What colour light is refl ected by leaves?

13 Which pigment absorbs sunlight during photosynthesis?

14 In which process is oxygen the substrate?


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In Biology, questions come in a range of different forms. It is essential that you understand what

information the question is asking for.

There are usually two sections to questions in Biology:

• The fi rst part will provide you with some background knowledge or information; it is the

stem of the question.

• The second part contains the instructions; what you need to do to answer the question.

It is very important that you read the instructions part carefully and provide the information that the

examiner requires. Do not give the information that you want to tell the examiner just because you

may know it very well. It is very common to assume you know what the question is about, when it

is actually asking something different, especially when under pressure in exam conditions.

Exam questions will contain key words used to introduce the question. If you know what these

keys words/terms mean, you will have a much greater chance of providing the correct answer.

The following terms are widely used in Biology exam questions, and give guidance as to the type

of answer needed. Make sure you familiarise yourself with these terms and are able to apply them

when answering the exam questions.

Term Description/Understanding

Analyse Use data from an experiment, usually from a graph or table, to draw valid

conclusions

Annotate On a diagram, drawing or graph, add brief notes to give an explanation

Apply Use of previous knowledge, equations, theories or principles to explain a

different or new situation

Calculate Use fi gures from tables or graphs to work out an answer; show all stages of

working out (steps used to reach an answer)

Compare Usually, explain similarities or differences between two or more sets of data,

objects or situations

Construct Usually, convert information provided into a graph, table or pyramid

Contrast Show the differences between different conditions

Defi ne Give a precise answer, normally the exact meaning of a biological term

Describe Give a full, detailed account with all the relevant information

Discuss Provide an answer that includes a range of views, or various factors that could

have an impact on a biological process

Evaluate Look at all the implications and limitations

Explain Give a reason, using biological knowledge or scientifi c information

Identify Look for the answer from a number of different possibilities, often from tables

or graphs

(Continued)

Examination Terminology5

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Term Description/Understanding

Illustrate Give an answer with clear examples with comparisons highlighted

List The answer does not need an explanation; it could be a sequence of names

or brief answers

Name Concise answer required, usually one word

Predict Give estimated or expected values, or write down what you think may happen

State You will need to give a specifi c name, term or numerical value, which usually

does not need any further explanation

Suggest Give any other biological possibilities or a valid logical explanation

Summarise Give a brief, concise or exact account; avoid too much detail

It is advisable to become familiar with these terms and to look through past exam papers from

your specifi c exam board to see which terms are frequently used.

Add further terms to the above list as you come across them.

Examination Terminology 5

Questions

Answer the following questions by paying particular attention to the terms used within the

questions. Refer to the table above to make sure you understand what is asked of you.

1 Explain what is meant by ‘activation energy’.

2 State the function of the nucleus and mitochondria.

3 Name the process by which plants convert light energy into chemical energy.

4 Describe how you would carry out a test to fi nd out if a solution contained glucose.

5 Briefl y describe the ‘lock and key’ model for enzyme action.

6 Suggest a limitation to the ‘lock and key’ model of enzyme action.

7 Compare the structure of plant and animal cells.

8 Lactose is present in milk. It is broken down by lactase into glucose and galactose.

This is shown by the following equation:

lactose � water lactase glucose � galactose

Name the type of reaction shown in the equation.

9 List three different substances required for healthy nutrition.

10 State the name given to small circles of DNA in prokaryotic cells.


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Units and Abbreviations6Why are units important in Biology?In Biology you will be making many observations and measurements that need appropriate units;

it is therefore important that a universal system is followed. Le Système International d’Unités

(abbreviated to SI) is a metric system that is used in science. It ensures that all scientists work in

the same standard units.

The table below shows the common measurements you will come across in Biology.

Name Unit Symbol

area square metre m2

concentration moles per cubic decimetre mol dm�3

energy Joule J

length metre m

mass kilogram kg

pressure Pascal Pa

temperature degree centigrade °C

time second s

volume cubic decimetre dm3

What do prefi xes mean?A prefi x can be used for units. This is usually a multiplier for that unit, such as ‘kilo’, which is 1,000

multiples of the unit – you could have kilometre, kilograms and kilojoules, for example.

Below is a list of the units (some with prefi xes) you will need to know.

Name Symbol

kilometre km

metre m

centimetre cm

millimetre mm

micrometre �m

nanometre nm

From the table above, you can see that nm is the smallest measurement used by the A-level biologist.

• To convert mm to �m, you need to multiply by 1,000.

• To convert �m to nm, you need to multiply by 1,000.



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• To convert �m to mm, you need to divide by 1,000.

• To convert nm to �m, you need to divide by 1,000.

An easy way to remember whether you need to divide or multiply by 1,000 is to…

1 Look at the fi gure and decide if it needs to be made bigger or smaller.

2 Then look at the units it needs to be converted into.

3 If the fi gure needs to be made bigger… MULTIPLY by 1,000 (or 100).

4 If the fi gure needs to be made smaller… DIVIDE by 1,000 (or 100).

�1,000 �100 �10 �1,000 �1,000

km m cm mm �m nm

�1,000 �100 �10 �1,000 �1,000

You will also need to be confi dent at recognising each of the units when they are written as

powers of ten (length has been used in the following table to illustrate this).

Name of unit Multiple or fraction of a metre

kilometre (km) 103 � 1000m

metre (m) 1m

centimetre (cm) 10�2 � 0.01m

millimetre (mm) 10�3 � 0.001m

micrometre (�m) 10�6 � 0.000001m

nanometre (nm) 10�9 � 0.000000001m

6Units and Abbreviations

Task and Questions

Task

Complete the table below showing conversions between different units. Some have already

been done for you, and these should help you to fi ll in the blanks.

Unit m cm mm �m nm

Height of a rosebush 145

Length of a fi ngernail 16

Diameter of a liver cell 25

Diameter of a cell membrane 10


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6 Units and Abbreviations

Questions

1 Convert 75�m into mm.

2 How many nm are there in 450mm?

3 Write 84�m in mm.

4 Susan uses an optical microscope to look at a liver cell that is 4mm in diameter.

a) How would she write this fi gure in cm?

b) How would she write this fi gure in �m?

5 Write 180�m as mm.

6 Write 0.2mm in �m.

7 Convert 2.5m into �m.

8 George measures the size of the nucleus in a plant cell as 30mm. What is this value in

nm?

9 The length of a salmon was measured as 0.5m. Convert this into �m.

10 A holly leaf was measured and had a diameter of 6.5cm.

a) What is its diameter in mm?

b) What is its diameter in m?

c) What is its diameter in nm?



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7Calculation Skills

You will be required to do some simple maths calculations in Biology, so you need to be confi dent

about using a calculator. You should familiarise yourself with some of the functions on a scientifi c

calculator. Not all calculators are identical – if in doubt, check the instructions that come with your

calculator.

In Biology you may be expected to work out calculations using data from a table or a graph. You

will need to be confi dent at calculating the following:

• Mean – this is the ‘average value’. To fi nd the mean, you will need to add up all the data

provided (ignoring anomalies if this is applicable), obtain an overall total, then divide it by the

number of data points within the group. For example, the mean for 14, 10, 20, 16 and 15

would be calculated as follows:

14 � 10 � 20 � 16 � 15 � 15 5

• Calculating percentage (%) – this gives you an idea of what proportion of a set of data

falls into any particular group.

Amount � 100 � percentage (%) Total

Worked example:

In a school 450 students took a Biology AS-level exam. Of these students, 58 obtained A*.

What percentage of students taking the exam obtained an A*?

Answer: Number of students with A* � 58

Total number of students taking the exam � 450

58 � 100 � 12.88% 450

This button is used to fi nd the square root of a

number

These are the standard buttons:� = add

� = minus� = multiply� = divide

�/� button lets you input a negative number


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• Calculating percentage increases (%) – this allows you to work out the percentage

increase in results, usually from the beginning of an investigation to the end of the

investigation.

total after increase � 100] � 100

total before increase

Worked example:

Last year 101 students took up AS Biology. This year the fi gure was 135.

What is the percentage increase between the two years?

Answer: Total number of students after increase � 135

Total number of students before increase � 101

135 � 100]�100 � 33.66% 101

There is a 33.66% increase.

• Calculating percentage decrease (%) – this allows you to work out the percentage

decrease in the fi gures you have at the end, such as in an investigation, compared to those

you started with.

total after decrease 100 � total before decrease

� 100] � percentage decrease (%)

Worked example:

In April this year, there were 167 snails in a pond. Four weeks later the number of snails had

dropped to 84.

What is the percentage decrease in the number of snails during these four weeks?

Answer: Total number of snails after decrease � 84

Total number of snails before decrease � 167

100 � 84 � 100] � 49.70%

167

There is a 49.70% decrease.

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tCalculation Skills7

Questions

1 Calculate the mean value for the following results:

105, 29, 78, 79, 95, 88, 74

2 Find out the mean height of boys in a class from the results below:

180cm, 140cm, 155cm, 144cm, 160cm, 188cm, 154cm, 172cm, 156cm

[

[

[

[



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Calculation Skills 73 Calculate the mean length of snail shells from different locations.

Location Snail shell length/mm

1 44

2 34

3 27

4 30

5 49

6 23

7 33

8 43

Mean

4 The table below shows the concentration of different ions inside algal cells and the

concentration of the pond water in which they were found.

IonsConcentration in the

pond water/mmol dm–3

Concentration inside algal cell/mmol dm–3

Sodium 1.1 4.8

Chloride 2.5 6.2

Nitrate 2.5 7.4

Potassium 1.2 5.7

Calculate the percentage increase in the concentration of nitrate ions inside algal cells.

5 Over the course of a year, a hospital checked 745 children for asthma. Of those

checked, 158 were found to be suffering from asthma. What percentage of the total

number of children examined were found to have asthma?

6 In an AS Biology exam paper the total marks were 60. If student A obtained 45 marks,

what percentage did this student obtain?

7 In an A2 Chemistry exam paper the total marks were 75. If student B obtained 53

marks, what percentage did this student obtain for the paper?

8 A blood cell with a diameter of 15.5�m was placed in concentrated salt solution for 24

hours. After this period the diameter measured was 8.9�m. Calculate the percentage

decrease in the diameter of the cell.


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There are different types of graphs that can be used to show data you may have obtained from a

practical or investigation.

All graphs have some common features:

1 Axes – make sure that you have the two axes the correct way around on your graph:

x-axis – (on the bottom/horizontal) has the variable you are changing; it is called the

independent variable (IV); y-axis – (on the side/vertical) shows the variable you are measuring; it is called the

dependent variable (DV).

2 All axes must be labelled correctly with the appropriate units.

3 Graphs should contain a title indicating what the graph is showing.

4 Scale – make sure that your scale is uniform, that is, always increases by a fi xed amount,

such as 0, 2, 4, 6, 8, 10 and not 0, 2, 5, 8, 10. It is usually much better to start the scales

for each axis at 0. You should use at least half the available space on the graph paper.

5 Plotting – make sure that you plot each reading or data point very carefully and accurately.

You can use a dot with a circle around it ( . ) or a cross (x).

6 Curve – it is easiest to join the data points with straight lines using a ruler, rather than trying

to draw the lines using ‘free hand’ (beware – other science subjects may like you to draw

curves using free hand).

7 Do not extend the lines beyond the data points; this is called extrapolating and you

should not do this unless specifi cally asked to do so.

Dependentvariable(DV)/units

Independent variable(IV)/units

x-axis

y-axis

8 Graph Plotting Skills



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Graph Plotting Skills 8What types of graph could you be asked to plot?A. Bar chart – this type of graph is plotted when the IV is not numerical (that is, there are no

numbers) but has categories, for example, human blood groups. The bars do not touch.

Percentageof UKpopulation/%

O A B AB

Human blood group

B. Histogram – this type of graph is used to show frequency distribution (effectively a tally

chart) with continuous data/measurements for the IV. When plotting a histogram, the bars

can be touching and the data is grouped into classes. For example, the height of students

in a science group, 140–144cm, 145–149cm, 150–154cm etc.

Frequency

Height/cm

C. Line graph – this type of graph is plotted to show the relationship between two variables

and indicates the effect of the IV on the DV – for example, the effect of temperature on

enzyme activity.

Enzymeactivity/min

Temperature/°C IV

DV

D. Ecological pyramid – this type of graph is used in ecology when you may be asked to

show different organisms in food relationships or different feeding levels. The pyramids

always have plants or producers at the base and the consumers (these eat others

organisms) above.


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Tertiary consumers (omnivores)

Secondary consumers (carnivores)

Primary consumers (herbivores)

Producers (plants)

The pyramid should always be symmetrical and the width of the bars proportional to the

number of organisms, the biomass or the energy at each level.

Graph Plotting Skills8

Questions

1 Name the type of graph you would plot to compare the volume of juice

produced from an orange, lemon, grapefruit and satsuma.

2 What type of graph would best show the results from an experiment looking at

the relationship between substrate concentration and enzyme activity? What

would the IV and DV be in this experiment?

3 What is at the base of ecological pyramids?

4 How would you present the results from an investigation looking at the hand-

span measurements of a class of students?

5 Plot a labelled graph for the data below.

Temperature/˚CVolume of apple juice

produced/cm3

0 0.5

10 5.0

20 12.5

30 20.0

40 35.5

50 28.0

60 15.0

6 What type of graph would you plot to show how many students in your class

are able to roll their tongue compared with how many cannot roll their tongue?



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8Graph Plotting Skills

7 Plot a graph showing the percentage of students from class 2HS obtaining each grade

at AS-level Biology.

A�15%, B�22%, C�35%, D�20%, E�8%

8 Plot a graph using the data below, obtained for a photosynthesis experiment carried

out at two different temperatures.

Time/minutesVolume of oxygen produced/cm3

At 20˚C At 30˚C

0.0 0.0 0.0

5.0 2.0 5.5

10.0 4.0 10.0

15.0 6.5 15.0

20.0 10.0 22.0

25.0 13.0 27.5


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In your Biology course you will need to be familiar and confi dent with certain formulae, structures

and equations. These will help you to summarise biological information into the most important

points.

Which biological molecules do I need to know?

Name Formula Structure

Glucose C6H

12O

6

H

6CH2OHOHO

OHOH

OHH

H

HH

5

4

3 2

1

Maltose C12

H22

O11

6CH2OHHO

OOHHO

OHH

H

HH H

5

4

3 2

1

6CH2OH

OH

O

OH

OHH

HH

H

5

4

3 2

1

Starch [CnH

2nO

n] – H

2O

O OH

CH2OHO

OH

OHH

HH

H O

O

O

H

CH2OHO

OH

OHH

HH

H OH

CH2

O

OH

OHH

HH

H OH

CH2OHO

OH

OHH

HH

H

H

CH2OHO

OH

OHH

HH

H

Amino acid NH2CHRCOOH H

HO

O

N C C

H H

R

amino group carboxylic acid group

Lipids C3H

8O

3

(glycerol) �

3 fatty acids

H O

H C CO

O

O

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C etc.

H C CO

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C etc.

H

H C CO

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C etc.

glycerol fatty acids

(Continued)

Formulae, Structures and Equations9



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Formulae, Structures and Equations 9Name Formula Structure

Nucleotide Phosphate, 5C

sugar (pentose)

� base phosphate

deoxyribose sugar

base

What reactions do I need to know?There are a number of types of reactions you may be asked about in your exams, so it is very

important that you know when to use these equations.

• Condensation reaction – this is when two monomers (single building units) are joined

together by covalent bonds and water is released. Condensation reactions are involved

when polymers are formed, as in protein synthesis. The bonds that join the molecules

together have special names depending on which biological molecule is being made. An

example is the formation of maltose from glucose monomers.

CH2OH

O

OHHO

OHH

C

C C

C

C

H

HH

OH

H

CH2OH

O

OHHO

OHH H H

C

C C

C

C

H

H H H

H

OH

H

CH2OH

O

OHHO

OH

HH HH H

CH2OH

O

O OH

OH

OH O

H H

maltoseglucose glucose water

��

��condensation reaction

H

• Hydrolysis reaction – this type of reaction is involved in the breakdown of large polymers

into single monomers. Large molecules are split using water (hydro � water and lysis �

split). Water and enzymes are required during this reaction. For example, when proteins are

digested, they are hydrolysed.

digestion

protease

proteins � water � enzymes amino acids

• Photosynthesis

light energy carbon dioxide � water glucose � oxygen

• Respiration

glucose � oxygen carbon dioxide � water � ATP


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Formulae, Structures and Equations9Questions

1 Name the type of reaction that involves the formation of polymers from

monomers.

2 How is the bond broken between two amino acids?

3 Write the formula for maltose.

4 Draw the structure of glucose.

5 What type of reaction is involved when you digest bread?

6 Spot the type of reaction shown below:

glycerol � 3 fatty acids triglyceride � water

You do not necessarily need to recognise the names of all the molecules present. Look for clues as to the general type of reaction occurring.

7 List the three molecules that make up a nucleotide.

8 What is removed during a condensation reaction?

9 When two amino acids join together through a condensation reaction, a water

molecule is released. How many water molecules will be produced if you join

eight amino acids in a chain?

10 Complete the reaction below:

enzyme maltose � water ? � ?

11 Draw a diagram showing the general structure of an amino acid.


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Using microscopes

Microscopes are used to observe and magnify cells. They enable us to have a clear, detailed

understanding of the different structures found within cells. There are two key features that

microscopes provide:

1 Magnifi cation – this is the extent to which the

image is larger than the object being viewed; it is

simply the factor by which the image is enlarged

compared to the original object.

size of imagemagnifi cation � _______________

actual size

It is important that the same units are used for both image and object. The size of the cell

or organelle is usually measured in millimetres (mm) and will generally need to be converted

to micrometres (µm) by multiplying this number by 1,000.

2 Resolution – this is the ability of a microscope to distinguish between structures that are

very close together. The resolving power of a microscope is the minimum distance apart

that two objects can be for them to appear as separate items.

Light (optical) microscopes

A light microscope uses lenses to focus rays of light through the specimen.

The light rays are then passed through a series of lenses that are responsible

for both the magnifi cation and resolution. Different parts of the specimen

absorb varying amounts and wavelengths of light. Light that is not absorbed is

transmitted to the eye through the objective lens and eyepiece.

(NB: A light microscope and an optical microscope are the same.)

• A light microscope has a resolution of approximately 0.2�m

• Magnifi cation is up to � 1,500

• For example, if objective lens � � 40 and eyepiece � � 10, then

overall magnifi cation � 40 � 10 � � 400

Microscopes15A-levelYou will need to know the main features and limitations of an optical microscope.

You should be able to identify the differences between optical and electron microscopes, including scanning and transmission electron microscopes.

You will need to use simple equations to work out the size and magnifi cation of a specimen.

GCSEYou may have used light microscopes to look at the general structure of cells and can identify larger organelles such as the nucleus.

You will know that to see internal structures and to identify roles within the cell, you need to use an electron microscope.

Important terms to remember:Magnifi cation � enlargement Resolution � detail

light

condenser lens

specimen

objective

projector lens

eyepiece

eye

final image

04_Topic Builders Ch15–35.indd 4404_Topic Builders Ch15–35.indd 44 10/04/2012 11:0610/04/2012 11:06


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15Microscopes

Electron microscopes

The electron microscope uses beams of electrons instead of light rays. The beams of electrons

are bent and focused by electromagnetic lenses. Electron beams have a much shorter wavelength and pass through objects that are very close together, therefore providing a much

better resolution. Electron microscopes also have a greater magnifi cation of � 500,000.

electron gun

anode

condenser

specimen

objective aperture lens

intermediate lens

projector lens

fluorescent screen

1. Transmission Electron Microscope (TEM)The beams of electrons produced by the electron gun are transmitted through the material. The section of the specimen being investigated has to be extremely thin, as electrons are much more easily blocked than light. Denser parts of the specimen absorb more electrons and appear darker on the image produced. Other parts of the image allow the electrons to pass through and hit the screen, which then fl uoresces, and these parts appear bright. The fl at, two-dimensional image produced is called a micrograph.

The preparation involves using chemicals to fi x the specimen, which may create artefacts. These are structures not originally present or that alter the specimen from its original condition.

2. Scanning Electron Microscope (SEM)The beams of electrons are bounced off the surface of the specimen and create an image from the electrons that are refl ected from the surface. The specimen used for SEM can be much thicker than that used for TEM, as there is no need for the electrons to penetrate through. The electrons are scattered by the specimen and as the scattering depends on the contours of the specimen surface, a three-dimensional image is produced.

Limitations of electron microscopy

Although electron microscopes produce a higher magnifi cation and greater resolution, there are a

number of limitations that need to be taken into account:

i. There must be a vacuum, as electrons can be scattered in all directions by air

molecules.

ii. Specimens are always dead.

iii. Tissue being observed has to undergo several stages of preparation, and each stage

may alter the ‘real’ appearance.

iv. Preparation may introduce artefacts.


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15 Microscopes

What are the differences between light and electron microscopes?

The following table summarises the main differences between the two types of microscope.

Feature Light microscope Electron microscope

Radiation Light rays Beams of electrons

Focusing Glass lenses – objective lens

and eyepiece

Electromagnetic lenses

Magnifi cation � 1,500 � 500,000

Resolution Low – 0.2 �m High – 0.0001�m

Preparation of specimen Lower chance of producing

artefacts

Greater chance of producing

artefacts

Specimen Dead or alive, as no vacuum Dead, as vacuum used

Image Viewed through eyepiece Viewed on fl uorescent screen

Equipment cost Relatively cheap Very expensive

Working out the size and magnifi cation of a specimen

Calculating the actual size

If the size of an image and the magnifi cation is known, then the actual size of the specimen can be

calculated using the following equation:

size of the image actual size � magnifi cation

Remember that the size of the image is usually measured in mm and should be converted into �m.

Worked example

X Y

matrix

cristae

innermembrane outer

membrane

If a mitochondria was magnifi ed by � 5,000, measure the diameter from point X to point Y and

calculate the actual size.

Size of the image from X to Y � 50mm

Actual size � 50 � 1,000

5,000

Answer � 10�m



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15Microscopes

Calculating the magnifi cation

Use the following equation to work out the magnifi cation:

magnifi cation = size of the image (mm � 1,000 to convert to �m) ÷ actual size

I

A M

I = image (usually in mm)

A = actual size (usually in m)

M = magnification

Questions

1 What is meant by ‘magnifi cation’?

2 Defi ne the term ‘resolution’, as used in microscopy.

3 Suggest two reasons why structures seen in electron microscopes might not

accurately represent those in a living cell?

4 Describe the difference between SEM and TEM.

5 If the diameter of cell is 35mm and magnifi cation is � 500, what is the actual size?

6 If the size of an image is 75mm and the actual size is 5�m, what is the magnifi cation?

7 What is the advantage of using an optical microscope?

8 What is the magnifi cation of an image of a chloroplast if the actual size is 8µm and the

image is measured at 24mm?

Now make more detailed notes about microscopes. Make a table of comparisons between optical

and electron microscopes, and a summary of the limitations of electron microscopes.

The questions below can help you to structure and organise your notes. Use your course text

book to add further information.

⇒ Name the different types of microscopes.

⇒ Which type of microscope has a better resolution?

⇒ What is the formula for calculating actual size?

⇒ Why are images obtained using electron microscopes not a true refl ection of the specimen?

⇒ How can you measure magnifi cation from a drawing provided?

⇒ List four differences between light microscopes and electron microscopes.

⇒ In a light microscope, if the eyepiece is � 10 and the objective lens is � 4, what is the

magnifi cation?

⇒ What is the maximum magnifi cation of optical microscopes used by A-level Biology

students in the laboratory?

Taking it Further


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What do I need to know about prokaryotic cells?

Prokaryotic cells are bacterial cells. They do not have a nucleus; instead they have naked DNA

in the cytoplasm, which is not membrane bound.

Key features of a prokaryotic cell (for example the bacterium E. coli)

• Small cells – approximately 5�m.

• Many have a slime layer/capsule that provides protection.

• Cell wall is made of murein.

• Contains plasma membrane.

• No nucleus.

• Has loop of DNA and tiny circles of DNA called plasmids.

• No membrane-bound organelles (that is, no chloroplast, mitochondria, endoplasmic

reticulum (ER) or Golgi body).

• Small ribosome is present free in the cytoplasm.

• Some have a fl agellum (plural: fl agella) – a tail-like structure that allows the bacterial cell

to move.

capsule (slime)

cell wall

plasma membrane

plasmid

nucleoid

DNA

flagellumm

Prokaryotic Cells16A-levelYou will need to familiarise yourself with the different types of cells. You should be able to state the similarities and differences between bacterial, plant and animal cells.

You should also remember an example of each cell type.

GCSEYou should know that cells are the building blocks for all organisms, and therefore make up all living things. Simple organisms, such as amoebas, are made up of one single cell, whereas complex multicellular organisms, such as humans, are made up of billions of cells. A prokaryotic cell has no nucleus.

You will need to be able to compare the features found in prokaryotic cells with those found in eukaryotic cells. Eukaryotic cells are discussed in detail in Topic Builder 17.



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Questions

1 What is the function of the fl agella?

2 Name the organelle present in the cytoplasm of prokaryotic cells.

3 Which feature provides protection for bacterial cells?

4 What are the tiny circles of DNA found in E. coli?

5 Do bacterial cells contain chromosomes?

Make more detailed notes about prokaryotic cells.



⇒ What are prokaryotic cells?

⇒ What is the cell wall of a bacterial cell made up of?

⇒ Explain the function of the slime layer.

⇒ What are the pieces of DNA that give bacteria protection against antibiotics called?

⇒ What is the approximate size of a bacterial cell?

⇒ What type of ribosomes are found in prokaryotic cells?

⇒ Give an example of a prokaryote.

Prokaryotic Cells

Taking it Further


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What do I need to know about eukaryotic cells?

Eukaryotic cells include protoctista (algae, amoebas), fungi, plant and animal cells. These are

more complex cells than prokaryotic cells (see Topic Builder 16) and contain more organelles.

Eukaryotic cells have a large nucleus that contains the DNA inside a membrane.

Plant and animal cells are examples of eukaryotic cells.

The similarities and differences between plant and animal cells

cytoplasm

cytoplasm

cell wall

cellmembrane

cell membrane

vacuole

nucleusnucleus

lysosome

nucleolus

nucleolus

nuclearmembrane

nuclearmembrane

chloroplast

mitochondriamitochondria

amyloplast

rough ER

smooth ER

rough ER

smooth ER

ribosomesribosomes

Golgi body

Golgibody

vacuole

Plant cell Animal cell

Key features of eukaryotic cells

• Approximately 50�m in size (range 10–100�m).

• Plasma membrane surrounds the cell content.

• Contain chromosomes/DNA inside the nucleus.

• Contain a nucleolus – a dark region inside the nucleus.

• Have a large number of membrane-bound organelles, such as endoplasmic reticulum

(ER), mitochondria, Golgi body/apparatus and lysosomes.

• Plant and animal cells contain large ribosomes. These are free in the cytoplasm or

attached to ER.

• Plant cells also have chloroplasts, a permanent central vacuole and a cellulose cell wall.

Eukaryotic Cells17A-levelYou will need to familiarise yourself with the different types of cells. You should be able to state the similarities and differences between bacterial, plant and animal cells.

You should also remember an example of each cell type.

GCSEYou should know that cells are the building blocks for all organisms, and therefore make up all living things. Simple organisms, such as amoebas, are made up of one single cell, whereas complex multicellular organisms, such as humans, are made up of billions of cells. A eukaryotic cell has a nucleus.



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Make more detailed notes about eukaryotic cells and any key features found in these cells.



⇒ What are eukaryotic cells?

⇒ List two main features of eukaryotic cells.

⇒ What similarities and differences exist between eukaryotic and prokaryotic cells?

⇒ What type of ribosomes are found in prokaryotic and eukaryotic cells?

⇒ Do all animal cells have a nucleus?

17Eukaryotic Cells

Task and Questions

Task

Complete the table below by placing a tick (✓) if the feature is present or a cross (✗) if it is

absent. You will need to have worked through Topic Builder 16 before attempting this task.

Feature E. coli Palisade cell Liver cell

Plasma membrane

Cell wall

Nucleus

Ribosomes

Golgi body/

apparatus

Chloroplast

Mitochondria

Permanent vacuole

Ring of DNA

Questions

1 Which cell type contains a cellulose cell wall?

2 What name is given to the outer layer of an animal cell?

3 What three organelles are found only in palisade cells?

4 List the four groups of eukaryotic cells.

5 Which organelle is only found in animal cells?

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There are a large number of membrane-bound organelles found within the cytoplasm of cells,

and each has its own specialised function.

Which organelles will I need to know and what are their roles within a cell?

An organelle is a component in the cytoplasm that is surrounded by a plasma membrane. The

table below shows a concise summary of the structures and functions of the main organelles you

will need to learn for A-level Biology.

The drawings below are general representations. Check with your exam board for the specifi c

features you will need to be able to recognise for each of the organelles.

Cell organelle Structure Function

Cell membrane(all cells)

Double layer of

phospholipids, with protein

• Selectively permeable

• Barrier between cell and

environment

• Controls movement of

material into/out of cell

Nucleus(eukaryotic cells only)

Largest organelle, enclosed

by a double membrane

called the nuclear envelope. Has a region

called the nucleolus

• Contains chromosomes

(genetic information)

• Controls all cellular activities

• RNA is manufactured and

ribosomes are assembled in

the nucleolus

(Continued)

Cell Organelles18A-levelYou will need to be able to relate structure to function for a range of membrane-bound organelles. You may be asked to draw an organelle and describe its precise role within the cell. Some of the organelles may be unfamiliar, such as mitochondria, Golgi body and endoplasmic reticulum (ER). It is important to learn their names and know where they are located within the cell.

GCSE knowledgeYou will have knowledge of some organelles found in plant and animal cells.

You should also know that organelles such as the nucleus, cytoplasm and cell membrane are found in both types of cell, but that organelles such as cell walls, chloroplasts and permanent vacuoles are only present in plant cells.



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Cell Organelles 18Cell organelle Structure Function

Endoplasmic reticulum (ER) (eukaryotic cells only)

A network of sac-like and

tubular cavities called

cisternae, formed from

membrane

• Two types of ER: smooth

and rough

• Rough ER (RER) has

many ribosomes attached;

involved in protein synthesis

and transports protein

• Smooth ER (SER) is involved

in lipid synthesis

Golgi body/apparatus(eukaryotic cells only)

Stack of fl attened sacs

surrounded by membrane.

Form from RER at one end

and vesicle (a membrane-

bound sac) breaks off at

the mature end

• Transports and modifi es

proteins

• Packages and exports

proteins

Mitochondria(eukaryotic cells only)

Double membrane

structure, with folded inner

membrane (folds are called

cristae). Fluid inside is

called matrix, and contains

DNA, proteins, lipids and

respiratory enzymes

• Responsible for aerobic

respiration

• Converts energy in organic

molecules into ATP, making

them the powerhouse of the

cell

Ribosome

(all cells)

Formed from two subunits:

protein and rRNA. May

be free in cytoplasm or

attached to ER

• Site of protein synthesis

Lysosomes(usually in animal cells)

Simple round sacs

surrounded by single

membrane containing

digestive enzymes

• Digestion of worn out cell

structures

• Ingests material through

a process called

phagocytosis

• Releases enzymes to

destroy and break down

cells

(Continued)


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Cell Organelles18Cell organelle Structure Function

Cytoplasm(all cells)

Clear, thick, jelly-like

material (cytosol), found

inside cell membrane.

Contains the cytoskeleton

• Site of many metabolic

reactions

• Macromolecules produced

by the cell are manufactured

here

Chloroplast(plants cells only)

Green oval-shaped

organelle containing

chlorophyll (the green

pigment). Double

membrane with inner

membrane modifi ed into

sacs called thylakoids.

Contains gel-like innermost

substance called stroma

• Carries out the process

of photosynthesis,

during which light energy

is converted to chemical

energy

Cell wall(in bacteria, called murein; in

plant cells, called cellulose)

Outer layer in plant cell.

Made from cellulose fi bres

• Provides strength and

support for the plant cell

Vacuole(plant cells – large; animal cells –

small)

Temporary in animal cells.

Permanent fl uid-fi lled sac

in plant cells, surrounded

by membrane called the

tonoplast

• May contain waste products

• Maintains internal

hydrostatic pressure and

helps provide turgor in plant

cells. Contains cell sap

Questions

1 Which organelles are found in all types of cell?

2 What is the function of the nucleolus?

3 How many layers does the mitochondria’s membrane have?

4 What type of ER has ribosomes on its surface?

5 What is the function of the cellulose cell wall?

6 What is the liquid part of the cytoplasm called?

7 What makes up the cisternae in the Golgi apparatus?

8 Which two subunits make up ribosomes?

9 What is required by chloroplasts for photosynthesis?

10 A cell surface membrane is made up of ………………..



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18Cell Organelles

Make more detailed notes about all the different organelles found within cells. It may be useful to

draw each organelle and then relate its structure to its function.

Some questions have been suggested below to help structure your notes. Use your course text


⇒ What is an organelle?

⇒ What does the mitochondrial matrix contain?

⇒ Give all the functions of the nucleus.

⇒ Find out at least four functions of the Golgi apparatus.

⇒ Compare the structure of a chloroplast with that of mitochondria.

⇒ What is the role of lysosomes in the animal cell?

⇒ What increases the surface area of the inner mitochondrial membrane?

⇒ Do all cells have vacuoles?

⇒ How is the cell wall different in plant and bacterial cells?

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What do I need to know about a cell membrane?

A cell (plasma) membrane provides a number of different functions.

• It excludes some substances permanently from the cell.

• It keeps some substances inside the cell.

• It allows some molecules to move freely into or out of the cell.

The membrane therefore determines, or regulates, what

moves into and out of the cell and can be described as

partially permeable, semi-permeable or selectively permeable. It is permeable only to specifi c molecules such

as water and some solutes.

Movement across the cell membrane may be either passive or active.

(See Topic Builder – 20: Movement Into and Out of Cells p59.)

A closer look at the structure of the membrane

There are number of compounds that help to make up the membrane:

• phospholipids (lipid bilayer)

• proteins

• cholesterol

• carbohydrate chains – if attached to protein � glycoproteins

– if attached to lipid � glycolipids

protein

cellmembrane

proteinchannel

Outside of cell

Inside of cell(cytoplasm) lipid bilayer

7nm

carbohydratechain

19 Structure of the Cell Membrane

A-levelYou will now need to have a good understanding of the different components that make up the cell (plasma) membrane and be able to identify them in a diagram. You will need to explain the role of the cell membrane in allowing movement of molecules into and out of the cell.

GCSEYou may know the importance of the cell membrane in determining what moves into or out of the cell. (It is a barrier between the cell content and the environment.)

You should recall that the cell membrane is found in all types of cell as well as in many organelles inside the cell.

The cell membrane has the same structure as the membrane that surrounds cell organelles. The structure may be described using the fl uid mosaic model.



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19Structure of the Cell Membrane

The structure of the membrane can be explained in terms of the fl uid mosaic model, in which

the phospholipids form a fl uid bilayer.

• The phospholipid molecules are partly hydrophilic (water soluble) and partly hydrophobic

(water insoluble). In water, the phospholipids form a double layer with the hydrophilic heads

in contact with water and the hydrophobic tails away from water.

• The proteins are arranged randomly within the phospholipids. The proteins form a mosaic

pattern that is constantly changing since the proteins are able to move around. The

proteins that span the membrane are called integral proteins, extrinsic proteins are on

the outside and those found on only one side are peripheral proteins.

• This structure is only a model – it is an idea put forward – as a membrane can only be

observed through an electron microscope.

The table below shows the main components of the cell (plasma) membrane and their role.

Component Role

Phospholipids • Form the lipid bilayer

• Provide a barrier to water

• Help to make the membrane fl uid

• Lipid-soluble molecules can diffuse through the phospholipid

layer

Proteins • Have specifi c shapes that determine function

• Water-soluble molecules and ions diffuse through protein pores

• Transport substances by acting as channels, carriers and

pumps

• Enzymes located in membrane

• Electron carriers in respiration and photosynthesis

• Hormone binding sites

Cholesterol • Provides stability to the membrane and stops it breaking up

Glycoproteins/Glycolipids

• Recognition and binding sites for some molecules

• Cell-to-cell recognition/communication

• Receptors for hormones

• Antigen recognition

• Found on the outside of the cell membrane


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Make more detailed notes about the structure of the cell (plasma) membrane. It may be useful to

summarise the role of the different components in a table. Drawing a diagram of the membrane

would also be good for revision.



⇒ What is a plasma membrane and why is it needed?

⇒ Defi ne the terms ‘hydrophilic’ and ‘hydrophobic’.

⇒ Explain why the fl uid mosaic model is used to describe the structure of the cell membrane.

⇒ Explain the different function of the proteins in the membrane.

⇒ What are the functions of the phospholipids in the cell membrane?

⇒ List the roles of glycoproteins in the membrane.

⇒ What types of molecules diffuse through protein pores?

Structure of the Cell Membrane19Task and questions

Task

Label the diagram of the membrane below.

A

B CD

E

Questions

1 The model of a cell (plasma) membrane structure is called the ………… ? (three words)

2 Defi ne ‘integral protein’.

3 What is the diameter of the membrane?

4 What is the function of cholesterol in the membrane?

5 What does the term ‘partially permeable’ mean?

6 What is a lipid referred to if it has a carbohydrate chain attached?

7 On which side of the membrane would you fi nd extrinsic proteins?

8 What type of molecules are able to diffuse through the phospholipid layer?

Taking it Further



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What different forms of transport are there?

1. Passive transport

Diffusion

• This is the net (overall) movement of molecules, or ions, from a region where they are highly

concentrated to a region where they are less concentrated, that is, down a concentration gradient.

• Each molecule or ion diffuses down its own concentration gradient, so it is possible to have

two substances diffusing in opposite directions.

• Molecules such as water (H2O) and carbon dioxide (CO

2) can move freely in gases and

liquids.

• Ions such as sodium (Na�) and chloride (Cl�) move freely in solution.

• Movement occurs until there is equilibrium between the two regions.

phospholipidHigh

concentration

Lowconcentration

Factors that increase the rate of diffusion:

• Difference in concentration gradient.

• Increase in temperature.

• Surface area for molecules to diffuse across.

• Size of the diffusing molecule or ion.

• Distance the molecule has to diffuse across.

Movement Into and Out of Cells 20A-levelYou will need to know that transport of substances into and out of cells occurs across the cell (plasma) membrane. The cell membrane is referred to as semi-permeable, partially permeable or selectively permeable. The movement can be either:

• passive transport, which requires no energy and includes diffusion and osmosis; or

• active transport, which requires energy.

GCSEYou will have learned that all cells need nutrients, which are used to generate energy or to build up cell structures, and that they require water and salts to carry out metabolic reactions.

You will have learned that cells need to remove waste substances such as carbon dioxide and urea, which, if accumulated in the cell, would inhibit the cell’s activities or may result in the death of the cell.

The movement of molecules is from an area of high concentration down to an area of low concentration.


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20 Movement Into and Out of Cells

Special type of diffusion – facilitated diffusion

Some large molecules (glucose and amino acids) normally pass into the cell through special proteins that speed up the rate at which these molecules move across the membrane. These

transport proteins are specifi c, as they have a particular shape and only allow the passage of one

type of molecule. This means that the membrane is selectively permeable.

Outside of cell

Inside of cell

Osmosis

• This is the movement of water molecules from a region where there are more water

molecules to one where there are fewer water molecules.

• Movement of water is from a dilute solution to a concentrated solution.

• Water moves from high water potential (�) to low water potential.

• Osmosis only involves the diffusion of water.

• The water potential of pure water (only water molecules are present) is zero.

watermolecules

solutemolecules

Dilute solution Concentrated solution

Low concentrationof solute

High concentrationof solute

membrane

Net movement of water

Osmosis in animal cellsa. If an animal cell in placed in dilute

solution (hypotonic solution), the cell would burst, due to lack of a cell wall, as there is net movement of water into the cell down a concentration gradient.

b. If an animal cell is placed in a solution with the same concentration as the cell (isotonic solution), then there is no net movement across the membrane and no change in the shape of the cell.

c. If the animal cell is placed in a concentrated solution (hypertonic solution), then the cell would shrink, as water moves out of the cell and into the surrounding solution.

Osmosis in plant cellsa. If a plant cell is placed in dilute solution (hypotonic

solution), it becomes turgid, as it takes up the water from the solution.

b. If the plant cell is placed in a solution with the same concentration as the cell (isotonic solution), there is no change in the shape of the cell.

c. If a plant cell is placed in a concentrated solution (hypertonic solution), it becomes fl accid, as water moves out of the cell and into the solution. It may become plasmolysed as the cytoplasm is pulled away from the cell wall.



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Movement Into and Out of Cells 20

Cells placed inconcentratedsalt solution

Plant cell placed in

concentratedsalt solution

Cells placed indistilled water

Plant cell placed in

distilled waterCells swelland burst

Cells shrinkand shrivel

H2O H2O

cell wallplasma membrane

cytoplasmvacuolenucleus

Cell stiffens butgenerally retains shape

Cell body shrinks andpulls away from cell wall

Solution effects on red blood cells Solution effects on plant cells

2. Active transport

• Active transport is the movement of molecules or ions against a concentration gradient.

• This process requires energy, usually in the form of ATP from respiration.

• Cells that carry out active transport have a large number of mitochondria, which are the site

of aerobic respiration.

• Movement involves a large number of carrier proteins that act as pumps.

• Each carrier protein is specifi c for a particular type of molecule or ion.

• Active transport is a highly selective process.

Lowconcentration

Highconcentration

ATP ADP + Pi

membrane proteinpumps


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20 Movement Into and Out of Cells

Task and Questions

Task

Complete the table below by placing tick (✓) if correct or cross (✗) if incorrect

Feature Osmosis Facilitated diffusion

Active transport

Diffusion

Movement

is down a

concentration

gradient

Requires

energy (ATP)

Proteins are

involved

Only water is

involved

Movement

is against a

concentration

gradient

Carrier proteins

act as pumps

Questions

1 What happens if an animal cell is placed in pure water?

2 Defi ne ‘diffusion’.

3 List three factors that increase the rate of diffusion.

4 What happens if a plant cell is placed in strong sugar solution?

5 What is the water potential (�) of pure water?

6 Explain the difference between active transport and facilitated diffusion

7 What would you expect to happen if an animal cell was placed in hypertonic solution?

8 How does facilitated diffusion differ from diffusion?

9 Which process involves the movement of water only?

10 What would you expect to see if a red blood cell was placed in isotonic solution?

11 Defi ne the term ‘turgid’.

12 In which transport process does movement occur against a concentration gradient?



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Now make more detailed notes about how molecules move across cell membranes. You may fi nd

it useful to create a table summarising the similarities and differences between types of movement

across the membrane.



⇒ What different types of transport are there across cell membranes?

⇒ What is the role of proteins in the transport of substances across the cell membrane?

⇒ Compare the results of osmosis in a palisade cell and a red blood cell.

⇒ Find out how bulk movement occurs across a membrane.

⇒ How does water potential affect osmosis?

⇒ Explain the terms hypotonic, isotonic and hypertonic.

⇒ Which process provides the ATP needed for active transport?

⇒ What does Fick’s Law state?

⇒ What equation is used in Fick’s Law and how does it relate to diffusion?

20Movement Into and Out of Cells

Taking it Further


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What will I need to know about carbohydrates?

Carbohydrates contain the elements carbon, hydrogen

and oxygen. Carbohydrates include sugars, starches,

cellulose and glycogen.

Carbohydrates can be subdivided into three groups:

1 monosaccharides

2 disaccharides

3 polysaccharides

What are monosaccharides?

• These are simple, single sugars, or monomers.

• They have a general formula CnH2nOn.

• They dissolve easily in water to form sweet solutions.

• The most common examples of a six-carbon sugar (hexose) are glucose, fructose and

galactose.

• Glucose has the formula C6H12O6.

• Glucose is the main respiratory substrate and building block for both disaccharides and

polysaccharides.

The structure of glucose can have two forms:

alpha (�) glucose beta () glucose

Both forms of glucose are important in the formation of

polysaccharides.

Structure and Function of Carbohydrates21A-levelYou will need to know the structural differences between sugars, starches, cellulose and glycogen.

You will need to be able to classify carbohydrates according to their size, and link functions of these carbohydrates to their structure and location.

GCSEYou will be familiar with different groups of carbohydrates in our diet, especially sugar and starch. You will recognise sucrose as the sugar found most commonly in our diet, as a source of energy. You should also know that starch is found in potato, bread, rice and cereals.

As carbon is the central element in carbohydrates, they are sometimes referred to as organic molecules.

OH

OHOHOH

OH

H

H

CH2OH

HH

OH

OH

OHOH

OH

H

H

CH2OH

H

H

The numbers in the diagrams, left, refer to the C (carbon) number in the formula.

The only difference between � and � glucose is the position of the hydroxyl (OH) group on C 1. A useful way to remember this is:

� � OH is pointing away from the oxygen

� � OH is beside the oxygen



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Structure and Function of Carbohydrates 21

Other hexose sugars you should be familiar with are fructose (C6H12O6) and galactose (C6H12O6).

These are structural isomers of glucose, as they have the same formula but different structures.

HO

OH

OH

O

C H

H

H

C C

C

CH2OH

CH2OH

HO OH

OH

OH

O

C C

C

C CH

H

H

CH2OH

fructose

H

galactose

How do monosaccharides join together?

Two monosaccharides are joined together by a condensation reaction in which water is

released. The bond formed between the two molecules is called a glycosidic bond.

HO

OH

OHOH

OH

H

H

H

CH2OH

H2O

H

HO

OH

OHOH

OH

H

H

H

CH2OH

H

HO

OH

OH

O

O

H

H

H

H

CH2OHH

OH

OHOH

OH

H

H

H

CH2OH

H

1– 4glycosidic

linkage

1 4

glucose glucose maltose

What are disaccharides?

• They have a general formula of C12H22O11.

• They transport energy or respiratory substrate.

• Two monosaccharides are linked together to form a disaccharide through a condensation

reaction (in which water is removed).

• They contain a glycosidic bond (see diagram above).

• Glycosidic bonds can be broken by hydrolysis reactions, using water and enzymes.

Disaccharide example

Source Enzyme required to break down disaccharide

Monosaccharides

maltose malt maltase glucose and glucose

sucrose cane sucrase glucose and fructose

lactose milk lactase glucose and

galactose

What are polysaccharides?

• They have a general formula Cn(H2O)n�1 � H2O (n � number)

• Many single sugars are joined together by condensation reactions to form large macromolecules.

• They can have a range of functions, including energy storage and structural roles in

plant cells.


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Examples of polysaccharides include:

1 Starch – found as a storage molecule in plant cells. It is a mixture of two types of polymers

of � glucose: amylose and amylopectin.

Different plants have different percentages of amylose (unbranched, spirals held by hydrogen bonds, or H-bonds) and amylopectin (branched, forms a slightly packed brush like molecule).

OH

OH

O

O

HH

CH2OH

O

OH

OH

OH

H

CH2OH

O

O

HO

OH

O

OH

CH2OH

OH

OH

O

O

CH2OH

O

OH

OH

O

O

CH2OH

amylose amylopectin

1,6 glycosidic linkagebranch point (shown here in grey)

1,4 glycosidic linkage (shown here in grey)

11

1

6

44

Starch is a compact structure, therefore many glucose molecules can be stored in a small

space. Starch is insoluble and does not affect the osmotic potential of the cell.

2 Cellulose – found in plant cells and is the main component of the cell wall, made from �

glucose. It forms long fi bres that provide structural support and extra strength, allowing the

cell wall to expand slightly when the cell takes up water by osmosis.

O O

OO O

CH2OH

CH2OH

O O

O

CH2OH

CH2OH

cellulose

3 Glycogen – a storage carbohydrate in animal cells made up of � glucose. It is very similar

to amylopectin but very highly branched.

OH

OH

OH

H

H

CH2OHH

OH

OH

OH

H

H

H

CH2OH

O

H

O

H

OH

OH

OH

H

H

CH2

H

OH

OH

OH

H

H

H

CH2OH

O

H

O

H

O

OH

OH

OH

H

H

CH2OHH

O

H

Obranchingoccurs here

glucosemonomer

Structure and Function of Carbohydrates21



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21Structure and Function of Carbohydrates

Glycogen can be hydrolysed to release glucose; this can be used in respiration to release energy

in the form of ATP.Tasks and Questions

Task 1

Complete the table below by placing a tick (✓) in the box if the statement is correct for each

type of molecule.

Statement Glucose Starch Cellulose Glycogen

Has the formula

C6H

12O

6

Formed by a

condensation

reaction

Building block is

glucose

Joined by

glycosidic bonds

Found as a

component of cell

wall

Highly soluble

Found as storage

molecule in

mammals

Made from

amylose and

amylopectin

Task 2

Use the diagrams below to help you answer the following questions.

OH

OHOHOH

OH

H

H

CH2OH

HH

A

OH

OH

OHOH

OH

H

H

CH2OH

H

H

B

OH

HOOH

O

O

H

H

H

CH2OHH

C

OH

OHOH

OH

H

H

H

CH2OH

HH

Which molecule/molecules (A, B or C):

1 Has a glycosidic bond?

2 Is the main respiratory substrate?


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Make more detailed notes about the structure and functions of different carbohydrates. You may

fi nd it useful to summarise the key differences in structure and relate these to their functions.

Drawing diagrams of these molecules would also be good for revision.



⇒ What are carbohydrates?

⇒ What is a structural isomer?

⇒ Name the three different hexose monomers?

⇒ What is the function of the disaccharides?

⇒ Explain what causes the difference in structure of amylose and amylopectin?

⇒ How does cellulose provide structural support to a plant cell wall?

⇒ What is the difference between amylopectin and glycogen?

⇒ Which polysaccharide has the monomer glucose?

⇒ Where would you fi nd starch in a leaf?

⇒ Find out what causes lactose intolerance in some people.

Structure and Function of Carbohydrates213 Is the monomer used to build cellulose?

4 Is formed by a condensation reaction?

5 Is only found in plant cells?

6 Has the formula C6H

12O

6?

Questions

1 What is the general formula of glucose?

2 Name the monomers that make up lactose.

3 What is the name of the carbohydrate storage molecule in animal cells?

4 What is released as a waste product during a condensation reaction?

5 Apart from lactose, what are the other two disaccharides?

6 How many monomers make up monosaccharides, disaccharides and

polysaccharides?

7 What reaction is involved in the breakdown of sucrose?

Taking it Further



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What do I need to know about lipids?

Lipids are a large, varied group of compounds that have

very important roles in all organisms.

• Lipids contain much smaller proportions of oxygen to carbon and hydrogen, compared

with carbohydrates.

• Lipids are insoluble in water.

• Lipids are soluble in organic solvents, such as ethanol.

• Lipids contain more energy per gram compared with carbohydrates, but are more diffi cult

to break down, or hydrolyse.

What are the main groups of lipids?

1. Triglycerides (fats and oils)

A triglyceride consists of:

i. a glycerol molecule (C3H

8O

3), which remains the same for all lipids; and

ii. three fatty acid chains. These are long chains of carbon with COOH (carboxylic acid) at

one end. They are sometimes represented as RCOOH. The R refers to the carbon part

of the chain; this part varies for different fatty acids.

H

H

H OHC C C

O

C

H

H

C

H

H

C

H

H

C

H

H

C H

H

H

H OH

HO

H OHC

C C

O H

H

C

H

H

C

H

H

C

H

H

C H

H

H

HO

C C

O H

H

C

H

H

C

H

H

C

H

H

C H

H

H

HO

H

H

H OC C C

O

C

H

H

C

H

H

C

H

H

C

H

H

C H

H

H

H

3H2O

O

H OC

C C

O H

H

C

H

H

C

H

H

C

H

H

C H

H

H

C C

O H

H

C

H

H

C

H

H

C

H

H

C H

H

H

simplified

glyc

erol

glycerol fatty acids triglyceride molecule

fatty acid

fatty acid

fatty acid

ester bonds

Glycerol and the three fatty acid chains combine by condensation reactions, with the release of

three water molecules and three ester bonds being formed.

Triglycerides are not polymers as they are not made up from repeating monomers such as

starch or proteins.

Structure and Function of Lipids 22A-levelYou will need to know the structural difference between fats and oils, which are examples of triglycerides. You will need to know how the structure of a triglyceride is important in relation to its function. You will also need to know how lipids perform their many different functions in living organisms.

GCSEYou may have some knowledge of fats and oils as sources of energy storage in living organisms. Fats are from animal sources and oils are obtained from plants. You may know that lipids also provide heat insulation and waterproofi ng.

Lipids are organic molecules made up of carbon, hydrogen and oxygen.


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Structure and Function of Lipids22Why are fats and oils different?

The difference between fats and oils is due to variations in the fatty acids. There are many different

fatty acids; the number of carbons and hydrogens in the chain varies but the COOH always

remains the same.

There are two types of fatty acid chains:

Saturated fatty acids – these have no double bonds between any of the carbon atoms along the chain. All the carbon atoms are joined with hydrogen atoms.

C

H

H

C

H

H

C

H

H

C

H

H

CCO

O

H

HH

C

H

H

C

H

H

C

H

H

C H

H

H

• Have higher melting point than unsaturated fatty acids

• Are solid at room temperature

• Found in animal fats

Unsaturated fatty acids – these have some double bonds between carbon atoms along the chain.

One double bond � monounsaturated

More than one double bond (can be many) � polyunsaturated

C

H

H

C

H

H

C

H

H

C

H

H

CCO

O

H

HH

C

H

C

H

C

H

HC H

H

H

• Have lower melting point

• Are liquid at room temperature

• Found in plant oils

2. Phospholipids (a special type of lipid)

Phospholipids are very similar to triglycerides except that one of the fatty acid chains has been

replaced by a phosphate group, therefore a phospholipid has two parts:

hydrophilichead

(phosphate-containing)

hydrophobic tail(fatty acid tail)

i. The part with the phosphate group is attracted to water –

hydrophilic – and is therefore water soluble. This is sometimes

called the hydrophilic head region.

ii. The part with the fatty acid chains repels water – hydrophobic –

and is therefore not soluble in water. This is sometimes also called

the hydrophobic tail region.

Phospholipids are an important component of the plasma membrane.

They arrange themselves in a double layer when mixed in water,

forming the bilayer arrangement.

They also allow lipid-soluble substances to pass across the membrane.

What are the main functions of lipids?

Lipids perform many different functions in living organisms. These include:

• Energy storage: very good stores of energy in animals and plant seeds, as they are

insoluble and will not be moved out of the cell easily.

• Thermal insulation: this is due to fats being slow conductors of heat; they help to retain

body heat in animals.



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22Structure and Function of Lipids

• Waterproofi ng: since the hydrophobic fatty acids repel water, lipids are good at preventing

water loss, as seen by plants and insects having a waxy cuticle.

• Buoyancy: lipids are less dense than water, making aquatic animals more buoyant.

• Cell membrane: the hydrophilic and hydrophobic regions help to maintain the bilayer of

the membrane.

Questions

1 What are the two compounds that combine to form a triglyceride?

2 Why is a triglyceride not a polymer?

3 How are triglycerides similar to carbohydrates?

4 Are lipids soluble in water?

5 How many fatty acid chains are there in a triglyceride?

6 What is the main difference between saturated fatty acids and unsaturated fatty acids?

7 How many water molecules are produced during the formation of a triglyceride?

8 How does a phospholipid differ from a triglyceride?

9 State four functions of lipids.

10 What is the formula for glycerol?

Make more detailed notes about the structure and functions of lipids. A useful revision tool may be

a fully labelled diagram of a triglyceride and annotations relating it to some functions in plants and

animals.



⇒ What are lipids?

⇒ What types of lipids are there?

⇒ What is a triglyceride?

⇒ What is a phospholipid?

⇒ Draw a diagram that shows where water is removed during a condensation reaction.

⇒ Which parts of a phospholipid are hydrophilic and hydrophobic?

⇒ What is the difference between monosaturated fatty acids and polyunsaturated fatty acids?

⇒ What are the main differences between fats and oils?

⇒ Explain the role of phospholipids in the cell membrane.

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What do I need to know about proteins?

Proteins all contain the elements carbon (C), hydrogen

(H), oxygen (O), nitrogen (N) and some contain sulphur (S).

Proteins are polymers made up from amino acids.

H

H

N C

R

H

O H

O

C

central carbon

amino group – NH2

functional groupcarboxylic acid

group – COOH

R is the only part that is different in the 20 amino acids and is called the functional group. For

example, if the R group is hydrogen, then the amino acid is glycine.

Large numbers of individual amino acids can be joined together by peptide bonds, which are

formed during condensation reactions; this results in a polypeptide chain.

H

H H

H20

H

R O

ON C C

H

H H H

R O

ON C C

H

H H

H O

N C C

H

H

H

O

OC C

HN

peptide bond

Proteins can have four different structures:

1 Primary structure

Arg

Asp

AlaGln

AsnGlyPheGlu This is the order/sequence of amino acids found in a

polypeptide chain. Different proteins have different amino acid sequences that determine their properties and shape.

Structure and Function of Proteins23A-levelYou will need to know the structure of monomers, which are the building blocks for proteins, and how their arrangement determines the structure and ultimately the function of proteins.

You will also need to know the different levels of organisation that proteins can have and the different bonds that hold protein structures together.

GCSEYou may have met proteins and learned how important they are in living organisms. In the human body, proteins have many different functions that are related their structure. You may be familiar with enzymes and how they control the rate of metabolism in cells and organisms.

Proteins are macromolecules, made up of monomers – amino acids. There are 20 different amino acids that make up all our proteins.



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23Structure and Function of Proteins

2 Secondary structure

Here, the amino acid chain forms a coil (�-helix) or �-pleated sheet, held in place by hydrogen bonds (H-bonds). Different parts of a polypeptide can have different secondary structures.

N H O CC H

R R

RR

H

H

HC

N

N

N

N N

H

H

H R

H

H

O

O CC

O CC

C

O C

O CC

O C

O CC R H C RCH R

H

H

N

HHN

-helix

-pleated sheetO O O O

O O

H

H

R

R

R

R

R

H H H

H

H

H HH

H

H

C

C

C

CC

C C

C

C CCC C

C C

O

O O

OR

C

C N

N

NN

NN N

N

H H

H

R

O R

R

R

H H

H

HH

H

HC

C C

C

CC C

C

O

O

NN

N N

H

R

O R

R

R

H H

H

HH

HC

C C

C

CC C

C CO

O

NN

N NH

examples ofamino acids

Proteins that only have a secondary structure can be called fi brous proteins, and have a

structural role, for example keratin in nails and hair.

3 Tertiary structure

This is the folding and twisting of the secondary structure, forming a 3D globular shape. The shape is held in place by a number of different bonds:

i. H-bonds (hydrogen bonds)

ii. Ionic bonds

iii. Disulphide bridges

iv. Hydrophobic interactions

Proteins with a tertiary structure are called globular proteins, such as enzymes, which are

involved in metabolic reactions.

CH2

CH2 CH2

CH

CH

CH2

CH2 CH2

H3C

H3C

NH3+

H3C

H3CO

O

-O

O

C

C

S S

H

HO

hydrogenbond

hydrophobicinteractions

polypeptide backbone

disulphide bridge


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23 Structure and Function of Proteins

4 Quaternary structure

haemoglobin

chains

� chains

Fe2+

Fe2+

Fe2+

Fe2+

A quaternary structure is made up from two or more polypeptide chains joining together to make a molecule. This structure may also contain a non-protein part, such as a haem group in haemoglobin.

These are large complex globular proteins, such as the hormone insulin, which is made

up of two polypeptide chains, and haemoglobin, which has four chains as well as a haem

group (containing iron).

Proteins are sensitive to changes in temperature and pH. Any extreme movement away from

optimum conditions in these factors causes the bonds holding the shape together to break.

This results in the shape changing and hence preventing the protein from functioning properly. The

protein is denatured.

Task and Questions

Task

Match the description with the name of the protein structure.

Description Structure

Globular compact 3D shape held in

position by ionic bonds, H-bonds and

disulphide bridges

Sequence of amino acids in a chain

forming the polypeptide

Can be either a helical structure or pleated

sheet, having a structural role with the

shape held by H-bonds

Structure due to the result of a combination

of more than two polypeptide chains,

forming a large complex molecule



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Make more detailed notes about the structure and functions of proteins. It may be useful to

summarise the different levels of protein structures and relate these to their functions.



⇒ What are proteins?

⇒ Which monomer units combine to form polypeptides?

⇒ Name the four chemical groups attached to the central atom of an amino acid?

⇒ What does the primary structure of a protein determine?

⇒ What is the secondary structure of a protein?

⇒ What is the tertiary structure of a protein?

⇒ What is the quaternary structure of a protein?

⇒ Give examples of fi brous and globular proteins.

⇒ What are the similarities and differences between amylase and insulin?

⇒ Which environmental factors can affect the structure of proteins?

23Structure and Function of Proteins

Questions

1 Name the type of bond formed between two amino acids.

2 What type of reaction is involved in the formation of a polypeptide chain?

3 How many different R groups are there?

4 What elements make up amino acids?

5 What is removed when a dipeptide is formed?

6 What is the protein structure of keratin?

7 Name the bonds that hold the tertiary structure in place.

8 What are the four levels of protein organisation.

9 Draw and label an amino acid.

Taking it Further


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What do I need to know about enzymes and the way they work?

Enzymes are highly specifi c and are involved in only one

reaction type, therefore showing specifi city. Biochemical

reactions in cells are controlled by enzymes and these reactions can be:

• catabolic – this is when large molecules are broken down into smaller subunits, such as in

the digestion of nutrients; or

• anabolic – this is when smaller subunits are used to build up larger molecules, such as

when the body makes proteins.

Enzymes lower the activation energy, which is the minimum energy needed to break chemical

bonds in molecules, and hence allow reactions to occur.

activation energy

lower activationenergy (usinga catalyst)Reactants

Products

Energy

Time

Enzymes speed up the rate of a reaction by lowering the activation energy necessary to start the reaction. Less energy is needed to start a reaction when an enzyme is present.

Why is the structure of enzymes important?

Enzymes are globular proteins with a compact 3D shape that is held in place by a number of

bonds, including hydrogen bonds, disulphide bridges and ionic bonds. The site on the enzyme

molecule where the reaction takes place is called the active site and it may have a precise shape

that only allows certain substrates to combine, which gives the enzyme specifi city.

How do enzymes work?

The active site, a region of the enzyme molecule that binds with the substrate (reactant)

molecule, is where the reaction takes place. An enzyme–substrate complex is formed when the

enzyme and substrate are in contact with each other.

Enzyme Structure and Mode of Action24A-levelYou will need to know that enzymes act as biological catalysts and are involved in a wide variety of reactions.

You will need to know that all enzymes are proteins, with a specifi c tertiary structure; this is important for the reaction they control as it provides specifi city.

You will learn that modes of action can be different for different enzymes.

GCSEYou will know that enzymes are catalysts and speed up reactions in cells. Without enzymes, reactions would occur very slowly, or may not occur at all.

You may know that enzymes are made up of proteins and that there are many different types of enzymes in living organisms.

Enzymes are globular proteins with a precise tertiary structure.



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Enzyme Structure and Mode of Action 24

substrate

active site

Substrate enteringactive site of enzyme

Enzyme–substratecomplex

Enzyme changes shapeslightly as substrate binds

Enzyme–productscomplex

Products leavingactive site of enzyme

products

E � S E�S complex E � P

eg, maltase � maltose � water maltase � glucose � glucose

There are two models for enzyme action:

1 Lock and key model – this is when the active site and the substrate are complementary

(molecules of the substrate fi t the active site), in the same way that the shape of a key is

complementary to the lock it fi ts in.

+

enzyme enzyme–substratecomplex

enzyme(unchanged)

product

active sitesubstrates

2 Induced fi t model – suggests that the active site is very similar to the substrate, and

when the substrate binds to the active site, it moulds around the substrate. As it alters its

shape, the enzyme puts the substrate molecule under strain, which results in the reaction

taking place.

active site

substrate

enzyme enzyme enzyme

Shape changes to accomodate the substrate

A B

}


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Enzyme Structure and Mode of Action24Task and Questions

Task

Complete the table below by placing a tick (✓) in the True or False column for each statement.

Statement True False

Enzymes are made up of carbohydrates

Activation energy is the minimum energy needed to start/

activate a reaction

An increase in temperature increases the kinetic energy of

molecules

Anabolic reactions are involved in the breakdown of large

molecules into smaller molecules

Enzymes can only be used once

In the induced fi t model, the active site and the substrate

are complementary

The shape of an enzyme is held in place by H-bonds,

disulphide bridges and ionic bonds

Questions

1 What is a catalyst?

2 Defi ne the term ‘activation energy’.

3 Describe the lock and key model for enzyme action.

4 What is the region on the enzyme molecule called where the substrate binds?

5 How do enzymes affect the activation energy level of a reaction?

6 What does the induced fi t model propose?

7 What molecules make up enzymes?

8 Explain the term ‘enzyme specifi city’.

9 Write an equation for the breakdown of sucrose.

10 Other than enzymes, what other molecule is needed for a hydrolysis reaction?



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Make more detailed notes about the structure of enzymes and their mode of action. It may be

useful to summarise how enzymes work in a table, highlighting the similarities and differences.



⇒ Why is an enzyme classed as a catalyst?

⇒ What types of bonds hold an enzyme in place?

⇒ With named examples, explain the difference between catabolic reactions and anabolic

reactions.

⇒ Which model states that the active site is rigid?

⇒ Find out what an allosteric region is, and where it is found on an enzyme molecule.

24Enzyme Structure and Mode of Action

Taking it Further


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Which factors affect enzyme activity?

The rate at which an enzyme-controlled reaction occurs is determined by a number of factors,

including:

• temperature;

• pH;

• substrate concentration;

• enzyme concentration;

• inhibitors.

1. How temperature affects enzyme activity

The kinetic energy of molecules increases with an increase in temperature; the molecules vibrate

more rapidly and are more likely to collide with each other.

In an enzyme-controlled reaction, both the enzyme and the substrate molecules move faster,

resulting in more collisions; the amount of substrate used up increases in the given time. This is

called the rate of reaction.

The optimum temperature is the temperature that brings about a maximum rate of reaction, that

is, the temperature at which the enzyme works best.

0 10 20 30 40 50 60 70Temperature (°C)

Increasingenzymeactivity

optimumtemperature

Above the optimum temperature, the enzyme molecules have too much kinetic energy and vibrate so much that the bonds creating the tertiary structure are broken. This causes a change in the shape of the enzyme molecule; the substrate is no longer able to bind with the active site because its shape has changed. So, the rate of reaction decreases and the enzyme is denatured.

Factors Affecting Enzyme Activity25A-levelYou need to know how a number of external factors infl uence enzyme-controlled reactions.

You should be able to explain that as enzymes have a precise tertiary structure, any change caused by external factors will interfere with enzyme activity.

You need to know how enzyme activity is altered by fl uctuations in the optimum conditions for enzymes.

GCSEYou may know that enzymes have exact shapes that are very important in allowing reactions to take place in living organisms.

You may be familiar with rates of enzyme-controlled reactions either increasing or decreasing, depending on the changes in the environment.



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2. How the pH affects enzyme activity

The pH of a solution is the measure of the hydrogen ion (H�) concentration of that solution.

Enzymes have an optimum pH for their activity, at which there is a maximum rate of reaction.

Enzymes function within a very narrow pH range, and any major change in pH above or below the

optimum pH can affect the structure of the enzyme molecule by:

• breaking the ionic bonds that keep specifi c tertiary structures in place, resulting in

denaturation of the enzyme;

• changing the charges on amino acid molecules so the substrate no longer fi ts.

1 2 3 4 5 6 7 8 9 10 11 12

arginaseEnzymeactivity

Acidic BasicpH

salivaryamylase

pepsin Different enzymes have different optimum pH values.

Pepsin � pH 2

Salivary amylase � pH 7

Arginase � pH 9.5

However, most work at neutral conditions in the human body.

3. How the substrate concentration affects enzyme activity

Rat

e of

reac

tion

Increasingconcentration does

not affect reaction rate

Substrate concentration

= point of saturation At low substrate concentrations, enzyme activity increases rapidly, as collisions between the substrate and active site happen more readily.

At high substrate concentrations, most of the active sites are occupied and the enzyme is working at full capacity. The rate of reaction is at a constant level.

4. How the enzyme concentration affects enzyme activity

With excess substrate, an increase in enzyme concentration results in a proportional increase in the rate of reaction. The substrate concentration may limit the rate of reaction.R

ate

of re

actio

n

Enzyme concentration

25Factors Affecting Enzyme Activity


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25 Factors Affecting Enzyme Activity

5. How inhibitors affect enzyme activity

substrate

enzyme

substrate

enzyme

no inhibitor

V

[S]

Competitive inhibitor

interferes with active

site of enzyme so

substrate cannot bind.

Non-competitive

inhibitor changes

shape of enzyme

so it cannot bind to

substrate.

Competitive inhibitor molecules have similar shapes to the substrate; therefore, they compete

with the substrate for the active site on the enzyme molecules. Because of this, competitive

inhibitors slow down rates of enzyme-controlled reactions. The effect of a competitive inhibitor

can be reduced by having an increased substrate concentration, as there will be more chance of

the substrate combining with the active site, and hence the rate of reaction will increase.

Non-competitive inhibitors bind to a different place on the enzyme molecule, called the

allosteric site and they distort the enzyme shape. This in turn affects the active site, so the

substrate is unable to form an enzyme–substrate complex. Increasing the substrate concentration

has little effect on the rate of reaction when a non-competitive inhibitor is present.

Task and questions

Task

Look carefully at the graphs below and use them to answer the questions.

Increasingenzymeactivity

Enzymeactivity

Rat

e of

reac

tion

Rat

e of

reac

tion

Rat

e of

reac

tion

A B C

D E



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Make more detailed notes about the range of factors that affect enzyme activity. It may be useful

to draw and fully annotate/label typical graphs that show the effect of external factors on enzymes.



⇒ What is a rate of reaction?

⇒ Explain how non-competitive inhibitors affect an enzyme-controlled reaction.

⇒ Does temperature have a similar effect on all enzymes?

⇒ What is meant by optimum conditions?

⇒ Do all enzymes have the same optimum temperature?

⇒ How does changing the pH affect the structure of enzymes?

⇒ Can the effect of a competitive inhibitor be reduced?

25Factors Affecting Enzyme Activity

Which graph or graphs shows the effect of:

i. non-competitive inhibitors on enzyme activity?

ii. enzyme concentration on enzyme activity

iii. temperature on enzyme activity?

iv. substrate concentration on enzyme activity?

v. change of pH on enzyme activity?

vi. change in the tertiary structure of the enzyme?

Questions

1 List two factors that affect the rate at which enzymes work.

2 What can happen to an enzyme if the pH changes from the optimum value?

3 How does an increase in temperature affect an enzyme-controlled reaction?

4 What is the term used to describe an enzyme when its shape has been altered and no

longer functions?

5 Why does the rate of reaction plateau?

6 How does a competitive inhibitor slow down a reaction rate?

7 Where does a non-competitive inhibitor bind on the enzyme molecule?

Taking it Further


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Structure and Function of Nucleic Acids26

What is the structure of nucleic acids?

Nucleic acids, such as DNA (deoxyribonucleic acid) and

RNA (ribonucleic acid), are made up of monomers called

nucleotides.

Each nucleotide is made up of three parts:

i. a pentose or deoxyribose sugar

ii. a phosphate group

iii. a nitrogenous or organic base

phosphate

deoxyribose sugar

base

NH2

C

CCC

C

HH

OO

O H

OH

HH

CH2

NN

NN

nitrogenous base

sugar

phosphategroup

H

OH

-O

O-

P

Simplified diagram Detailed structure

In all DNA molecules, the pentose sugar and the phosphate group remain the same and are

sometimes called the sugar-phosphate backbone. It is the nitrogenous base that is the different

component in different nucleotides.

How many different nucleotides are there within DNA?

There are four different nucleotides in the DNA molecule, with four different nitrogenous bases:

• Adenine (A)

}

these are double rings – purine• Guanine (G)

• Cytosine (C)

} these are single rings – pyrimidine

• Thymine (T)

A-levelYou will need to know the structure of the building blocks, or monomers, that make up DNA.

You will also need to explain that these monomers are nucleotides, which are made up of three different molecules through condensation reactions. It is the order, or sequence, of these monomers that results in variation within any population.

GCSEYou may know that the nucleus contains the genetic information, in chromosomes.

You should remember that chromosomes are made up of deoxyribonucleic acid (DNA), found in the nucleus. You may also know that chromosomes have genes that cause individuals to be different from each other, causing genetic variation.

Nucleic acids are organic molecules containing carbon, hydrogen and oxygen, as well as nitrogen and phosphate groups.



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The nucleotides are linked together by condensation reactions to form a polynucleotide;

therefore DNA is a polymer.

What is the structure of DNA?

• DNA consists of two polynucleotide strands linked by H-bonds.

The complementary base pairing rule �

Adenine � Thymine

Guanine + Cytosine

T

T

T

T

A

A

A

C

C

C

C

G

G

G

C G

T A

G C

hydrogen-bonded

bases

sugar-phosphate

backbone

phosphate

backbone

nucleotide bases

• Within DNA, A must always pair with T, so they are

always present in identical amounts. The same is true

for G and C. For example, if a sample of DNA has 22%

base A, then base T must also be 22%. The remaining

56% must be made up of bases G and C together,

meaning each is 28%.

• Each species is unique, or ‘genetically different’, and will have different amounts of base

pairing.

• The exact order, or sequence, of the different bases along the polynucleotide chain/strand

varies, and this forms the genetic information stored by the DNA. The two strands are

twisted and form a double helix.

• DNA is a large stable molecule, highly coiled and condensed, which remains constant in

the cell.

What is the function of DNA?

• Contains coded information in the form of genes.

• Genes determine the code for proteins.

• DNA determines the characteristics for the organism.

• Passes genetic information from one generation to the next.

26Structure and Function of Nucleic Acids

In order to form the hydrogen bonds, the two polynucleotide chains are anti-parallel – that is, the strands run in opposite directions.


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26 Structure and Function of Nucleic Acids

How is RNA different from DNA?

Ribonucleic acid (RNA) is composed of nucleotides, but is single stranded.

The main difference is that the pentose ribose has a hydroxyl group on carbon atom 2. The purine

bases are adenine (A) and guanine (G). The pyrimidine bases are cytosine (C) and uracil (U).

There are three types of RNA:

1 Messenger RNA (mRNA) is made in the nucleus and moves into the cytoplasm. The

length and base sequence varies. It is involved in protein synthesis.

2 Transfer RNA (tRNA) is single stranded and clover leaf shaped. Many different types of

tRNA are found in the cytoplasm. It is also involved in protein synthesis. It has three bases,

called the anticodon, which codes for an amino acid attached to one end.

H-bonds

modifiednucleotides

anticodon loop

Anticodon

G

GG

GG G

G GG

GG

GG

G

G

UA

AA

A

A

A

A A

A

HA

A AC

C

CC

CC

C

C

CD

D C

CC U

UU U

U

U

U

A

ACC

GG

TG C

P

amino acidattached here

5’ end

3’ endOH

3 Ribosomal RNA (rRNA) is made in the nucleolus and is part of the ribosome in

cytoplasm.



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26Structure and Function of Nucleic Acids

Tasks and Questions

Task 1

Complete the table below to summarise the differences between DNA and RNA.

Feature DNA RNA

Number of polynucleotide

chains

Nucleotide bases

Pentose sugar

Type of nucleic acid present

Location in cell

Task 2

Complete the missing complementary bases for the following double helix.

oPA

T

o P

G

oPo P

T

C

oP

o P

oP

o P

oP

o P

5’

5’

3’

3’

Questions

1 What are the three components of a nucleotide?

2 Name the bases that are classed as pyrimidines?

3 What type of bonds are formed between bases in DNA?

4 How many different types of RNA are present in a cell?

5 What is the name given to the coiled/twisted structure of DNA?

6 Name the monomer of DNA.

7 If a sample of DNA has 15% base T, how much G is there?

8 What is the function of tRNA?

9 What are the nitrogenous bases in DNA?


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Make more detailed notes about the structure and functions of nucleic acids. You may fi nd it

useful to draw the structure of a nucleotide and to list the differences between DNA and RNA in

a table.



⇒ What is DNA and what is its role?

⇒ List three differences between DNA and RNA.

⇒ What is the base pairing rule?

⇒ What two components make the backbone of the strand, and what links the two strands of

DNA?

⇒ If a sample of DNA had 19% of cytosine nucleotide, what percentage of its nucleotides will

be adenine? How can this be worked out?

⇒ Find out about the roles of different RNA nucleotides in protein synthesis.

⇒ What does ‘anti-parallel’ mean?

26 Structure and Function of Nucleic Acids

Taking it Further



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How does DNA make copies of itself?

DNA replication is a way of making copies of the DNA, producing new molecules that are identical

to the parent strand. It occurs during the interphase of the cell cycle.

The replication is called semi-conservative replication, as the new molecule formed has one

new strand and one of the original parent strands of DNA.

Replication occurs in the nucleus.

For replication to take place, a number of molecules must be present:

• Original DNA

• Free nucleotides

• Enzyme – DNA helicase

• Enzyme – DNA polymerase

• Enzyme – DNA ligase

DNA replication involves a number of stages:

1 The double helix is separated, or unzipped, by the enzyme DNA helicase, which breaks

the hydrogen bonds between the complementary bases.

2 Each strand now acts as a template for the formation of new/daughter DNA.

3 Free DNA nucleotides are joined with the exposed bases on the template strand by the

enzyme DNA polymerase.

4 The four different types of DNA nucleotides are joined in the following way: adenine pairs

with thymine, and guanine pairs with cytosine.

5 In the leading strand, the free nucleotides are joined continuously in one go.

6 In the lagging strand, the free nucleotides are joined in short strands and these sections are

joined by the enzyme DNA ligase.

27Replication of DNA

A-levelYou should know that when new cells are made during cell division, each new cell must have an exact copy of the instructions to carry out its function properly.

You will need to know how DNA makes a copy of itself in order to enable the inheritance of genes from the parents.

GCSEYou may have some understanding of the structure of DNA and its importance in controlling cell activity.

You should also know that DNA is copied and passed from one generation to the next, allowing genes to be passed from parent to offspring.


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3’

3’

5’

5’

leading strand

DNA polymerase

DNA helicase(breaking bonds)

lagging strandparental strand

primer

replication fork

DNA ligase

Overall direction of replication

7 Each new strand is complementary to the template on which it is made.

Tasks and Questions

Task 1

Complete the table below by fi lling in the missing gaps for either the enzyme responsible or

the function.

Enzyme Function

? Attaches the free nucleotides to the template strand

? Breaks the hydrogen bonds between the complementary

bases pairs in the double helix

DNA ligase ?

Task 2

Label the diagram showing the replication of DNA.

A

F

E

B

C

D

Replication of DNA27



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27Replication of DNA

Make more detailed notes about how DNA replication takes place. It may be useful to draw and

label diagrams of the different stages of the replication process.

The questions below may help you to structure and organise your notes. Use your course text


⇒ Where in the cell is DNA found?

⇒ Why is it necessary for DNA to undergo replication?

⇒ How does replication occur and what molecules need to be present?

⇒ What would the results be if DNA replication occurred by conservative replication and

dispersive replication? Use diagrams to show the possibilities of these types of replication.

Taking it Further

Questions

1 What name is given to the model of DNA replication?

2 In what part of a human cell does DNA replication take place?

3 How many strands are there at the end of one cycle?

4 Explain the term ‘semi-conservative replication’.

5 What is the function of DNA helicase in the replication process?

6 What type of bond is present between complementary bases on the two strands?

7 List the components required for DNA replication.

8 Which strand is replicated as one continuous process?

9 When does DNA replication occur?

10 What is the function of DNA ligase?


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What features are needed for effi cient gaseous exchange?

For effi cient gas exchange, the surface it occurs at must show

special features: large surface area; moist, thin membrane; be

permeable; and have good transport systems. These features

all help to increase the rate at which diffusion of gases occurs

across the exchange surface.

Structure of the mammalian respiratory system

All mammals breathe air and have specially adapted lungs to exchange gases with the

atmosphere. The lungs are located in the thorax,

a region separated from the abdomen by the

diaphragm and contained within the ribcage.

The respiratory system comprises:

• The trachea – epithelial cells have cilia and

mucus. Cartilage prevents the tube from

collapsing during breathing.

• Bronchi – each bronchus enters the lungs.

Cartilage rings keep the bronchi open during

breathing.

• Bronchioles – contain many smaller tubes with

little or no cartilage.

• Alveoli – tiny air sacs that form the gas

exchange sites.

• Intercostal muscles – the external and internal rib muscles are involved in ventilation.

• Diaphragm – sheet of muscle important in ventilation.

What happens during ventilation?

Ventilation (breathing) is the movement of air into and out of the lungs. Air is inhaled into the

lungs through the trachea, bronchi and bronchioles. It is exhaled via the same route, in reverse.

The movement of air into and out of the lungs is due to pressure and volume changes caused by

the intercostal muscles and the diaphragm.

28 Structure and Function of Mammalian Lungs

A-levelYou will need to know the gross structure of the mammalian respiratory system and the mechanisms that enable ventilation to take place.

You will need to be able to explain the mechanism for breathing and understand how the distinct features of the alveolus enable rapid movement of gases.

GCSEYou may have some knowledge and understanding of the breathing system and how it is involved in ventilation.

You should know that the alveoli provide the site of gas exchange and that they are specially adapted to enable the effi cient movement of gases into and out of the body.

Ventilation is the movement of gases into and out of the lungs.

Gas exchange is the exchange of gases in the lungs at the alveoli.

Respiration is the release of energy from organic molecules and occurs in all cells.

trachea

bronchus

bronchiole

alveoli

diaphragm

lungs

heart

rib muscle

rib



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Inhalation Exhalation

External intercostal

muscles contract,

moving the ribcage up

and outwards

Internal intercostal

muscles contract,

moving the ribcage

down and inwards

Diaphragm contracts,

becoming fl atter and

moves down

Diaphragm relaxes,

rising to a dome

shape

Volume in the thorax

increases

Volume in the thorax

decreases

Pressure inside the

thorax drops below

atmospheric pressure

Pressure inside the

thorax rises above

atmospheric pressure

Air fl ows into the lungs Air is pushed out of

the lungs

Active process Passive process

What is the site of gas exchange in the lung?

The site of gas exchange is a tiny sac called the alveolus. The ends of the bronchioles contain

many alveolar sacs, similar to a bunch of grapes, therefore increasing the surface area for diffusion

of gases. There are over 300 million alveoli per human lung.

The alveolus

• One-cell-thick layer made up of squamous epithelia (minimum distance for diffusion).

• Each cell contains collagen and elastin fi bres.

• Blood capillaries surround the alveolus (maintains the concentration gradient).

• There are millions of alveoli (thus creating a large surface area for diffusion).

• Special cells in alveolar wall secrete fl uid that contains detergent, which prevents the wall

from sticking together during exhalation.

Ventilation brings fresh supplies of air into

the lungs. This is called tidal air.

• Blood arriving to the alveolus in the

artery is deoxygenated and moves

slowly, so has more time for diffusion.

• Oxygen moves rapidly from the

alveolus into the blood capillaries and

into the red blood cells.

• Carbon dioxide moves out of the red

blood cells and into the alveolus and is

exhaled. The blood leaving the alveoli is

now oxygenated.

Structure and Function of Mammalian Lungs 28

chestexpands

diaphragmrelaxes

diaphragmcontracts

capillary

film of moisture

to pulmonary vein

from pulmonary artery

ventilation

epitheliumof alveolus

red blood cell

carbon dioxide escapesinto alveolus

oxygen enters red blood cells

diffusion of carbon dioxide

diffusion of oxygen


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Structure and Function of Mammalian Lungs28Constituent Inhaled air Exhaled air

Oxygen 20.9% 16%

Carbon dioxide 0.03% 4.0%

Water vapour Variable Variable but more

than in inhaled air

Nitrogen 78.1% 78.1%

Noble gases 0.94% 0.94%

Task and Questions

Task

Label the diagram below showing an alveolus and the blood capillary.

C

B

AD

E

air movement

direction ofblood flow

Questions

1 List the route oxygen molecules take from the nose to red blood cells.

2 What is the function of alveoli?

3 Which two sets of muscles are involved in the mechanism of breathing?

4 How many cells thick is the alveolar wall?

5 What is the difference between ventilation and gas exchange?

6 Why does carbon dioxide diffuse from air in the alveoli to the tidal air?

7 What is the condition of the blood arriving at the alveoli?

8 How do the alveoli increase surface area for gas exchange?

This table shows the composition of inhaled air and exhaled air. Note particularly the difference in oxygen and carbon dioxide levels in inhaled and exhaled air.



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28Structure and Function of Mammalian Lungs

Make more detailed notes about the structure and function of mammalian lungs. You may fi nd it

useful to make a table comparing inhalation and exhalation, and to label a diagram of the alveolus.



⇒ What is ventilation?

⇒ What is gas exchange?

⇒ Describe the structure of the lungs.

⇒ What features of alveoli make them the ideal gas exchange site?

⇒ Why is it better to breath through your nose than through your mouth?

⇒ What causes the carbon dioxide concentration in the blood arriving at the alveolus to be so

high?

⇒ What causes air to fl ow into the lungs?

⇒ How can lung capacity be measured?

⇒ What is meant by ‘ventilation rate’ and how is it calculated?

⇒ How can breathing be controlled in humans?

Taking it Further


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What is the structure of the heart?

The heart is a double pump, with the right side pumping blood to the lungs, while the left side

pumps blood to all the cells/organs of the body. The heart is made of special muscle called

cardiac muscle, which is myogenic; this means it can contract on its own without being

stimulated by a nerve. Cardiac muscle has a high demand for energy, which is produced via

aerobic respiration; the muscle has a dense capillary network to bring oxygenated blood and

glucose needed for respiration.

superiorvena cava

pulmonary artery

pulmonary vein

pulmonary valve

tricuspid valve

mitral valve

aortic valve

inferior vena cava

aorta

rightatrium

rightventricle left

ventricle

left atrium

The functions of each structure found in the heart are summarised in the table below.

Structure Function

Right atrium Receives deoxygenated blood from the lower part of the body via

the inferior vena cava, and from the head/shoulders via the superior vena cava

Right ventricle Pushes deoxygenated blood into the lungs through the pulmonary arteries (the only arteries that carry deoxygenated blood), one to the left

lung and one to the right

Left atrium Receives oxygenated blood from the lungs via the pulmonary veins

(the only veins that carry oxygenated blood), one from the left lung and

one from the right

(Continued)

Heart Structure and the Cardiac Cycle 29A-levelYou will need to know the internal structure of the heart and be able to identify and name the main blood vessels and valves associated with the two sides of the heart. You will also need to be able to explain the sequence of events that occur during the cardiac cycle.

GCSEYou should know that the heart is a pump that produces most of the pressure to move blood through vessels in the body.

You may also know that the right side of the heart has deoxygenated blood passing through it, while the blood in the left side is oxygenated.

The heart has four chambers:

Right atrium

Two atria

Left atrium

Atria contract at the same time, pushing blood downwards into the ventricles.

Right ventricle

Two ventricles

Left ventricle

Ventricles contract at the same time, pushing blood upwards into the blood vessels and out of the heart.



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29

Structure Function

Left ventricle Most muscular chamber of the heart. Pushes oxygenated blood to the

organs via the aorta

Valves Atrioventricular (AV) valves are between the atria and the ventricle;

the tricuspid valve is on the right side, and the mitral (bicuspid) valve

is on the left side. Valves in the blood vessels are collectively called

semilunar valves: the pulmonary valve is in the pulmonary artery on

the right side, and the aortic valve is in the aorta on the left side

Blood vessels Right side of the heart – the vena cava enters the heart and the

pulmonary artery leaves the heart

Left side of the heart – pulmonary vein enters the heart and the aorta

leaves the heart

Coronary arteries from the aorta supply oxygenated blood to the heart

What is the cardiac cycle?

The cardiac cycle is the sequence of contraction and relaxation of the chambers of the heart in

one heartbeat. The cardiac cycle occurs in the sequences outlined below.

Systole – term used to describe the heart muscle when it is contracting.

Diastole – term used to describe the heart muscle when it is relaxing.

Atrial andventricular

diastole

1

Atrial systole;ventricular

diastole

2

Ventricular systole;atrial diastole

3

0.1 sec

0.3 sec0.4 sec

c

semilunarvalves closed

semilunarvalves openAV valves

open

AV valvesclosed

diastole atrial systole ventricular systole

AV valve --- open AV valve --- open AV valve --- closed

Semilunar valve --- closed Semilunar valve --- closed Semilunar valve --- open

Heart Structure and the Cardiac Cycle

Sequence of events occurring during the cardiac cycle:

1. Diastole – a relaxation phase. The atria are relaxed and fi ll with blood. Ventricles are also relaxed. The atrioventricular (AV) valves are open and the stage lasts for approximately 0.4 seconds.

2. Atrial systole – atria chambers contract, forcing blood out of the atria into the ventricles when the atrioventricular (AV) valves open, lasting 0.1 seconds.

3. Ventricular systole – ventricle chambers contract, pumping blood into the large arteries and out of the heart as the semilunar valves open. This stage lasts approximately 0.3 seconds.


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29 Heart Structure and the Cardiac Cycle

Valves only open in one direction and only allow blood to fl ow in one direction. Relative pressure in

the heart chambers determines if the valves are open or closed, as follows:

• If there is a high pressure behind the valve, then the valve is forced open.

• If the pressure is high above the valve, the valve is then closed.

Blood always moves from an area of high pressure to an area of lower pressure. Changes in

pressure associated with the cardiac cycle are caused by the contraction and relaxation of the

cardiac muscle.

The left ventricle is more muscular than the right and produces high pressure, which effectively

transports blood, oxygen and nutrients to all the cells of the body.

How much blood is pumped out of the heart in a minute?

The cardiac output can be worked out using the equation below.

cardiac output � stroke volume � heart rate{ { {Volume of blood Volume of blood Number of beats

pumped out of heart pumped out of heart per minute (min�1)

per minute (cm3min�1) per beat (cm3)

Task and Questions

Task

Label the diagram of the heart using the terms provided in the box.

Questions

1 Which side of the heart carries oxygenated blood?

2 What does ‘myogenic’ mean?

3 What is the equation to calculate cardiac output?

4 What is the function of a valve?

semilunar valves, superior vena cava, inferior vena cava, aorta, pulmonary artery, left atrium, right ventricle, tricuspid valve, right atrium, pulmonary veins, left ventricle, coronary arteries



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29

Make more detailed notes about the structure of the heart and relate the different parts to their

functions. It may be useful to draw a fl ow diagram showing the sequence of events in the cardiac

cycle.



⇒ What structures are found within the heart and what are their functions?

⇒ How do valves perform their role of preventing blood from fl owing the wrong way?

⇒ What is the cardiac cycle?

⇒ What causes the ‘lub dup’ sound of the heart?

⇒ Why you would expect to have higher blood pressure in the aorta than in the pulmonary

artery?

⇒ Why is it important that deoxygenated and oxygenated blood are kept separate in the

heart?

Heart Structure and the Cardiac Cycle

5 Why are there two pulmonary veins entering the left atrium?

6 Why is the left ventricle more muscular than the right ventricle?

7 What is the name of the artery that carries deoxygenated blood?

8 During which stage(s) of the cardiac cycle are the semilunar valves closed?

9 Which is the only vein that contains oxygenated blood?

10 What is the name of the valves that separate the atria and the ventricles?

11 What is the name of the arteries that supply the heart with oxygenated blood?

12 What terms are used to describe contraction and relaxation of the cardiac muscle?

Taking it Further


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What are blood vessels?

Blood vessels are tubes that carry blood to all parts of the body. There are three main types of

blood vessel; these are connected together to form a closed system, allowing blood to fl ow

continuously to cells, tissue and organs.

The three main types of blood vessel are:

1 Arteries – carry blood away from the heart.

2 Capillaries – very small in diameter and form the junction between arteries and veins.

3 Veins – carry blood towards the heart.

vein

venulearteriole

artery

capillaries

Direction ofblood flow

Overall fl ow of blood:

site of exchanges

heart → artery → arteriole → capillaries → venule → vein → heart

oxygenated blood deoxygenated blood

30 Blood Vessels and Blood Cells

A-levelYou will need to know the structural differences between arteries, veins and capillaries, and be able to discuss the relationship between these blood vessels.

You will also need to know the function of blood cells in exchanging the nutrients and waste products at the respiring cells/tissues.

GCSEYou should know that the circulatory system transports many important substances around the body.

You should know that the heart acts as a pump, keeping the blood in continuous motion in the blood vessels. You may be familiar with the role blood cells have in transporting nutrients to respiring cells.



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30Blood Vessels and Blood Cells

What is the difference between an artery, a vein and a capillary?

Artery

• transports oxygenated blood away from the heart

to the organs

• thick wall and narrow lumen

• blood fl ows under high pressure, and in pulses

• elastic fi bres allow the wall to stretch and recoil

after pulses

• has smooth inner lining, giving less resistance

Vein

• transports deoxygenated blood back to the heart

• thin wall, less muscle and wide lumen

• low pressure and not in pulses

• many valves to prevent backfl ow of blood

Capillary

• very small and wall is only one cell thick

• blood becomes deoxygenated; site of nutrient

exchange

• found around cells, tissues and muscles

• pressure drops through the capillary network

• allows tissue fl uid to be formed

What is the structure and function of blood?

Blood is made up of a range of cells (45%) that are transported in a liquid called plasma (55%).

The plasma consists of about 90% water. The remaining 10% contains molecules that include:

• nutrients – eg, glucose, lipids, amino acids, mineral salts and vitamins

• hormones – eg, insulin, adrenaline and anti-diuretic hormone (ADH)

• respiratory gases – eg, oxygen and carbon dioxide

• plasma proteins – eg, albumin and fi brinogen

• waste products – eg, urea

The function of blood plasma is to transport these substances around the body. It also transports

heat from parts of the body to the skin, where it is lost to the environment.

tunical media(smooth muscle)

tunica interna(endothelial cells)

tunica externa(elastin and collagen)

18mm (aorta)

30mm (vena cava)

tunical media(smooth muscle)

tunica interna(endothelial cells)

tunica externa(elastin and collagen)

5 m

endothelial cells


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30 Blood Vessels and Blood Cells

Plasma contains platelets. Theses are involved in the blood clotting process and prevent loss of

blood from a wound.

What types of blood cell are there?

There are two types of cell found in the blood:

1 Red blood cells (erythrocytes). The main function of these cells is to transport oxygen

to the cells of the body. They are able to do this because they contain the pigment

haemoglobin. Key features of red blood cells include:

2.0 m

7.5 m

side view

top view

* biconcave shape – this increases the surface area

to volume ratio

* round shape – allows cell to squeeze through the

small capillaries

* thin cell surface membrane – provides a short

diffusion distance

* packed with the pigment haemoglobin – this

carries the oxygen

* no nucleus – allows more haemoglobin to be

transported

* very small size – 7.5�m, with approximately

5 million per mm3 of blood

2 White blood cells (leucocytes). There are a number of different types of leucocyte, all

containing a nucleus and cytoplasm. They are involved in defence and provide long-term immunity. They are larger than red blood cells but are fewer in number.

Leucocytes are divided into two groups – phagocytes and lymphocytes – both of which

have different functions.

Phagocyte Lymphocytenuclearlobes

Golgi apparatus

nucleus

cytoplasm

cytoplasm cell membrane

Functions: Engulfs bacteria by Secretes antibodies against a specifi c

phagocytosis; it is non-specifi c, pathogen and can provide

occurring regardless of the infection type. long-term immunity.



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30Blood Vessels and Blood Cells

Make more detailed notes about the structure of the blood vessels and blood cells. It may be

useful to summarise the key features in a table, highlighting any similarities and differences

between them.



⇒ What are the functions of arteries, veins and capillaries?

⇒ How are arteries, veins and capillaries connected?

⇒ Draw fully labelled diagrams of the three types of blood vessel.

⇒ How are the structural adaptations of an artery related to their functions?

⇒ What is the process of phagocytosis?

Task and Questions

Task

The table below refers to some features of the two types of cell present in blood. If a feature is

present, place a tick (✓) in the box, if it is not, place a cross (✗).

Feature Leucocytes Erythrocyte

Contains haemoglobin

Can engulf bacteria

Contains a nucleus

Biconcave shape

Questions

1 Give two similarities between phagocytes and lymphocytes.

2 Which blood vessel carries blood away from the heart?

3 What feature of veins prevents the backfl ow of blood?

4 What is the name of the structure where exchange of nutrients takes place?

5 Which vessels have the wider lumen – arteries or veins?

6 List three features of erythrocytes that make them effi cient at transporting oxygen.

7 What is the function of platelets?

8 Which fi ne blood vessels lead to the capillaries?

9 Is the pressure high or low in the arteries?

Taking it Further


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What is tissue fl uid and how is it formed?

Tissue fl uid is a watery liquid that surrounds all the cells of the body. It is formed from blood plasma and it has an essential role in the normal functioning of cells, tissues and muscles.

Tissue fl uid contains many substances, including:

• glucose

• fatty acids

• amino acids

• vitamins

• salts

• oxygen

• water

• waste products such as carbon dioxide

These substances can move into and out of capillaries, depending on their concentration gradient.

Blood in the arteries and arterioles is under pressure due to the pumping action of the heart.

As blood enters very narrow capillaries, it creates hydrostatic pressure (hydro means water),

as there is a large volume of the watery plasma trying to squeeze through very narrow tubes,

therefore the pressure increases.

Blood in the arteriole end of the capillary contains:

• water and dissolved substances, such as glucose, amino acids and salts; and

• large plasma proteins and blood cells.

The high hydrostatic pressure causes small molecules and water to be forced out through the

thin capillary wall. Large molecules are too big to pass though the capillary wall and remain in the

blood.

As water leaves the arteriole end of the capillary, it lowers the water potential of the blood and

raises the water potential of the tissue fl uid.

Tissue Fluid and Lymph31A-levelYou will need to know the components that make up tissue fl uid.

You will need to understand and explain how tissue fl uid is formed, the importance of water potential and the role of osmosis.

You will need to be able to describe the formation of lymph.

GCSEYou may know that there is fl uid surrounding or bathing all the cells of the body and that this fl uid provides the cells with the essential nutrients they require for their functions and removes their waste products.



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Blood in the venule end of the capillary contains:

• more plasma proteins; and

• less water.

As blood moves along the capillary, the hydrostatic pressure falls; the water potential is lower due

to the higher concentration of plasma proteins, therefore water moves back into the capillary by

osmosis from the surrounding tissue fl uid.

Waste products also move into the venule end of the capillary.

from arteriole

redbloodcells

to venule

Osmotic pressure is greater than bloodpressure therefore fluid moves from the

tissue into the venule.

Blood pressure is greater than osmoticpressure therefore fluid moves from the

capillaries into the tissue.

water(H2O)

aminoacids

water(H2O)

wastemolecules

oxygen(O2)

carbondioxide

(O2)

glucose(C6H12O6)

plasmaproteins

What is lymph?

Lymph is the liquid (fl uid) that forms when excess tissue fl uid moves into vessels called

lymphatic vessels. These lymphatic vessels are very close to the cells and capillaries, and carry

any excess fl uid and waste products away from the cells.

arteriole

tissue cells

tissue fluid

tissue spaces

venule

lymphatic vessel

lymph capillary

lymph

opening

endotheliumof lymphaticcapillary

tissue fluid

anchoringfilament

31Tissue Fluid and Lymph


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Tissue Fluid and Lymph31Lymph is made up of three main components:

1 Tissue fl uid that has not moved back into the blood at the venule end of the capillary

2 Fat droplets

3 White blood cells

Lymph drains into a series of lymph nodes that contain large numbers of white blood cells

(lymphocytes and macrophages) that ‘clean up’ the lymph by destroying any bacteria or foreign

material. Larger lymphatic vessels drain their contents into the neck.

Lymphatic vessels are very similar to blood vessels but contain many valves to prevent any

backfl ow. Lymph is moved in the lymphatic vessels due to the following: hydrostatic pressure,

muscle contraction and breathing action (increasing the size of the thorax).

The table below shows some comparisons between tissue fl uid and lymph.

Feature Tissue fl uid Lymph

Location Found surrounding cells Within lymphatic vessels

Cells present None White blood cells

Made from Blood plasma Excess tissue fl uid

Method of being transported Hydrostatic pressure and

osmotic forces

Hydrostatic pressure, muscle

contraction, breathing action

Direction of fl ow Out of the arteriole end of the

capillary into the venule end of

the capillary

Towards the neck and then

the heart, from the tissue

Task and Questions

Task

Label the diagram below showing the formation of tissue fl uid and lymph.



Topic

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Make more detailed notes on how tissue fl uid and lymph are formed, highlighting the importance

of both hydrostatic and osmotic pressure. It may be useful to summarise the similarities and

differences between tissue fl uid and lymph.



⇒ What is tissue fl uid?

⇒ What is lymph?

⇒ How is tissue fl uid formed?

⇒ How is lymph formed?

⇒ What are the difference between the composition of tissue fl uid and lymph?

⇒ What causes the movement of lymph in the body?

⇒ What are the different stages involved in returning tissue fl uid to the circulatory system?

⇒ What would happen to the return of fl uid in the venule end of the capillary in a person who

had high blood pressure?

⇒ Describe what would happen to the return of fl uid in the venule end of the capillary in a

person who has a very low protein diet.

31Tissue Fluid and Lymph

Questions

1 In which two ways can tissue fl uid be returned to the bloodstream?

2 Name four substances found in tissue fl uid.

3 How is hydrostatic pressure produced?

4 What happens to excess tissue fl uid?

5 By what process does water return in the venule end of the capillary?

6 What prevents the backfl ow of lymph in the lymphatic vessels?

7 What are the main components of lymph?

8 What does lymph drain into?

9 Is hydrostatic pressure higher or lower at the venule end of the capillary?

10 What do the larger lymphatic vessels drain their contents into?

11 Is osmotic pressure higher or lower in the arteriole end of the capillary?

Taking it Further


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What happens during a cell cycle?

Cells go through regular cycles of growth that is followed by division. This is called the cell cycle

and has three distinct stages: interphase, mitosis and cytokinesis.

• Interphase – this is the preparation and growth stage of the cell cycle. Interphase is the

longest stage of the cell cycle and there are three phases within this cycle that enable two

identical copies of the cell to be made. These stages are:

Mitotic phase First growth phase

Sec

ond

grow

th p

hase

growthand

preparationfor mitosis

growthand

normalmetabolic

roles

DNAreplication

M

S

G2 G1Prophase

Metaphase

Anaphase

Telophase

Synthesis phase

Interphase

Cytokinesis i. G1 – fi rst growth phase. Cell increases in

size, synthesising new proteins and cell

organelles.

ii. S – synthesis phase, replication of

DNA occurs by semi-conservative replication (DNA doubles).

iii. G2 – second growth phase. Organelles

grow and divide.

• Mitosis – this is when the nucleus divides into two, each part having the same number of

chromosomes as the original nucleus.

• Cytokinesis – this is when division of the cytoplasm occurs and the cell divides into two cells.

Changes in the DNA content during the cell cycle can be shown as a graph, such as the one

below:

Incr

easi

ng c

ell m

ass

0 0

0 12 24 36 48

1

1

2

2

Time/hours

Incr

easi

ng D

NA

mas

s pe

r nu

cleu

s

cell

mass

DNAmass

Cell Cycle and Mitosis32A-levelYou will need to know the events that occur in both the cytoplasm and nucleus of a cell as it prepares to divide.

You will need to explain key features of the different stages during mitosis, and understand the importance of mitosis for growth and repair as well as asexual reproduction.

GCSEYou should know that all cells are made from pre-existing cells through cell division, and that the new cells formed are called daughter cells and contain all the organelles. You should also know that a process called mitosis produces identical daughter cells.

Cell mass increase is due to the G1 and G2 phases in interphase. The mass returns back to original when cytokinesis splits the cell into two.

DNA mass increase is due to the S phase of interphase, when DNA replicates and doubles in mass. The mass returns to original indicating that the cell has split into two cells.



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What happens during mitosis?

Mitosis is a process that results in the production of two identical daughter nuclei from one original

nucleus, and always follows on from the interphase.

Mitosis can be divided into four stages:

i. Prophase (P) – pairing of the chromosomes

ii. Metaphase (M) – chromosomes meet at the equator (middle) of the cell

iii. Anaphase (A) – chromatids pull apart

iv. Telophase (T) – chromatids reach the opposite poles and two nuclei form

interphase P➝M➝A➝T cytokinesis

}

stages of mitosis

chromosomes

chromatids

centromere

spindle

centriole

Prophase

Telophase Interphase

Metaphase Anaphase

Prophase – the DNA condenses and shortens, and chromosomes become visible. The

nucleolus disappears and the nuclear membrane disintegrates. Centrioles move to

opposite poles, forming spindle fi bres and structures called ‘asters’.

Metaphase – chromosomes line up at the equator (middle) of the cell, attached to the

spindle fi bre by its centromere.

Anaphase – the spindle fi bres contract, pulling the sister chromatids apart towards the

opposite poles.

Telophase – each chromatid has reached the opposite pole (now sometimes called

the chromosome), spindle fi bres break down, and the nuclear membrane reforms. The

chromosomes unwind.

Finally, cytokinesis occurs when the original cell divides into two cells. The cytoplasm splits into

two and the plasma membrane pinches around the two cells; this is sometimes referred to

as cleavage.

32Cell Cycle and Mitosis


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Cell Cycle and Mitosis32What is the signifi cance of mitosis?

Cell division by mitosis is very important in organisms as it enables new cells to be produced that

contain identical DNA to the parent cell. It is known as diploid division, as the chromosome

number remains the same in the new cells. Therefore, clones will be produced, as long as there is

no mutation.

Mitosis is important for three reasons:

i. growth

ii. repair

iii. asexual reproduction

Task and Questions

Task

Label the stages (A –E) and structures (1–10) in the diagram showing mitosis below.

A B

C

D

E

12

3

4

5

6

910

7

8

Questions

1 What are the three stages of the cell cycle?

2 Explain why the chromosomes are not visible during interphase?

3 During what stage of the cell cycle would you expect the following events to take

place:

a. formation of spindle fi bres

b. DNA replicates

c. nuclear membrane reforms

d. cell organelles synthesised



Topic

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Make more detailed notes on the different stages involved in the cell cycle. It may be useful to

summarise the key events that occur during interphase and mitosis.



⇒ What is the cell cycle and why is it necessary?

⇒ What occurs during each stage of the cell cycle?

⇒ What is the function of the centromere during mitosis?

⇒ What is the signifi cance of mitosis with reference to growth, repair and asexual

reproduction?

⇒ What are the four phases of mitosis?

⇒ What happens at each of the stages of mitosis?

⇒ Find out if the process of mitosis is the same for plant cells.

⇒ Describe two differences between cell division in plant and animal cells.

32Cell Cycle and Mitosis

4 What happens during cytokinesis?

5 Defi ne the term ‘mitosis’.

6 What is the importance of mitosis?

7 What structures are required to pull the sister chromatids apart during anaphase?

8 During which stages of mitosis is the nuclear membrane not present?

Taking it Further


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Why is water important in plants?

Plants need water for many important functions, such as photosynthesis, turgidity,

germination and other metabolic processes. It is also the transport medium for molecules

such as sucrose and mineral ions. Water lost through the leaves by transpiration needs to be

replaced.

The route of water movement through the plant is shown below.

Soil water ➝ root hair ➝ root cortex ➝ xylem ➝ leaves ➝ diffusion through stomata ➝ atmosphere

root hair

epidermis

ground tissue (cortex)

xylem

phloem

path of waterendodermis

Water and mineral ions are taken up from the soil

through long, fi ne extensions of the epidermal

cells, which form root hairs. Soil has a higher water potential compared with the root hair

cells, and so the water moves into the cell down

its water potential gradient by osmosis.

Once absorbed into the root cells, water passes

into the cortex and fi nally into the xylem via two

main pathways (discussed further below).

How does water cross the cortex of the root into the xylem?

Water crosses the cortex into the xylem by:

• The apoplast pathway – as the cellulose cell wall is permeable, water passes through the cell walls of adjacent cells. The cohesive property of water (the molecules are

attracted to one another and form hydrogen bonds to stick together) results in a tension

being created that pulls the water along the cell walls of the cells in the cortex. Therefore,

movement occurs by cohesion and is through the cellulose cell wall.

• The symplast pathway – this occurs through the cytoplasm of the cells in the cortex as

a result of osmosis. The cell walls have very small gaps, or pores, called plasmodesmata

(singular plasmodesma) through which cytoplasm is able to pass between adjacent cells.

Water passes along the cytoplasm of the cells in the cortex by osmosis to the xylem

vessels. Therefore, movement occurs by osmosis and is through the cytoplasm.

Transport of Water and Minerals in Plants33A-levelYou will need to know the different movement processes involved in transporting water from the soil into plant cells. You will also need to know the function of the xylem and phloem in transporting molecules through the plant.

GCSEYou many know that water moves from the soil into the root and up into the leaves where it is used for photosynthesis. You may also know that sugars move from the leaves to roots.



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The cell walls of the endodermal cells contain a strip called the casparian strip, comprising a

waterproof substance called suberin. Water is unable to cross this strip and the apoplast pathway

is blocked; movement at the endodermis is therefore by the symplast pathway.

cell wallCasparian strip

cytoplasm

vacuole

plasmodesma

apoplastic pathway(through cell wall)

symplastic pathway(through cytoplasm)

vascular cylinder

endodermis and Casparian strip

cortex

epidermis

root hairpathway A

pathway B

Mineral ions such as sodium ions, nitrate ions and ammonium ions are taken up from the soil

into the root hairs by active transport. They are moved into the cells against a concentration

gradient, therefore energy is used in their uptake.

How does water move up the stem?

Transpiration is the main force that pulls water up the stem and into the leaves. Water molecules

evaporate through the stomata on the lower surface of the leaves. There are a number of ways

water moves through the stem:

1 Cohesion-tension theory – water molecules have a tendency to form hydrogen bonds

with each other, sticking together by cohesion. Water then forms a continuous column. As

water evaporates from the mesophyll, this pulls more water molecules from the xylem as

a result of transpiration. This is called the transpiration pull. The transpiration pull puts the

xylem under tension.

2 Capillarity – as the xylem vessels are narrow, there is a large surface area to volume ratio,

which means that many water molecules are in contact with the xylem wall. The narrower

the xylem vessels, the more water sticks to the wall. As water is attracted to the wall, this is

called adhesion.

3 Root pressure – at the endodermis, salts are moved into the xylem vessel; this lowers the

water potential in the xylem, therefore water moves into it. This creates root pressure

that forces water up the xylem.

It is the combination of all three forces that causes water to move up the xylem from the roots,

through the stem and into the leaves.

33Transport of Water and Minerals in Plants


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Transport of Water and Minerals in Plants33Task and Questions

Task

Label the diagram of the root below.

A

B

C

D

E

Questions

1 By which process does water enter a root hair cell?

2 Is the water potential of soil high or low?

3 Which property of water is important for the apoplast pathway?

4 Where in the leaf is water lost to the environment?

5 State two differences between the apoplast and symplast pathways?

6 What is the name given to the small gaps in the cell walls fi lled with cytoplasm?

7 Why is water moved by the apoplast pathway unable to cross the endodermis in the

cell wall?

8 State the three main forces responsible for the movement of water up the stem in the

xylem?

9 Defi ne the term ‘cohesion’.

10 Is the water potential in a root hair cell high or low?

11 Give four functions of water in plants.

12 By which process is water lost from the leaf?



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Make more detailed notes on the different ways in which water is taken into and through plants. It

may be useful to draw a fully labelled diagram of a root showing the pathways involved.

The questions below can help you structure and organise your notes. Use your course text book

to add further information.

⇒ Why is water needed by plants?

⇒ How does water enter and travel through plants?

⇒ How would you measure the rate of water loss through transpiration?

⇒ How have some plants adapted to living in dry conditions and preventing water loss?

⇒ Describe the role of cohesion-tension theory, capillarity and root pressure in the movement

of water through a plant.

⇒ How is the structure of xylem related to its function of movement of water?

⇒ What are the main differences in the structure of the epidermis and endodermis?

⇒ What are the differences between the structure of roots and stems?

⇒ How are the products of photosynthesis transported in the phloem?

33Transport of Water and Minerals in Plants

Taking it Further


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What is biodiversity?

Biodiversity is the variety of life, and can be subdivided

further into:

• Species diversity – the number of different species

and the numbers within each species.

• Genetic diversity – the range of genes within any one species.

• Ecosystem diversity – the variety of different habitats within any given area.

Biodiversity therefore includes all the different micro-organism, fungus, plant and animal species as

well as the ecosystem they are found in.

The ecosystem can be divided into:

• Abiotic factors – the non-living, physical and chemical components such as water

availability, soil pH, temperature, oxygen.

• Biotic factors – all the living components and their interactions with each other, which

may lead to competition for food, space and mating partners.

How can biodiversity be measured?

There are number of factors that need to be considered when measuring the biodiversity of any

given habitat. For example:

i. which species are present in a given area at a given time;

ii. how much of each species there is (abundance) in this given area and at the given

time; and

iii. the distribution of each species in the area at the time.

To measure biodiversity, you will need to collect, or estimate, the number of individuals of each

species as well as how many different species are present.

Sampling techniques

These enable representative samples to be collected using quadrats (frames of known size).

Measuring Biodiversity34A-levelYou will need to know some key defi nitions related to diversity and how species diversity can be measured in an ecosystem.

You will also need to understand and explain species richness and species evenness in a habitat.

GCSEYou should know that there is a wide variety of organisms living in a range of different habitats.

You should also know that even within species there are differences.

There are three components that make up biodiversity:

• Species diversity

• Genetic diversity

• Ecosystem diversity



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34Measuring Biodiversity

1 Random sampling is used to estimate the population of stationary organisms. It involves:

• Dividing the areas in a grid.

• Generating random coordinates, then placing a quadrat at those coordinates.

• Counting the number of individuals of the species that are present in the quadrat.

• Collecting multiple quadrats in order to estimate the population size – a large sample

increases the reliability of the data.

(For aquatic habitats, a container of known volume is used to remove samples at random points.)

2 Line transect sampling enables you to measure the distribution of species in a straight

line, across changes in a habitat.

3 Mark-Release-Recapture (MRR) techniques are used to estimate the population of

mobile species and involve the following steps:

• Catching a random sample of animals and counting them.

• Marking the animals caught (M1).

• Releasing the marked animals.

• After some time, catching another random sample of the animals (M2).

• Recording the proportion of animals that are marked in the second sample (M3).

• Then, using the formula below, estimating the population size.

M1 � M

2

estimated population size �

M3

What is the difference between ‘species richness’ and ‘species evenness’?

These two factors both contribute to biodiversity.

• Species richness – this is related to species diversity. It refers to the number of different

species present in the community and an environment.

• Species evenness – this refers to how evenly distributed the different species are. It is a

measure of the relative abundance of the different species.

For example, an environment with three species – A, B, C – contains the following:

A � 38, B � 34, C � 55

This environment is low in species richness but high in evenness.

If there is an increase in the species richness and evenness, then diversity also increases.


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Measuring Biodiversity34How is species diversity measured?

Species diversity takes into account both the number of species and their abundance within an

ecosystem. Species diversity shows the stability of an ecosystem and allows you to compare the

diversity between two habitats/ecosystems.

Species diversity (the index of diversity) can be measured using Simpson’s Diversity Index, as

shown below.

N (N�1) d � index of diversity

d = N � total number of organisms of all species

� n(n�1) n � total number of organism of each species

� the sum of (ie, all added together)

Worked example

The species diversity was investigated in two different ponds. The results were as follows:

Species Pond A n(n�1) Pond B n(n�1)

Water snails 5 5(4) � 20 2 2(1) � 2

Mayfl y larvae 4 4(3) �12 6 6(5) � 30

Tadpoles 4 4(3) � 12 12 12(11) �132

Pond skaters 6 6(5) � 30 2 2(1) � 2

Goldfi sh 5 5(4) � 20 2 2(1) � 21

Number of species 5 5

Total number of all species

24 n(n�1) � 94 24 n(n�1) �168

Index of diversity

24 � 23 552Pond A d � ➝ � 5.87 94 94

24 � 23 552Pond B d � ➝ � 3.29 168 168

High index of diversity (d) values mean there is greater species diversity. This indicates that there is greater food variety and the environment is less harsh and more stable.

Low d values indicate a less stable and harsher environment, with lower species diversity.



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Measuring Biodiversity 34Task and Questions

Task

Calculate the index of diversity for the two habitats below. Show all the steps of your

calculations.

Species Open fi eld n(n�1) Wild meadow n(n�1)

Buttercup 4 9

Daisy 5 2

Clover 5 9

Dandelion 6 8

Number of species

Total number of all species

Questions

1 What is meant by ‘species diversity’?

2 Why is the species diversity in the Sahara desert low?

3 What is the difference between abiotic and biotic factors?

4 Defi ne ‘abundance’.

5 State why you would carry out random sampling.

6 Write the formula for the Mark-Release-Recapture technique.

7 What do the components N and n refer to in the index of diversity equation?

8 What is ‘species richness’ and ‘species evenness’?

9 If the index of diversity has a high value, what does that indicate about the ecosystem?

10 List two ecosystems where you would expect to fi nd a high value for the index of

diversity.

11 Give three examples of abiotic factors.

12 What would help to increase the reliability of data collected via sampling?


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Measuring Biodiversity34Taking it Further

Make more detailed notes on the different sampling techniques that can be used to estimate

population sizes. It may be helpful to learn components of all the equations used to calculate the

size of populations.



⇒ What is sampling?

⇒ Which sampling techniques are available?

⇒ How could you estimate the population of woodlice in a given area?

⇒ What are the defi nitions for the following terms: species, habitat, ecosystem, biodiversity?

⇒ Give four ways in which the impact of an ecological investigation can be minimised.

⇒ Which factors can cause an increase in genetic diversity?

⇒ How can the environment have an impact on diversity?

⇒ How could you investigate the population sizes and species diversity at two points of a

river?



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What is conservation?

Conservation is the preservation and active management of species, habitats and natural

resources. It is a dynamic process, adapting in response to changes in human activities or other

environmental pressures.

Conservation measures can be carried out in order to preserve endangered plant and animal

species. It may also be a measure that allows for the restoration or creation of new habitats.

For example: tropical rainforests have been under great threat from loss of habitats and species

extinction over the last few decades. There are many reasons for their conservation, including:

• Ecological – large amounts of carbon dioxide are ‘fi xed’ here; without rainforests,

increases in the greenhouse effect, global warming, soil erosion and fl ooding could occur.

• Economic – creation of new commodities, new crops, ecotourism.

• Ethical – wildlife has a cultural importance to local people; it is wrong to reduce biodiversity

and many consider all species to have a right to life on Earth.

• Aesthetic – beautiful species of plants and animals are found in the rainforest.

What is the difference between in-situ and ex-situ conservation?

In-situ conservation

This is conserving the organisms in their own habitats, or on site. It

involves protecting an endangered species in its natural habitat by

setting up nature reserves, or by protecting the species from predators

or threats. The species may increase in numbers in their natural

habitat and continue to show the process of evolution and adaptation within their environment.

Therefore, genetic diversity is maintained within the population. Large nature reserves promote

conservation of biodiversity more effectively than smaller reserves.

In-situ conservation has number of advantages:

• Species remain adapted to their own natural habitats.

• Greater genetic diversity can be conserved.

• Natural behaviour patterns are maintained.

• Species interact with other species, therefore the whole ecosystem is conserved.

35Conservation

A-levelYou will need to describe how species can be conserved in their own habitats, such as in nature reserves, zoos, botanic gardens and seed banks. You will need to understand that conservation enables biodiversity to be maintained so that plants and animals are able to adapt to changes in the environment.

GCSEYou will be aware of the importance of conserving different species and their habitats. You may be able to give an example of an ecosystem, such as the tropical rainforest, where many species are under threat of becoming extinct, along with some reasons for its conservation.

In-situ – in the natural environment or habitat

Ex-situ – out of the natural environment or habitat


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Conservation 35Disadvantages of in-situ conservation include:

• Large areas of land are required.

• Human interference.

Ex-situ conservation

This involves removing the populations from an area under threat to a new, safe location. Some

species become very rare and are at great risk of becoming extinct in the wild. The natural

habitats may sometimes be destroyed (due to fi res, volcanic eruption, fl ooding, human activity), so

it is essential to remove the threatened species to increase their chances of survival.

Ex-situ measures include:

• Botanic gardens – sites where there are many different species of plants cultivated in

greenhouses or in the open.

• Captive breeding – in zoos. This is a conventional method of conserving endangered

animal species. The animals are encouraged to breed and when numbers are suffi ciently

high, some individuals are returned to the wild to re-establish a natural population. These

facilities have an educational value, too, providing valuable information to the public about

threatened or endangered species.

• Seed banks – seeds from endangered plants are kept in cold, dry storage for many years.

They can be used as a source of genes in biotechnology. Some seeds that may not be

viable can be germinated and grown to produce replacement seeds. The Kew Millennium

Seed Bank (in Wakehurst Place, Sussex) maintains stocks of seeds from any endangered

plant species.

Disadvantages of ex-situ conservation include:

• Impossible to create the habitat as it should be in the natural environment.

• Natural evolution and adaptation processes may be hindered or altered by putting the

species into an unnatural habitat.

• Seed banks are ineffective for some plant species with recalcitrant seeds, that is those

with short viability.

The importance of international cooperation in global conservation

The loss of habitats and its consequences on the populations of different species is of international

concern. Effective conservation programmes require international cooperation. There are a

number of ways that this can be achieved:

1 Convention on International Trade in Endangered Species (CITES) – this is an

international agreement aimed at ensuring that the international trade of wildlife specimens

does not threaten their overall survival.

2 International conservation of fi sh – measures are needed to promote fi sh conservation,

as most fi sh live in international waters and ships from any country are able to catch them.

Enforcement is diffi cult and relies on international trust.



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35Conservation

3 Convention on Biological Diversity (CBD) – an international treaty concerned with

sustainable development. It has three aims: to conserve biological diversity; sustainable use

of biological resources; and fair and equal benefi ts from genetic resources.

Questions

1 What is conservation?

2 What are the main reasons for conservation?

3 List all the differences between in-situ and ex-situ conservation.

4 Give an example of a habitat that is being conserved.

5 How are seeds kept in seed banks?

6 Give two reasons why ex-situ conservation may be used.

7 Outline three methods of ex-situ conservation.

8 What are the main aims of the Convention on Biological Diversity?

Make more detailed notes on different conservation methods used in different parts of the world.

Highlight the main reasons for conserving an ecosystem, such as tropical rainforests.



⇒ What is meant by the edge effect of a nature reserve?

⇒ How have in-situ and ex-situ approaches been used to conserve named species?

⇒ What is the importance of hedgerows in conserving habitats in farming/agriculture?

⇒ What is active conservation management?

Taking it Further


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36The use of enzymes from bacterial and fungal cells has occurred for over a thousand years in

processes such as baking, brewing and cheese-making.

Enzymes are very useful in industry due to some special features. They:

• allow reactions to occur at lower temperatures;

• are specifi c;

• can be used over and over again;

• can be produced in bulk, using micro-organisms.

Enzymes are used in a variety of processes, such as food manufacture, the production of

medicines (pharmaceutical compounds) and analytical methods. There are hundreds of different

enzymes that are used in many different commercial applications.

You may have carried out a number of practical investigations using enzymes during your biology

course and may be familiar with the role of enzymes in breaking down large molecules.

Task

Research the commercial uses of enzymes, with one example from each of the following

industries:

• Food industry

• Analytical sciences

• Pharmaceutical industry

Prepare a poster, PowerPoint presentation or a leafl et that covers the following points:

• What example you have chosen and the reasons behind your choice.

• The substrates, enzymes and products involved in your choice.

• The key features of the enzymes chosen.

• A discussion of the importance of the commercial application of these enzymes.

• Explanations of any advantages and disadvantages in using these enzymes.

Use of Enzymes in Industry

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Infectious Diseases – Causes, Transmission and Prevention 37Infectious diseases kill many millions of people worldwide. They are caused by a variety of different

micro-organisms, which can be found in almost any environment – in air, soil and water. Disease-

causing organisms are called pathogens. These pathogens can be spread in a number of ways,

including touching, eating, drinking and breathing. They can also be spread through animal and

insect bites, kissing and sexual contact. There are four main types of pathogenic organism:

bacteria (eg cholera), fungi (eg athletes’s foot), viruses (eg HIV) and protozoa (eg malaria).

Vaccines, sanitation, hygiene and medicines can help prevent infections.

Task

Write an informative article for a newspaper highlighting the harmful effects of pathogenic

infections in the human body. Using a named example for each of the four types of micro-

organisms, explain the impact (major or minor) of the disease on the individual and society as

a whole. You may use the examples mentioned above or use different examples that may be

of interest to you.

Your article must include the following points:

• The diseases you have chosen.

• The micro-organism that causes each disease.

• How each disease is transmitted.

• Why it is important worldwide and how many people it affects globally.

• How each disease can be prevented.

• What controls need to be in place both for the micro-organisms and for humans

at risk.

You may wish to look at the importance of education programmes to raise awareness

regarding transmission and prevention of the diseases.

Your article should also include information on any medical advances in the controlling or

curing of the diseases you have discussed.


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Lifestyle choices have a great impact on individuals and on society. Lifestyle diseases are more

evident, and occur more frequently, in industrialised societies. These diseases linked to our

lifestyles are on the increase and there are now many campaigns highlighting the dangers and

health issues linked with the different ways we live our lives.

Prevention is usually the best way to approach these problems and it is very important to make

people aware of the consequences of an unhealthy lifestyle and the risk of many associated

diseases. Some of the major factors that have had a dramatic impact on health over recent years

include:

Smoking Alcohol Unbalanced diet Lack of exercise

All these risk factors are linked to an increased chance of developing diseases such as lung

cancer, coronary heart disease, liver cirrhosis and diabetes.

Lifestyles and Diseases38

Task

Write an informative leafl et highlighting the importance of a healthy lifestyle, that could be

used as part of a healthy lifestyle awareness campaign in your school or college.

You should discuss a range of lifestyle choices and explain how they are linked to diseases.

Your leafl et must include the following points:

• What is health and what is a disease.

• The effects of smoking cigarettes on the body (link this to some of the chemicals found

in tobacco).

• How drinking alcohol affects your physical and mental abilities.

• What is an unbalanced diet.

• The links between high fats and cholesterol in diet and coronary heart disease.

• What are cardiovascular diseases.

• The benefi ts of regular exercise on the body.


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Human Impact on the Environment 39The natural environment is very important to humans, both for economic and social reasons.

Humans have exploited the environment and its natural resources for many commodities,

including resources for food, energy and medicine, and land for agriculture and building.

A rapid increase in the human population has led to a growing demand for the Earth’s precious

resources and has resulted in widespread damage to a range of ecosystems. Raw materials such

as coal, oil and natural gas are being used up at an alarming rate.

Task

Prepare a research project titled ‘Human Impact on the Environment’.

You will need to research how humans have exploited the environment for resources and be

able to explain the consequences both of extracting and of actually using these resources.

You should prepare a poster or PowerPoint presentation to share your research with your

fellow classmates.

You must highlight some of the following points:

• What are non-renewable energy resources.

• How the combustion of fossil fuels affects the atmosphere.

• What the greenhouse effect is.

• How greenhouse gases are linked to global warming.

• The effect of global warming on plants and animals.

• The long-term effect of deforestation on the environment and on biodiversity.

You may use other examples to explain human impact on the environment.


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Stem cells are special types of cells that have the ability to self-renew by mitosis and to

differentiate. There has been great interest in stem cells due to their potential for tissue repair and

for treating a range of degenerative conditions, such as multiple sclerosis and Parkinson’s disease,

where neurones and other cells in the nervous system have been damaged or have lost their

ability to function. There is great potential to use stem cells to replace damaged cells.

Although most of the therapeutic uses of stem cells are at an early stage, many scientists and

researchers believe that stem cell treatment will one day be able to cure many human diseases

and relieve suffering.

in vitro fertilised egg

blastocyst stage

inner stem cell mass

cultured undifferentiatedstem cells

muscle cells

neural cells

blood cells

40 Therapeutic Uses of Stem Cells

Task

Carry out research on the therapeutic uses of stem cells and prepare a presentation in the

form of a poster or on PowerPoint.

Your presentation should cover the following points:

• What stem cells are and how they are different from body cells.

• Where stem cells are found.

• The difference between embryonic stem cells and adult stem cells.

• An explanation of the therapeutic use of stem cells in treatment of lymphomas.

• A discussion of at least three potential uses for this type of therapy.

• A discussion of any risks of stem cell therapies.

• Those who would benefi t most from stem cell therapy.

• Why there are such varied attitudes to stem cell research.


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