biology lesson notes cells

Biology Lesson Notes

Form 3/SS+NS/ss/12-13 Cells

Cells

Under microscope, plant or animal parts are seen to be made of small distinct units.

These units are called cells.

A cell is a distinct mass of living material (protoplasm) enclosed by a membrane.



Some details about cells

There are different shapes and sizes of cells.

Cells are very small, usually too small to see

with naked eye.

They are small enough for thousands to fit on

full-stop.

But sometimes they can be as large as a tennis

ball.

The unit of measurement used is

the micron (equal to one millionth of a metre)

(1m = 1000,000 micons, 1mm = 1,000 microns).

The average human cell is about 10 microns in

diameter.

Smallest bacteria are about 0.2 microns in

diameter (2 thousandths of a millimetre).

Each cell is adapted to carry out a particular

function.



Different types of Animal Cells



Different types of Plant Cells



Red Blood Cells White Blood Cell Muscle Cells Root Hair Cell

Since there are so many differences between cells, it is difficult to speak of

a “typical” cell. However, there are certain common features.

Nerve Cell Sperm Cell Egg Cell



A Typical Animal Cell

Cell Membrane

Cytoplasm Contains glycogen granules)

Nucleus (contains chromatin)

Mitochondria (visible only under electron microscope)

Nuclear Membrane

Vacuoles (food or contractile)



A Typical Plant Cell

Mitochondria

Chloroplasts (contains chlorophyll)

Vacuole (contains cell sap)

Nucleus

Cellulose Cell Wall

Cytoplasm (contains starch granules)



Parts of a Typical Cell

Cell Cell

Membrane

Cytoplasm

Nucleus

Vacuoles

Mitochondria

Chloroplasts



The Cell Membrane

The Cell Membrane is a semi-permeable layer which holds the cell together and controls what goes in and out of the cell.

• The cell membrane is a semi-permeable layer. • This means that some substances can pass through it, but others cannot. • The cell can take up and excrete certain small chemicals through these “holes” in the membrane.

• It is very thin but tough • It is made of protein and phospholipid (a kind of fat) molecules. • It encloses the other cell structures.

• In plant cells only, the cell membrane is also called the plasmalemma • It is surrounded by the Cell Wall, which is is a rigid, protective wall made of cellulose. • It gives the plant its stiffness and helps it grow tall. • It is secreted by the plant cell's cytoplasm. • Cell walls may contain lignin (a complex aromatic compound).

This is an important component of wood.



The Cytoplasm

The Cytoplasm is the jelly-like, transparent substance inside the cell. It is the site where most reactions take place.

• A jelly-like, transparent substance.

• Consists mainly of water (between 65% and 95%).

• Solids present in cytoplasm include protein granules, carbohydrates, droplets of fat, and pigments (e.g. chlorophyll).

• The cytoplasm is either watery (liquid) or syrupy (semi-solid), depending on the concentration of solids dispersed in the fluid.

• Much of the chemical work of the cell is done in the cytoplasm.



The Nucleus

The Nucleus is an organelle which contains the genetic material of the cell. It controls most of the functions of the cell.

• The nucleus normally has a round or oval shape.

• The nucleus contains chromatin. These consist of strands of DNA and protein.

• The chromatin is enclosed in a nuclear membrane in most cells.

• A strand of DNA is a long series of genes or “instructions” used to control all the functions of plants and animals.

• Genes determine the characteristics of a cell.

• The nucleus is the control centre of the cell.

• It controls growth and division

• It also controls most of the chemical reactions taking place in the cell.



The Vacuoles

• Vacuoles are non-living, fluid-filled cavities in the cytoplasm.

• In animal cells, they are small and change size and position.

• Food vacuoles contain food molecules

• Contractile vacuoles used to pump out water. (e.g. Amoeba)

• Plant cells have one large central vacuole.

• It is filled with a fluid called cell sap.

• It contains salts, sugars and pigments dissolved in water.

• It has a high pressure to keep the plant rigid and upright.



The Mitochondria

The Chloroplasts

• Mitochondria are sausage-shaped structures with semi-permeable membranes.

• Mitochondria function as energy production centres

• Glucose (sugar) in the cytoplasm is oxidized (burnt) in the mitochondria for energy.

• This process is called respiration.

• Chloroplasts are green, oval structures containing chlorophyll (a green pigment).

• They are found only in plants (and some one-celled animals)

• In the presence of sunlight, the chloroplasts carry out photosynthesis.

• All life on Earth depends on photosynthesis.

• Without green plants animals would not have food.

• Fossil fuels are formed from green plants of the past that died and changed into oils/coal by large pressures as they became buried

• These form today’s major fuels (coal, gas, and oil).



Differences between Plant and Animal Cells

Animal Cells Plant Cells

Usually smaller than plant cells Usually larger than animal cells

No cell wall and no cellulose (shape is often irregular and outline less visible)

Cellulose cell wall (gives cells a well-defined outline and regular shape)

Consist almost entirely of cytoplasm Thin lining of cytoplasm

Small vacuoles which change shape, size and position (concerned only with excretion, secretion or food intake)

Large central vacuole filled with fluid (sap) (concerned with rigidity of cell and therefore whole plant)

No chloroplasts (and no chlorophyll) Chloroplasts (containing chlorophyll)

Food stored as glycogen granules Food stored as starch granules



Levels of Organisation

All Living organisms are made of cells.

But living organisms can be grouped into several levels by considering the amount of complexity and organisation they show.

There are four levels of organisation:

Organisms exist at all of the 4 levels

Level of organisation

Cellular

Tissue Organ

System



Cellular

Level

• Organism consists of a single cell.

• The cell is capable of carrying out all the functions necessary for living.

• E.g. unicellular animals Amoeba, Paramecium and most Bacteria.

Tissue Level

• A tissue consists of hundreds of cells of one or few types.

• Each cell has more or less the same structure and function.

• E.g. hydra, jellyfish, and most plants (higher plants have leaves and reproductive structures, but most of their body functions are still carried out by cells and tissues).

• Higher organisms also have tissues e.g. bone, muscle & nervous tissue.



Organ Level

• Organ a functional unit consisting of several tissues grouped together

• An organ usually has a particular function(s).

• Examples earthworm, higher plants (have simple organs like leaves and reproductive structures)

• Higher organisms also have organs e.g. kidney, pancreas, muscles, heart.

Organ

System Level

• Organ system several organs which together perform a specific function.

• Examples the great majority of animals (insects, fish, mammals)

• Organ systems e.g. the digestive system includes structures like the mouth, stomach, intestines, liver, pancreas, blood, nervous system, etc. The circulatory system includes the heart, arteries, veins and capillaries.



Comparing Unicellular and Multicellular Organisms

Unicellular Organisms Multicellular Organisms

Growth is limited.

A cell can only grow up to a certain size.

Then it has to divide into 2 smaller organisms.

Organism can grow much bigger.

This makes them stronger, faster, and more difficult to kill.

A cell is non-specialized.

It has to be able to carry out all the functions of living.

This means that it cannot be particularly good at one job. Its functions remain primitive and inefficient.

Different groups of cells can specialize at particular jobs so they are more efficient at that particular job.

This “division of labour” makes the whole organism more efficient, allowing it to develop better structures and functions.



Unicellular Organisms Multicellular Organisms

Cells are independent since they can carry out all functions of living.

Cells lose their independence.

This means that they cannot survive without each other.

This is beneficial to whole organism but a disadvantage to individual cells.

Each cell has easy access to nutrients and respiratory gases through its surface

This is because it has a large surface area-to-volume ratio.

Cells in the centre of the body cannot get nutrients and respiratory gases directly from surroundings.

The larger the organism, the greater the problem.

This is because large organisms have a small surface area-to-volume ratio.

Transport systems are needed to supply these cells.



The diagrams below explain the effect of the surface area-to-volume ratio:

Cell can easily take in and remove substances from its

surroundings.

The outer cells have less surface available for exchange and the central cell has no contact

with the surroundings.



Calculating the surface area-to-volume ratio:

S. Area = 16 cm2

Volume = 2x2x1 = 4 cm

3

SA to Vol. Ratio = 16/4

= 4:1

S. Area = 24 cm2

Volume = 2x2x2 = 8 cm

3

SA to Vol. Ratio = 24/8

= 3:1

2

2 1

2 1

2

2

2 2

2



Fill in the blanks The larger the organism, the smaller the surface area to volume ratio.

Elephants have a small surface area to volume ratio. To make up for this, they have

large flat ears so that they can lose heat from them.

Artic foxes have small ears. This prevents them from losing too much heat. Desert

foxes have large ears. This helps them to loose excess heat from their bodies.

A large penguin is better adapted at living in very cold conditions than a small penguin

since it has a smaller surface area to volume ratio. This means it loses less heat.



The Light Microscope

Fine Focusing

Adjustment

Rotating

Nosepiece

Objective

Lenses

Slide with

Specimen

Stage Iris Diaphragm

Mirror

Stand

Eyepiece Lens

Rough Focusing

Adjustment

Tube



Parts of the Microscope

The Iris Diaphragm controls the intensity

(amount) of light which enters the Microscope.

The Mirror directs light into the Microscope. Some Microscopes have an in-built light below the stage instead of

an adjustable mirror.

The Stage has two Clips (not shown) which hold

the slide in place.

The Small Objective Lens is the Low Power Lens while the Large Objective Lens is the High Power Lens.

When using the high power lens, be careful that you do not crack the slide!

The Magnifying Power of the microscope is calculated by multiplying the magnifying power of the eyepiece by the

magnifying power of the objective lens being used. You can calculate this for your microscope.



Using the Microscope

Switch on the microscope.

Place the slide on the stage and clip it in place.

Turn the nosepiece to the low power objective lens (x10). Be careful not to break the slide.

Turn the rough focusing knob until the LP objective lens is at its lowest position.

Focus by turning the rough focusing knob to raise the lens upwards until a sharp image is observed.

To use high power, turn the nosepiece to the high power objective lens (x40 or x60).

Use ONLY the fine focusing knob when using high power.



Taking Care of the Microscope

Handle the microscope only from the stand.

Be careful not to scratch the lenses - clean them only with lens tissue.

Keep the microscope covered when not in use.

If possible, store the microscope in a cupboard.

Make sure that the objective lenses do not make contact with the slide.



Calculating the actual size of an object from a scale drawing

Actual size = Size of diagram

Magnification

Measure this distance on your paper in cm or mm, then change to metres

(Magnification x 4000)

Actual size = 0.067 m

= 0.00001675 m

(or 1.675 x 10-5

m) 4000

Actual size in µm (x1 000,000) = 16.75 microns



Summary

Nearly all plants and animals are made up of thousands or millions of microscopic cells.

All cells contain cytoplasm enclosed in a cell membrane.

Most cells have a nucleus.

Cytoplasm contains organelles such as mitochondria and chloroplasts.

Many chemical reactions take place in the cytoplasm to keep the cell alive.

The nucleus directs the chemical reactions in the cell and also controls cell division.

Plant cells have a cellulose cell wall and a large central vacuole.

Cells are often specialized in their shapes and activities to carry out particular jobs.

Large number of similar cells packed together form a tissue.

Different tissues arranged together form organs.

A group of related organs make up a system.

As a cell’s size increases its surface area to volume ratio gets less. Eventually the surface area to volume ratio becomes too small for enough food and oxygen to get into the cell. Transport systems are then required.

biology lesson notes cells

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