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Plants are living things. They need water and nutrients, they use gases from the air and they produce waste materials. They grow, they reproduce and they die. Like all living things, plants are made up of cells.

Structure of a plant cellAlthough animals must eat to survive, plants do not. Plants make their own food in a process called photosynthesis. Energy from sunlight combines carbon dioxide (CO2) from the air and water (H2O) from the soil to make a type of sugar called glucose (C6H12O6) and oxygen gas (O2). The word equation and unbalanced formula equation for photosynthesis is:

carbon dioxide + water + sunlight → glucose + oxygen gasCO2 + H2O + sunlight → C6H12O6 + O2

The plant then uses the glucose produced as its food and energy source.

The cell wall of a plant cell contains a tough fibrous material called cellulose. The cell wall needs to be thick and rigid to provide the plant with support. Plant cells contain a large vacuole filled with sap.

light energy

water (from soil)

to all parts ofthe plant

carbon dioxide(from air)

released into the air

glucose

chlorophyllin cells

oxygen+

Sun

Fig 5.2.1 The process of photosynthesis

Photosynthesis occurs in structures called chloroplasts inside cells on the surfaces of leaves. Chloroplasts can be seen using a light microscope. Chloroplasts contain a green chemical, chlorophyll, which traps the light energy plants need to run the photosynthesis reaction.

Specialised plant cellsPlants contain different types of cells, each performing a different job. Plants need cells to stand upright, cells to suck water from the soil, cells to reproduce and cells to allow photosynthesis to take place. Following are some of the specialised plant cells that plants need.

Fig 5.2.3Typical plant cells

A typical plant cell

large vacuolecontainingcell sap

cell wall

cell nucleus

chloroplast

mitochondrioncell membrane

cytoplasm

Fig 5.2.2

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>>>Plant cellsPlant cells

chloroplastsnucleus

Fig 5.2.4 A photosynthetic cell

• Photosynthetic cells: These cells make up a layer near the surface of a leaf. Most of a plant’s photosynthesis happens here.

• Root hair cells: Plants and photosynthesis need water, which is absorbed from soil by specialised root hair cells.

• Conducting cells: Special conducting cells form tubes or vessels that transport water and nutrients to all parts of the plant.

water-conductingtube

food-conductingtube

sieve

Fig 5.2.5Conducting cells

• Guard cells: Plants take in carbon dioxide and give out oxygen, but they lose some valuable water when they do so. Special guard cells lie on the underside of leaves. Their job is to open and close small openings called stomata and so reduce water loss.

guardcell

Fig 5.2.6 A guard cell

A single stomata, properly called a stomaFig 5.2.7

Cunning cactusCactus plants have needles for leaves.

The needles have little space for the stomata through which water

is lost. This adaptation allows the cactus

to survive in desert conditions.

Fig 5.2.9Root hairs increase the surface area through which water is absorbed into a plant.

A root hair cell

cell in rootroot hair

nucleussoil

Fig 5.2.8


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5.25.2Plant systemsThe cells of plants group together to form organs and systems. Leaf cells group together to form the organ that you would know as a leaf. Several leaves form the food-making system for the plant. Some other plant systems are:• the reproductive system consisting of the parts of

a flower• the food storage system, often in the form of a bulb

or tuber• the root system for securing the plant in the

ground and obtaining water and nutrients• the food and water transport system consisting of a

network of veins.

Plant pathwaysThere are two types of tubes that transport food and water inside plants.• Xylem tubes carry water and minerals

(e.g. phosphorous, potassium, nitrogen, sulfur, calcium, iron and magnesium) from the soil, up into the stems and leaves. Xylem tubes are made of dead cells strengthened with a woody substance. Unlike animals, a plant does not have a heart to pump liquid through its tubes. Instead, water is pushed upwards by pressure

in the roots. Evaporation through the stomata (tiny holes in the leaves) further assists by sucking the water upwards.

• Phloem tubes are made from living cells. Their function is to transport the food (glucose) that is produced by photosynthesis in the leaves to the stem and roots. Some plants store glucose directly for use when required (e.g. to produce new buds in spring). Lettuce and cabbage can store glucose in their leaves and celery can store it in its stem. A carrot plant stores glucose in the carrot. Other plants store the food in the form of starch. A potato plant stores starch in potatoes, which is why potatoes are not as sweet as carrots and other vegetables.

Xylem and phloem tubes are grouped together in vascular bundles, separated by a layer of cambium cells. Cambium cells are able to become either new xylem or new phloem cells as required.

water enters root hairs

water travels through xylem vessels

glucose made by photosynthesis

water evaporates out of stomata

Fig 5.2.10 Water flow through a plant

Homework book 5.1 Water movement in trees

Ripening bananasA green banana contains

starch. This starch changes into glucose

as it ripens, making the banana sweeter.

xylem

vascular bundle

phloem

cambium

Fig 5.2.11Cross-section of a stem, showing vascular bundles

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Plant skeletonsAnimals have skeletons to hold them upright. Plants use other means.

Firm or floppyThe soft parts of a plant are supported by water in its cells. The plant will be upright and its cells firm (turgid) if enough water is present. The plant’s stem and leaves may droop and become flaccid if the water content in the cells falls.

Fig 5.2.13 Growth rings in mature trees

Stomata control the flow of oxygen and water vapour out of a leaf, and also control

carbon dioxide intake.

straight guard cells

guard cells

stoma in open position

wilted leaf

cell magnified

closed stoma

underside of leaf

cell magnified

Fig 5.2.12

many annual rings

xylem

xylemphloem

vascular bundle

cambium

cambium joins up

phloemvascular cylinder formed

A B C D

Fig 5.2.14Formation of growth rings in a tree

WoodTrees are just big plants and so they too contain xylem and phloem cells. Vascular bundles in the stem eventually link up to form a vascular cylinder. Phloem cells stay in the outer layer of a tree, just under the bark. These phloem cells are the pathways for nutrients to reach all parts of a tree. The tree may die if they are damaged. Ringbarking removes a layer of phloem cells and will quickly kill a tree. Each year a new layer of xylem cells is produced, and the inner layers of old xylem cells combine with other plant substances to form wood. A cut cross-section of a tree trunk can reveal these yearly rings of growth.

Plant cellsPlant cells


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[ Questions ] 21 Plants are usually green. Suggest why.

22 Study the plant diagram in Figure 5.2.15 and state which part contains each of the following systems.

flowerleaf

bulb

roots

Fig 5.2.15

a reproductive systemb food-making systemc food and water transport systemd root systeme food storage system

23 What would happen to a plant without:a cellulose in its cellsb chlorophyll in its cells?

24 A plant would die if it did not have the following cells. Suggest how it would die if it had no:a guard cellsb photosynthetic cellsc conducting cellsd root hair cells.

25 Growth rings are produced because trees grow at different rates during a year, producing different patterns within the tree. Why do trees grow at different rates during a year?

Revision questions

Structure of a plant cell 1 Why do plants need photosynthesis?

2 What is the chemical formula for:a carbon dioxide b oxygen gas?

3 What chemicals do these formulas represent?a C6H12O6 b H2O

4 For the photosynthesis chemical reaction, write its:a word equationb unbalanced chemical equation.

5 What is the green substance in plant cells and what does it do?

6 Which part of a plant cell contains:a sapb fibrous materialc chlorophyll?

Specialised plant cells 7 Which type of plant cell is:

a ‘hairy’b the ‘gatekeeper’c a ‘transporter’?

8 Construct a table that shows what each type of plant cell does.

Plant systems 9 List four systems that a plant has.

10 A leaf is really an organ of a plant. Explain why.

Plant pathways 11 Name the two types of plant transport tubes and

describe what they do.

12 Plants need minerals. List five of them with their element symbols.

13 What gets fluid moving through a plant?

14 What is a vascular bundle?

15 State two ways plants store energy for future use.

16 What is so special about cambium cells?

Plant skeletons 17 Why do some plants wilt in hot weather?

18 What provides support for a tree?

Thinking questions

19 Suggest why photosynthetic cells are normally located on the top of a leaf and not under it.

20 Animals do not need the ability to carry out photosynthesis. Suggest why.

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[ Practical activities ]5.2

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Prac 1 Unit 5.2

You will needMicroscope, potassium iodide stain, lamp, filter paper, glass slide, eye dropper, water, cover slip, samples of onion skin, banana and rhubarb, wooden craft stick

What to do 1 Slice or peel a thin layer of skin from an onion.

2 Prepare a wet mount of the onion skin using the method described on pages 129 to 130.

3 Obtain a clear image with the microscope and sketch what you see.

4 Sketch a few of the onion cells that you see.

5 Smear a thin layer of banana onto a clean slide and stain the specimen.

6 Add a drop of water and a cover slip.

7 Obtain a clear image with the microscope and sketch what you see.

8 Peel some of the outer layer from a piece of rhubarb.

9 Prepare a wet mount and observe the rhubarb cells under the microscope.

10 Sketch its image.

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at workInvestigating

Geranium shootsRingbark one geranium shoot, cover the ringbarking with Vaseline to prevent drying and place the shoot in a container of water. Place a similar, non-ringbarked shoot in another container of water. Leave them for two weeks and record any observations made about root growth. Root growth requires food that is produced in the leaves. What can you conclude about food pathways in geranium shoots?

Present your work as an experimental report. Include all the normal features, such as aim, materials, method, results and conclusion.

Radish seedsPlace some radish seeds on moist cotton wool and observe the roots that develop over a few days. What does this arrangement allow you to observe that might be lost when pulling a plant out of the ground?

5.2 SCIENCEPresent your work as an experimental report. Include all the normal features, such as aim, materials, method, results and conclusion.

Constructing

Plant cellsUse Plasticine to make a three-dimensional model of a plant cell. Slice it to produce a two-dimensional (flat) view similar to that shown in Figure 5.2.2 on page 137.

Surfing

Researching paperSurf your available resources (textbooks, encyclopaedias, Internet, etc.) to find out how paper is manufactured from wood.

Present your work as a flow chart showing every major stage in the process from tree to paper.

26 Which plant tubes are most at risk when a rabbit nibbles the base of a small tree?

27 Putting cut flowers in a vase of water keeps them fresh for longer. Suggest how.

28 Suggest how removing the leaves from a plant might affect it.

29 Plants contain a large amount of carbon. Suggest where this comes from.

Plant cellsPlant cells


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5.25.2

Water transportYou will need

Celery stick, two beakers, razor blade, dye

What to do 1 Arrange the apparatus as shown in Figure 5.2.16.

2 Leave it overnight before observing the celery stalk closely.

3 Cut the celery stick lengthways and across the stalk and note the presence of any dye.

4 Design a modification to the set-up to investigate what effect the leaves have on the movement of the dye.

Questions

1 Where did the dye get to? In which direction did it move?

2 Draw a diagram showing the dye and its movement.

3 Suggest why one half of the celery stalk was placed in water with no dye.

Prac 2 Unit 5.2

2 Which cells were easier to observe. Why?

3 Describe some of the similarities and differences you saw between banana, onion and rhubarb cells.

Questions

1 Why was stain recommended for viewing banana cells, but not for onion cells?

celery

dye water

Fig 5.2.16Water transport in celery


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5.35.3Animals are made up of cells just like plants. Animals are far more complex than plants, however, and need a greater variety of cell types to allow them to live, move and reproduce.

Specialised animal cellsThe skin, muscles, organs and blood of animals are all made up of different types of cells, most of them so small that hundreds would fit inside a full stop.

Each animal cell is specialised to perform its particular job. Whatever the job, there is a cell specifically for it. Some of the more important cells that animals need are:• blood cells to carry food and oxygen around

the body and to fight off infection from bacteria and viruses

• muscle cells to give movement

• nerve cells to send messages from the brain to the muscles and back from nerve receptors to the brain

• skin cells to cover bodies and to provide a barrier for infection

• bone cells to help support

the body and protect internal organs• fat cells to insulate the body and store energy• specific cells to make up the different organs of an

animal such as the heart, brain, lungs and liver• sperm and egg cells that can combine to produce a

new animal.

Body cellsYour body contains

over a hundred million million cells.

Various types of human cells, each specialised for a different purpose

involuntarymuscle cells

nerve cell

part of a skeletal muscle cell

red blood cell

bone cell

fat cell

white blood cell

Fig 5.3.1

Fig 5.3.2Red and white blood cells, magnified 3000 times

Structure of an animal cellAlthough there are similarities, animal cells are usually more complex than plant cells. All animal cells, whether they are from a human, pig or a frog, have several characteristics in common.


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cell nucleus

cytoplasm

mitochondrion(not normally visible)

cell membrane

vacuole

Fig 5.3.3 The structure of an animal cell

Stained human cheek cells showing clearly the cell nucleiFig 5.3.4

Homework book 5.2 Cell diagrams

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[ Questions ]Revision questions

Specialised animal cells 1 To start a new animal, a sperm cell must fertilise an

_______ cell.

2 How many cells are in your body?A less than 1000 C about 1 billionB exactly 100 million D over 100 million million

3 List any five types of body cells and explain what they do.

4 Which type of human cells:a help keep out infection c carry oxygenb send messages d assist with movement?

5 Draw two different types of human cell.

Structure of an animal cell 6 What part of an animal cell:

a is so small it cannot usually be seen with a light microscope

b is a jelly-like liquidc contains coded instructionsd are energy capsulese is the outside wall?

• Thecell membraneisathinouterlayerthatcontainsthecellandcontrolswhatgoesinorcomesout.Animalshavesomeformofskeletontoholdthemuprightandsodonotneedthethickcellwallsthatplantshave.

• Thecytoplasmisajelly-likeliquidcontaininghundredsofchemicals.Itfillsmostofthecell.Newsubstancesaremadeandenergyisreleasedandstoredhere.Thecytoplasmcanbethoughtofasthechemicalfactoryofthecell.

• Vacuolesarestorageareasthatmaycontainair,water,wastesandfoodparticles.Animalcellsoftencontainseveralsmallvacuoles.

• The‘controlroom’ofthecellisthecellnucleus.Thenucleuscontrolsallchemicalreactionsinacellandhowthecelldevelops.Thenucleuscontainsinstructionsinchemicalcodesforbuildingnewcells.

• Mitochondriaaresmallobjectsthatcanbethoughtofasenergycapsules.Eachmitochondrioncontainssugarandoxygen,whichcancombinetoreleaseenergy.Mitochondriaaresosmalltheycannotusuallybeseenusingalightmicroscope.

Thinking questions

7 There are fewer types of plant cells compared to animal cells. Suggest why.

8 Muscle cells contain large numbers of mitochondria. Why do you think this is so?

9 Suggest why animal cells do not need the tough cell wall that plant cells need.

10 Which cells do humans only produce after puberty?

11 Suggest what humans would look like if their cells contained chloroplasts full of chlorophyll.

Analysis question

12 The digestive systems of animals like sheep, cows and rabbits contain bacteria that help break down a substance found in the cell walls of plants but not in animal cells. Humans are unable to break down this substance. Deduce what this substance is.

5.35.3

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