B2.3 Photosynthesis

B2.3 Photosynthesis

Green plants and algae use light energy to

make their own food. They obtain the raw

materials they need to make this food from

the air and the soil. The conditions plants are

grown in can be changed to promote growth.

You need to be able to:

■ interpret data showing how factors affect

the rate of photosynthesis

■ evaluate the benefits of artificially

manipulating the environment in which plants

are grown.

Key words

Photosynthesis

Glucose

Chlorophyll

Chloroplasts

Algae

Carbon dioxide

Leaves

Function of leaves

• Trap light energy for photosynthesis

Producing sugar from photosynthesis

• Exchange of gases –

oxygen and carbon dioxide

Structure

Wide

Helps to catch more light

energy

Thin

Help get carbon dioxide

from bottom to top of

leaf for photosynthesis

Leaf structure

Greener on top

CO2 gets in

here

Leaf diagram – palisade layer

CO2

Most

chlorophyll

Leaf cell - palisade

Position?

Upper surface of

leaf

Features?

Box shape

Chloroplasts

Function?

Photosynthesis

Gas exchange

• Leaves are designed to allow carbon

dioxide to get to the main chlorophyll layer

at the top of the leaf

• They have small holes called stomata on

the under surface

• Each hole is open & closed by 2 guard

cells

Leaf diagram –

stoma and guard cells

Stoma position

Stoma is a small hole

Its size is controlled by 2 guard cells

closed open

Stoma function is for gas

exchange in the leaf

Carbon dioxide

oxygenGuard cell

Provided plant is photosynthesising

Stomata open and

close at different

times of the day

When it is light the

plant needs CO2 for

photosynthesis so

the stoma open

At night (darkness)

they close

Gas exchange

Cross section of a leaf

Add the labels to your diagram

The top surface has

a thick waxy

cuticle to stop water

evaporating from the

cells

The top layer of cells is

closely packed and

contain lots of

chloroplasts because

these carry out most of

the photosynthesis

(Palisade layer)

Cross section of a leaf

Veins contain xylem

tubes to carry water to

the leaf cells from the

roots and via the stem

Pores called stoma let

gases in/out the leaf. They

are on the bottom so they

are not blocked and are

shaded to prevent

evaporation. Guard cells

can shut them to control

water loss

The lower layer of

cells has air spaces

between to let carbon

dioxide move

towards the top

surface and a big

surface area for gas

exchange

Add the labels to your diagram

Roots – function and structure

• Hold plant in position

• Absorb water and minerals from the soil

• Specialised cells to increase surface area for water intake

Wheat seed

Root hairs

Fragile parts of

cells that grow

from the main root

They massively

increase the

surface area for

absorption

Root hair cells (x150)

Root ‘B’ has had

the hairs damaged -

Toughened root cap

e) The glucose produced in photosynthesis may be

converted into insoluble starch for storage. Plant cells

use some of the glucose produced during

photosynthesis for respiration.

f) Some glucose in plants and algae is used:

■ to produce fat or oil for storage

■ to produce cellulose, which

strengthens the cell wall

■ to produce proteins.

g) To produce proteins, plants also use

nitrate ions that are absorbed from the

soil.

In a science fiction book, a villain threatens to

spray the countryside with a chemical that

destroys chlorophyll (the green substance in

plants).

What effect will this have on plant life?

Explain your answer as fully as you can.

A farmer cuts his meadow for hay.

He noticed that the grass he cut was

green, but the stalks were yellow.

How do you explain this?

A small tree is planted in a meadow. After

20 years it has grown into a big tree,

weighing 250 kg more than when it was

planted.

Where do you think the extra 250 kg come

from?

Explain your answer as fully as you can.

Plants and minerals

A number of plants seedlings are placed into a water culture and fed with

various nutrients.

One is fed with only distilled water, the others are fed with nutrients but one

without Nitrogen, another without Iron, another without Magnesium and

another without Phosphorous. The final one is fed with all the nutrients.

Describe the appearance of the plants in

each tray

Which plant looks healthiest?

What can you conclude?

Very

little

growth

Poor

growth

Yellowish

leaves

■ to produce fat or

oil for storage

■ to produce

cellulose, which

strengthens the

cell wall

■ to produce

proteins.

Carbon dioxide + water -> Glucose + Oxygen 6CO2 + 6H2O -> C6H12O6 + 6O2

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