1

Photosynthesis — limiting factors

What you should know by the end of this unit:

Photosynthesis is the process by which green plants make their own food.

Light energy is trapped by the green pigment chlorophyll.

Chlorophyll is found in structures called chloroplasts.

Green plants make sugar from carbon dioxide and water.

The energy to synthesise (make) the food is obtained from the sunlight absorbed by chlorophyll.

Oxygen is a by-product of photosynthesis.

The sugar made by plants during photosynthesis is a high-energy chemical

called a carbohydrate.

A plant uses this sugar as a source of energy to grow.

A plant can store sugar as starch.

Iodine solution turns blue-black when starch is present.

Light, chlorophyll and carbon dioxide are the three requirements for photosynthesis.

A limiting factor is an environmental factor which slows down the rate of

photosynthesis if it is in short supply.

Light intensity, carbon dioxide concentration and temperature limit the rate of photosynthesis if they are in short supply.

Skills that you should be able to carry out by the end of this unit:

Design a table with appropriate headings.

Draw a line graph from a table of results.

Interpret information presented as a line graph.

Work as part of a group and take responsibility to support the work of the group.

Write up experiments with an aim, method, results and conclusion.

Appreciate the importance of variables in an experiment and how to improve the reliability of results.

2

Photosynthesis copy this heading

Photosynthesis is the process by which green plants make their own

food.

During photosynthesis green leaves convert light energy to chemical

energy which is contained in sugar.

The green plants make sugar from dioxide and water.

This process involves the trapping of light energy from the sun.

Light energy is trapped by the green pigment chlorophyll. Chlorophyll

is found in structures called chloroplasts.

Photosynthesis goes on mainly in the leaves, though any green part of

the plant can photosynthesise.

Oxygen is given off as a by-product.

Word equation for photosynthesis

Carbon dioxide + water sugar + oxygen

raw materials products

Essential requirements

Chlorophyll Light

3

Water and minerals are absorbed from the soil by the roots and

drawn up the stem through tubes to the leaves where the water is used

for photosynthesis.

Carbon dioxide is absorbed from the air through tiny pores mainly on

the underside of the leaf.

Activity 1 Collect the photosynthesis true/ false/ don’t know cards. Work in pairs and use the resources available to sort the cards.

roots

pores

Open Closed

4

Activity 2 Answer these questions in sentences

1. Name the process by which green plants make food.

2. Where does the green plant get the energy to synthesis food?

3. Name the pigment that traps this energy and where it can be found.

4. What energy change takes place during photosynthesis?

5. (a)Name the two raw materials green plants use to make

food. (b) Explain how these raw materials get into the plant.

6. (a)Name the products of photosynthesis.

(b) Which of these is the by-product?

Food storage in plants copy this heading

The food (sugar) made by plants during photosynthesis is a high-energy

chemical and is a type of carbohydrate. If a plant does not

immediately use this sugar as a source of energy to make new cells and

grow, the plant will store the sugar as starch.

This starch is the plant’s store of food and can be converted back to

sugar for energy when needed.

Some sugar will be stored as starch in the leaves and some will be

transported to other parts of the plant.

Cells in a leaf

5

Stored sugar

Fruit is an examples of a part of a plants containing stored sugar e.g.

grapes, plums, oranges.

Other examples of parts of plants containing stored starch are tubers

and seeds such as potatoes, beans, corn, wheat and rice.

Activity 3 copy and complete this note

The sugar made by plants during photosynthesis is a high energy

c______________.

The sugar can be used by the plant immediately as a source of energy

to make n____ c______ and g____ or the plant can store the sugar as

s______.

This starch can be c_______ back into sugar when the plant needs

e___________.

Starch can be stored in the leaves or in s______ or f________.

6

Testing leaves for starch copy this note

We can use the presence of starch in a leaf as proof that photosynthesis

has taken place. We test the leaf with iodine solution.

Iodine solution turns blue-black when starch is present.

It is important that the plants have no starch in them at the start of the

experiment. Potted plants are de-starched by leaving them in a dark

cupboard for two or three days

Activity 4

Collect this instruction sheet and stick it into your jotter

Procedure to test a leaf for starch:

1. Boil the leaf in a beaker of water for one minute (to

burst cell membranes).

2. Switch the Bunsen burner off.

3. Place the leaf into a boiling tube of

ethanol. Place the boiling tube into

the freshly boiled water. Leave until

all the chlorophyll is dissolved out.

3. Remove the leaf from the boiling

tube and rinse it in the beaker of

water. Spread the leaf out over a

white tile and test it for starch using

iodine solution.

7

Activity 5 Complete the starch test sort cards

Activity 6 Collect the results sheet shown below.

You are going to test a leaf from plant A and plant B for starch. Record your result and your conclusion on the sheet.

Is light necessary for photosynthesis?

Set up starch present? Conclusion

Tidy away all your equipment and get your teacher to check

your results and conclusion.

.

Clear plastic

bag

Green plant that has been destarched

Plant B

Green plant that has been destarched

Black plastic bag

Plant

A

8

Activity 7 Collect the results sheet shown below.

You are going to test a leaf from plant A and plant B for starch. Record your result and your conclusion on the sheet.

Is carbon dioxide necessary for photosynthesis?

Set up Result Conclusion

Tidy away all your equipment and get your teacher to check

your results and conclusion.

Clear plastic

bag

Sodium hydroxide

solution

Water

Plant B

Plant A

Green plant that has been destarched

Green plant that has been destarched

Clear plastic bag

9

Activity 8 Collect the results sheet shown below.

You are going to test a leaf from a variegated plant for starch.

Record your result and your conclusion on the sheet.

Tidy away all your equipment and get your teacher to check

your results and conclusion.

Is chlorophyll necessary?

Set up Result Conclusion

Green part contains chlorophyll White part does not contain chlorophyll

Clear plastic bag

Variegated plant that has been in the dark for 2 days to destarch

Green (chlorophyll) White (no chlorophyll)

10

Activity 9 Answer these questions in sentences

1. Name the chemical reagent used to test for starch.

2. Describe the colour change shown by this reagent if starch is

present.

3. Copy and complete

Photosynthesis will not take place in the absence of

c_________ d__________ l_____________

c__________________

Limiting factors copy this note

A limiting factor is an environmental factor which if in short supply slows down the rate at which a plant can carry out photosynthesis

Activity 10 Group activity

THINK/ PAIR/ SHARE

Can you think of an environmental factor which will affect the rate of photosynthesis? Share and discuss with your partner and the other groups in the class. Make a list in your jotter. Use the help cards if you find this difficult.

11

Activity 11 The effect of light intensity on the rate of photosynthesis Look at the equipment shown in the diagram below.

This equipment can be used to find the effect of changing light

intensity on the rate of photosynthesis.

We can measure the rate of photosynthesis using aquatic plants like Elodea and Cabomba. During photosynthesis, these plants give off bubbles of oxygen. The rate of photosynthesis can be measured by counting the number of bubbles produced or by collecting the oxygen and measuring its volume.

Copy this heading

12

Discuss in your groups how you could change the light intensity

the plant is receiving.

Discuss what variables have to be kept the same in order to make it a fair experiment.

Think about how your group would measure the rate of

photosynthesis.

Use the information gathered during your group

discussion to Copy and complete this table.

What is the aim of this experiment?

What are you changing?

How are you changing it?

What will you keep the same?

How will you measure the rate of photosynthesis?

13

Use the webpage “water weed”

Set the carbon dioxide levels to 2.0

Set the light intensity to 1 and note the rate of photosynthesis in your jotter.

Repeat for a light intensity of 2 to a light intensity of 10, remember to note down your results.

Copy and complete the table below into your notes.

Light intensity

Rate of photosynthesis

1

2

3

4

5

6

7

8

9

10

Collect graph paper and draw a line graph of your results.

Answer the following questions in sentences

1. What happens to the rate of photosynthesis as the light

intensity increases?

2. Why does the graph level off?

3. What could be limiting the rate of photosynthesis at this point?

14

Activity 12 The effect of carbon dioxide concentration

on the rate of photosynthesis

Look at the equipment shown in the diagram below.

This equipment can be used to find the effect of changing Carbon

dioxide concentration on the rate of photosynthesis.

Discuss in your groups how you could change the carbon dioxide

concentration the plant is receiving.

Discuss what variables have to be kept the same in order to make it a fair experiment.

Think about how your group would measure the rate of

photosynthesis.

Copy and complete this table

What is the aim of this

experiment?

What are you changing?

How are you changing it?

What will you keep the same?

How will you measure the rate of photosynthesis?

Copy this heading

15

Use the webpage “water weed”

Set the light intensity to 7

Set the carbon dioxide levels to 2.0 and note the rate of photosynthesis in your jotter.

Repeat for a carbon dioxide level of 2 to a level of 10; use the results to copy and complete the table below

Carbon dioxide concentration

Rate of photosynthesis

2

3

4

5

6

7

8

9

10

Collect graph paper and draw a line graph of your results.

Answer the following questions in sentences

1. What happens to the rate of photosynthesis as the carbon dioxide

concentration increases?

2. Why does the graph level off?

3. What could be limiting the rate of photosynthesis at this point?

16

Activity 13 The effect of temperature on the rate of photosynthesis

Design an experiment to find the effect of changing temperature

on the rate of photosynthesis.

Discuss in your groups how you could change the temperature the

experiment is at.

Discuss what variables have to be kept the same in order to make it a fair experiment.

Think about how your group would measure the rate of

photosynthesis.

Copy and complete this table

What is the aim of this experiment?

What are you changing?

How are you changing it?

What will you keep the same?

How will you measure the rate of photosynthesis?

Activity 12 Copy this table of results into your notes.

Temperature (OC) Rate of photosynthesis

5 1

10 4

20 9

30 26

40 35

50 30

60 0

70 0

Collect graph paper and draw a line graph of these results.

17

Answer the following questions in sentences

1. What happens to the rate of photosynthesis as the temperature

increases?

2. What is the optimum temperature?

3. Why does the rate of photosynthesis fall above the optimum

temperature?