CCEA GCSE Biology - Denmour Boyd, James Napier 2017
Unit 1
Photosynthesis and plant leaves
Specification points
This chapter covers specification points 1.2.1 to 1.2.6. It covers understanding of the process of photosynthesis, knowledge of the equations involved, investigations into the factors which affect the rate of photosynthesis and how the structure of a plant leaf is adapted for gas exchange and light absorption.
Photosynthesis
Photosynthesis takes place in the green parts of plants, particularly in the leaves, where the raw materials of carbon dioxide and water are made into glucose (sugar). The glucose is usually immediately converted into starch and stored in cells. The green pigment chlorophyll plays an important role in photosynthesis as it traps the light from the Sun that is needed to drive the process. This absorption of energy means photosynthesis is described as endothermic. Oxygen is produced as a waste product.
Photosynthesis can be summarised by the word equation:
Tip
An endothermic process or reaction is one which absorbs energy (light energy in the case of photosynthesis) from its surroundings.
and the balanced chemical equation:
In photosynthesis, light energy from the Sun is converted into chemical energy (food in the form of sugar and starch).
The glucose that is produced in photosynthesis can be used in several ways or converted into a range of products that the plant requires. Uses include:
• respiration — the glucose is used in respiration to provide energy
• storage — in many plants the glucose is converted into starch and oils for storage
• useful substances — the glucose can be converted into a range of useful products including cellulose (for cell walls), chlorophyll and protein for growth.
Leaves (and other parts of the plant) that are carrying out photosynthesis in bright light will take carbon dioxide into the leaves and oxygen will pass out. Not surprisingly, the brighter the light, the faster the process will take place.
Photosynthesis is important for animals as well as plants, as it provides a source of food and releases oxygen back into the atmosphere.
Photosynthesis experiments
It is possible to carry out investigations to show that photosynthesis is taking place or that particular raw materials are needed for the process.
The starch test
This test is used to show the presence of starch in green leaves and allows the conclusion that photosynthesis has taken place. The starch test consists of a series of steps. Remember to wear eye protection for this test.
1 A leaf is removed from a plant that has been in bright light and is placed in boiling water for at least 30 seconds. This kills the leaf, stopping any chemical reactions.
2 The leaf is then placed in boiling ethanol (Figure 2.1). This procedure must take place using a water bath as ethanol is flammable and must not be exposed to a direct flame. The ethanol removes the chlorophyll from the leaf making it a whitish-green colour and brittle.
3 The leaf is then dipped into water again to soften it.
4 The soft leaf can then be spread on a white tile and iodine solution added.
If starch is present, the yellow-brown iodine solution will change to a blue-black colour. If starch is absent the iodine solution will remain yellow-brown.
Test yourself
1 Where is chlorophyll found in plants?
2 What are the products of photosynthesis?
3 How does a plant use the products of photosynthesis?
4 Why are the products of photosynthesis also important to animals?
5 How is a plant destarched?
Destarching
To carry out these experiments it is necessary to destarch the leaves of the plant first. Leaves can be destarched by leaving the plant in the dark for at least two days. This will ensure that any starch already in the leaves will either be used by the plant cells or removed and stored elsewhere in the plant. The importance of this is that if the starch test at the end of the experiment is positive, it shows that starch has been produced during the period of the experiment.
Show you can
Explain why, after destarching a plant, it is necessary to test one of the plant leaves for the presence of starch before starting the experiment.
Prescribed practical
Biology Practical 1.2 Double Award Science B1: Investigating the need for light and chlorophyll in photosynthesis
Investigate the need for light in photosynthesis
Procedure
1 Destarch a plant by keeping it in a cupboard for two days. Remember to wear eye protection.
2 Test a leaf from the destarched plant for the presence of starch. If the leaf tests negative continue to 3.
3 Cover a part of one leaf of the plant with lightproof paper or foil as shown in Figure 2.2a. Make a drawing of the destarched leaf at the start of the experiment.
4 Place the plant in bright light for several hours.
5 Test the leaf from the plant for starch. Make a drawing of the leaf after it is tested for starch.
Sample drawings of the leaf
Questions
1 Describe and explain the results.
2 Which part of the leaf acts as a control in this experiment?
Investigate the need for chlorophyll in photosynthesis
Procedure
1 Destarch a variegated plant by keeping it in a cupboard for two days. Remember to wear eye protection.
2 Test a leaf from the destarched plant for the presence of starch. If the leaf tests negative, continue to step 3 to proceed with the investigation.
3 Make a drawing of one of the leaves.
4 Place the plant in bright light for several hours.
5 Test the leaf from the plant for starch.
6 Make a drawing of the same leaf after it is tested for starch.
Sample drawings of the leaf
Questions
1 Describe and explain the results.
2 Which part of the leaf acts as the control in this experiment?
Investigating the need for carbon dioxide
To show that carbon dioxide is an essential raw material for photosynthesis it is necessary to compare a leaf that is deprived of carbon dioxide with a leaf that has a good supply of carbon dioxide. This can be achieved by comparing two leaves as shown in Figure 2.4. Sodium hydroxide will absorb the carbon dioxide from the air surrounding the experimental leaf. The control leaf will only have water (or a chemical that increases carbon dioxide levels) in its flask and therefore there will be carbon dioxide present.
Test yourself
6 In Figure 2.4, which flask has the lowest concentration of carbon dioxide? Explain your answer.
7 Why is the cotton wool in the neck of the experimental flask soaked in sodium hydroxide?
8 Explain why the experiment uses leaves from the same plant.
9 Describe and explain the steps involved in testing the leaves for starch.
Show you can
If carbon dioxide is essential for photosynthesis, what results would you expect?
Measuring the rate of photosynthesis
Using apparatus like that shown in Figure 2.5, it is possible to demonstrate that oxygen is produced by photosynthesis. The rate of photosynthesis will affect the rate at which the bubbles of oxygen will be given off and this can be used to compare photosynthesis rates in different conditions. For example, by moving the position of the lamp it is possible to investigate the effect of light intensity on photosynthesis.
Test yourself
10 What measurements would be required to calculate the rate of photosynthesis using the apparatus in Figure 2.5?
11 How is the rate of photosynthesis calculated and what units would be used?
12 What effect would moving the lamp further away from the beaker have on the rate of photosynthesis?
13 Why is counting bubbles not an accurate method of measuring the volume of oxygen produced?
The rate of photosynthesis can be more accurately calculated by measuring the volume of oxygen produced after five minutes. Alternatively, an oxygen electrode connected to a data logger can be used to measure the change of oxygen concentration.
Show you can
Suggest why it is necessary to wait for five minutes each time the lamp is moved before taking measurements.
Factors affecting the rate of photosynthesis
The rate at which photosynthesis occurs depends on the availability of the raw materials needed for the process, carbon dioxide, water and light. As we have seen from the equations (page 12), carbon dioxide and water are the substrates which join together chemically to form the product of photosynthesis (sugar). Increases in these substrates will increase the rate at which the products of photosynthesis are formed. Light provides the energy needed to join the substrates together so again more light means more products formed.
Test yourself
14 Why does temperature affect photosynthesis?
15 What other factors affect photosynthesis?
Temperature affects all reactions. Increasing temperature also gives molecules energy (kinetic energy), making them move faster and collide more often, which also increases the number of reactions.
For photosynthesis to take place at its maximum rate, all of these environmental factors must be present at peak or optimum levels. However, if one (or more) factor is in short supply, the rate of photosynthesis will be limited. These raw materials become limiting factors and the rate of photosynthesis will be determined by whichever factor is in the shortest supply.
Test yourself
16 Why is it important for farmers to understand limiting factors?
17 What is a limiting factor in the process of photosynthesis?
Figure 2.6 shows the effect of light intensity on the rate of photosynthesis. Part a) shows how temperature (cold and hot days) can further influence the rate.
Tip
A limiting factor in photosynthesis is any of the environmental factors (light, carbon dioxide or temperature) which prevents the reactions of photosynthesis taking place at their maximum rate.
As light intensity increases, irrespective of temperature, the rate of photosynthesis increases up to a point where the graph begins to level off and form a plateau. As an increase in light intensity causes an increase in photosynthesis at the lower light levels, the amount of light must be limiting the rate at which photosynthesis occurs. Within the plateau part of the graph, further increases in light intensity do not lead to an increase in photosynthesis, therefore something else must be limiting the rate.
Show you can
Use the information in Figure 2.6 to help you decide which factor might limit the rate of photosynthesis in the following situations:
a) during a bright winter afternoon in a British grassland
b) in a cornfield in mid-summer sunshine in Southern France.
The effect of temperature can be explained by comparing the rates of photosynthesis on cold and hot days. On a hot day, photosynthesis occurs at a higher rate at higher light intensities when compared to a cooler day. Therefore, we can conclude that temperature is limiting the rate of photosynthesis at the higher light intensities on the cooler day.
It is possible that the rate of photosynthesis may still not be at its maximum where the rate has plateaued on a hot day at the highest light intensities. It is possible that carbon dioxide could be a limiting factor in these conditions. To test this, we would need to increase carbon dioxide levels to see if this has any effect.
Figure 2.6b shows how light intensity affects the rate of photosynthesis at different carbon dioxide levels. Again, light intensity is limiting the rate of photosynthesis at low light levels. The fact that an increased carbon dioxide level leads to a higher rate of photosynthesis at higher light intensities shows that the low carbon dioxide level was a limiting factor once light levels had ceased to be limiting.
It is important for farmers and growers to understand how photosynthesis affects their crops as reducing the effects of limiting factors will increase the growth of a crop.
The leaf: the site of photosynthesis
In most plants the process of photosynthesis takes place in the leaves. Leaves come in many shapes and sizes but, to allow photosynthesis to take place efficiently, they are usually adapted for:
• light absorption
• gas exchange
The way in which leaves are arranged on a plant ensures that each leaf can absorb as much light as possible and that as far as possible each leaf is not in the shade of other leaves. The section through a leaf shown in Figure 2.7 shows many other ways in which a leaf is designed to aid light absorption and encourage gas exchange.
Tip
Leaves are also adapted for defence against disease by the cuticle and cell walls of the epidermis providing a physical barrier, which reduces the entry of disease-causing organisms.
Use a microscope to investigate a cross section of a mesophytic (typical unspecialised) leaf. The leaf you examine may be different from the one described in this book, but you should look for as many adaptations for maximising light absorption and gas exchange as possible.
Light absorption in a leaf is maximised by:
• the short distance between the upper and lower surfaces, which allows all the cells to receive light
• the large surface area
• the thin transparent cuticle covering the epidermis, which reduces water loss by evaporation, while allowing light to enter the leaf
• the epidermis which lacks chloroplasts and so also allows light into the leaf
• the presence of chloroplasts rich in the pigment chlorophyll which absorbs the light
• the regular structure of the palisade mesophyll (end on to the upper surface), which ensures that many cells rich in chloroplasts are packed together near the upper surface of the leaf.
Tip
Water also moves through the intercellular air spaces as a gas (water vapour). It is formed by evaporation from the moist surfaces of the spongy mesophyll cells and then diffuses through the air spaces and stomata out of the leaf. This is part of the transpiration stream discussed in Chapter 8.
Gas exchange in a leaf takes place by diffusion and is maximised by:
• the intercellular air spaces in the spongy mesophyll, which allow carbon dioxide to enter and oxygen to leave the photosynthesising cells, which are mainly concentrated in the palisade layer
• stomata, which allow carbon dioxide and oxygen to enter and leave the leaf. Stomata are small pores that can occur between cells in the epidermis on the lower surfaces of leaves. Each stoma is surrounded by two guard cells that regulate the opening and closing of the stoma. In many plants the stomata are open during the day and closed at night.
Test yourself
18 Why are leaves thin?
19 Name the waxy layer on the upper surface of a leaf.
20 Which layer of cells has the highest density of chloroplasts?
21 Which process causes gases to move through the intercellular air spaces and through the stomata?
In some plants stomata can occur on both the upper and the lower leaf surface. Some plants have all their stomata on their upper leaf surfaces.
Show you can
Water lilies have leaves which float on the surface of water. Suggest why they only have stomata on their upper leaf surfaces?
The balance between photosynthesis and respiration
All living organisms respire. In plant respiration, the glucose produced in photosynthesis is broken down to release energy. Plants require oxygen to respire and they produce carbon dioxide as a waste product. These gases enter the leaves through the stomata.
During the night when there is no light for photosynthesis, respiration will be the only process involving gas exchange that takes place. Therefore, oxygen will enter the leaf and carbon dioxide will leave. However, during the day when photosynthesis is occurring both processes will take place. When the light intensity is high the rate of photosynthesis will exceed the rate of respiration. When this happens, carbon dioxide enters the leaves and oxygen moves out.
There will be times during the day when the light intensity is low, causing photosynthesis to take place very slowly. At these points, usually at dawn and dusk, the rates of respiration and photosynthesis are equal and there will be no overall, or net, gas exchange. This point is called the compensation point.
The movement of carbon dioxide into and out of plants can be determined using hydrogencarbonate indicator. Hydrogencarbonate indicator is bright red in normal concentrations of atmospheric carbon dioxide. If there is an increase in the carbon dioxide concentration the indicator will change colour to yellow. A decrease in the carbon dioxide concentration will turn the indicator purple. Figure 2.8 shows how the indicator can be used to show gas exchange in living organisms.
Test yourself
22 How does the colour of hydrogencarbonate indicator change when the concentration of carbon dioxide increases?
23 What has happened to the concentration of carbon dioxide if hydrogencarbonate indicator becomes purple?
24 Which environmental factor causes a compensation point?
25 What happens to the rates of photosynthesis and respiration at the compensation point?
Show you can
How could the compensation point be demonstrated using Tube B?
The results are explained in Table 2.1.
Practice questions
1 Photosynthesis occurs in plant leaves.
a) Name the chemical in leaves that absorbs light for photosynthesis.
(1 mark)
b) What term is used to describe reactions such as photosynthesis that require energy?
(1 mark)
c) Copy and complete the boxes in the word equation for photosynthesis.
(3 marks)
2 Figure 2.9 shows a section of a leaf.
a) Name parts A, B, C and D.
(4 marks)
b) Explain how parts A and B adapt the leaf for photosynthesis.
(2 marks)
c) Describe and explain how carbon dioxide moves from the air into the site of photosynthesis in B.
(4 marks)
3 Figure 2.10 shows the setup of an investigation into the effect of light on the rate of photosynthesis.
Three sealed tubes containing pondweed and equal volumes of red indicator solution were placed in different light conditions for 24 hours.
After 24 hours the indicator in A was purple, B was red and C was yellow.
a) Describe the contents of a suitable control for this investigation.
(1 mark)
b) Suggest one reason why it may not be valid to compare the results for each tube.
(1 mark)
c) Name the indicator solution.
(1 mark)
d) Explain the colour change in tube A.
(2 marks)
e) Tube B represents the compensation point. Explain what the compensation point is in this experiment.
(2 marks)
4 Figure 2.11 shows a graph of the carbon dioxide used by photosynthesis and produced by respiration during one day.
Use evidence from the graph to help explain the relationship between respiration and photosynthesis at:
a) A
(3 marks)
b) B
(3 marks)
c) C
(3 marks)