IGCSE Biology | Practice Test — Answers & Worked Solutions
Write down the definition of photosynthesis.
What is chlorophyll? State where it is found in plant cells.
State what chlorophyll does with light energy during photosynthesis.
Write the word equation for photosynthesis.
Write the balanced chemical equation for photosynthesis.
Name the carbohydrate that plants use to build their cell walls.
State one use of sucrose in plants.
State what is meant by a limiting factor in photosynthesis.
Name three factors that can limit the rate of photosynthesis.
Before testing a leaf for starch, a plant must be destarched first. Why is this step necessary?
The plant is kept in the dark for 24–48 hours. During this time, it uses up all the starch already stored in its leaves.
A variegated leaf has green patches and white patches. After being left in bright light for several hours, the whole leaf is tested with iodine solution.
(a) Predict the colour of the iodine on the green patches. Explain your answer.
(b) Predict the colour of the iodine on the white patches. Explain your answer.
A well-destarched leaf is partially covered with black foil and left in bright light for several hours. When tested with iodine, only the uncovered part turns blue-black.
State the conclusion you can draw from this result.
The covered part received no light and produced no starch — photosynthesis did not occur. The uncovered part received light, photosynthesised, and stored starch. Since the only difference between the two parts was light, we can conclude that light is required for photosynthesis.
A well-destarched plant is placed in a sealed transparent container along with soda lime, which absorbs all the CO2 from the air. The container is left in bright light for 24 hours. A leaf is then removed and tested with iodine.
(a) What result would you expect from the iodine test?
(b) Explain why this result is expected.
Soda lime removed all the CO2 from the air inside the container. CO2 is a raw material for photosynthesis. Without it, the plant cannot carry out photosynthesis, so no glucose is produced and no starch is stored in the leaf.
A student places a piece of Elodea (a water plant) near a lamp. She counts 10 oxygen bubbles per minute. She then moves the lamp further away from the plant.
(a) Predict what will happen to the number of bubbles per minute.
(b) Explain your prediction.
Moving the lamp further away reduces the light intensity reaching the plant. With less light energy available, the rate of photosynthesis decreases. Less oxygen is produced, so fewer bubbles are released per minute.
Explain why plants store excess glucose as starch rather than keeping it as glucose inside cells.
Starch is insoluble — it does not dissolve in the water inside cells. This means it does not affect the water balance (osmosis) of the cell.
Glucose is soluble. If glucose accumulated in the cell, it would affect the water balance inside the cell, which could disrupt normal cell function.
A plant at 5°C photosynthesises more slowly than an identical plant at 25°C, even when both have the same light intensity and CO2 concentration. Explain why.
Photosynthesis is controlled by enzymes. At 5°C, there is very little kinetic energy, so enzyme molecules and their substrates collide less frequently and less successfully. The enzymes work much more slowly.
At 25°C, the temperature is closer to the enzymes’ optimum. There is more kinetic energy, more successful collisions occur, and the enzymes work faster.
The rate of photosynthesis in a plant increases as CO2 concentration rises from 0.01% to 0.04%. When CO2 rises further from 0.04% to 0.08%, the rate stays the same. Explain why the rate stops increasing.
When CO2 rises above 0.04%, CO2 is no longer the limiting factor. Another factor — either light intensity or temperature — has now become limiting.
Hydrogencarbonate indicator (starting colour: orange-red) is added to two sealed test tubes, each containing a piece of Elodea. Tube A is placed in bright light. Tube B is covered with foil to block all light. After one hour:
(a) State the expected colour of the indicator in Tube A and explain why.
(b) State the expected colour of the indicator in Tube B and explain why.
The plant photosynthesises in the light, absorbing CO2 from the water in the tube. As CO2 decreases, the solution becomes more alkaline, causing the indicator to turn purple.
In the dark, the plant cannot photosynthesise, but it continues to respire. Respiration releases CO2 into the water. As CO2 increases, the solution becomes more acidic, causing the indicator to turn yellow.
A greenhouse farmer wants to increase the rate of photosynthesis in his tomato plants during winter. Suggest two changes he could make and explain why each would help.
Install artificial lighting in the greenhouse. Light provides the energy for photosynthesis. More light means more energy is available for chlorophyll to absorb, so the rate of photosynthesis increases.
Inject extra CO2 into the greenhouse. CO2 is a raw material for photosynthesis. More CO2 available means the reaction can proceed faster, increasing the rate.
A student says: “If I give a plant more light, the rate of photosynthesis will always increase.” Explain why this statement is not always correct.
More light will only increase the rate if light is the limiting factor. If CO2 concentration or temperature is already the limiting factor, adding more light will have no effect — the rate is being held back by something else.
Photosynthesis produces glucose, which the plant converts into other useful substances.
(a) Name the two raw materials needed for photosynthesis.
(b) State where each raw material enters the plant.
(c) Name the two products of photosynthesis.
(d) For each carbohydrate below, state one use in the plant: Starch, Cellulose, Sucrose.
Carbon dioxide enters through the stomata — small pores on the surface of leaves.
Water is absorbed through the roots.
Starch — stored as an energy store in cells (it is insoluble, so it does not affect osmosis).
Cellulose — used to build plant cell walls, giving the plant structure and strength.
Sucrose — transported through the plant via the phloem to carry energy to other parts.
A student investigates the effect of temperature on the rate of photosynthesis using Elodea. She counts oxygen bubbles per minute at different temperatures, keeping light intensity and CO2 concentration constant.
| Temperature (°C) | Bubbles per minute |
|---|---|
| 10 | 5 |
| 20 | 12 |
| 30 | 19 |
| 40 | 8 |
| 50 | 1 |
(a) Describe the trend between 10°C and 30°C.
(b) Explain what is happening at 40°C and 50°C.
(c) State what the student is measuring and explain why counting oxygen bubbles is a suitable method.
As temperature increases from 10°C to 30°C, the rate of photosynthesis increases — the number of bubbles rises from 5 to 19 per minute. Higher temperatures give enzyme molecules more kinetic energy, increasing the frequency of successful collisions and speeding up the reaction.
Above the optimum temperature (~30°C), the rate drops sharply. The high temperature causes the enzymes controlling photosynthesis to denature — they lose their three-dimensional shape and can no longer function. At 50°C, nearly all enzymes have denatured and the rate is almost zero.
The student is measuring the rate of oxygen production (bubbles per minute). This is a suitable method because oxygen is a direct product of photosynthesis — more bubbles per minute means photosynthesis is occurring faster.
IMAGE NEEDED: Graph showing rate of photosynthesis (y-axis) vs temperature (x-axis) — curve rises to a peak around 30°C then falls sharply
Google Images Search: “IGCSE biology rate of photosynthesis temperature graph labeled educational”
A student designs an experiment to show that carbon dioxide is needed for photosynthesis.
(a) Describe how the experiment should be set up, including what acts as the control.
(b) State how you would test for the product of photosynthesis and describe the expected results.
(c) Name one variable the student must keep constant and explain why.
- Take two plants of the same type and size. Destarch both by keeping them in the dark for 24–48 hours.
- Experimental plant: Place in a sealed transparent container with soda lime (which absorbs all CO2 from the air).
- Control plant: Place in an identical sealed transparent container without soda lime. This plant has normal CO2 levels.
- Leave both containers in the same bright light at the same temperature for 24 hours.
Remove a leaf from each plant and test both with iodine solution.
Experimental plant (soda lime, no CO2): Leaf stays orange-brown — no starch, because photosynthesis could not occur without CO2.
Control plant (normal CO2): Leaf turns blue-black — starch is present, because photosynthesis occurred.
Any one of the following (with explanation):
Light intensity — must be the same for both plants. Different light levels would change the rate of photosynthesis, making it impossible to conclude that the result was caused by CO2 alone.
Temperature — must be the same. Temperature affects enzyme activity; if different, results could be due to temperature, not CO2.
A plant is given increasing light intensity while CO2 concentration and temperature are kept constant.
(a) Define the term limiting factor.
(b) Describe and explain what happens to the rate of photosynthesis as light intensity increases from very low to very high.
(c) The rate eventually stops increasing even though light intensity is still rising. Name one factor that may now be limiting the rate, and explain how increasing it would help.
At very low light intensity: The rate of photosynthesis is low. Light is the limiting factor — there is not enough energy for chlorophyll to absorb, so the reaction proceeds slowly.
As light intensity increases: More energy becomes available. The rate of photosynthesis rises — the graph increases steeply.
At high light intensity: The rate levels off and stops increasing. Light is no longer the limiting factor. Another factor — CO2 or temperature — is now in shortest supply and is holding back the rate.
Carbon dioxide concentration: If CO2 is now limiting, increasing the CO2 supply gives the plant more raw material. The rate of photosynthesis can then increase again.
OR Temperature: If temperature is limiting, increasing it (up to the optimum) speeds up enzyme activity, allowing the rate to rise further.
A student wants to show that light is needed for photosynthesis using the starch test.
(a) Why must the plant be destarched before starting the experiment?
(b) Describe the experimental setup, including what acts as the control.
(c) State the expected result for the experimental leaf (no light) and the control leaf (in light).
(d) A second student forgets to destarch her plant before starting. Explain why this makes her results unreliable.
If the plant is not destarched, it already contains starch from before the experiment. Any starch found after the test could have been there already, not newly made during the experiment.
- Keep the plant in the dark for 24–48 hours to destarch it.
- Cover part of one leaf tightly with black foil to block all light. This is the experimental part (no light).
- Leave the rest of the same leaf (or another leaf on the same plant) uncovered. This is the control — it receives normal light.
- Leave the plant in bright light for several hours, then remove the leaf and test both parts with iodine solution.
Experimental leaf (covered, no light): Orange-brown — no starch. Photosynthesis could not occur without light.
Control leaf (uncovered, in light): Blue-black — starch present. Photosynthesis occurred and starch was stored.
If the plant was not destarched, both the covered and uncovered parts already contain starch. After the experiment, both parts would test blue-black — even the covered part that received no light.
