Question 1
Biology only (what this means)
Biology only: only in International GCSE Biology. Double Award students can skip it. What the labels meanWhich cells control the opening and closing of the stomata?
- Palisade cells
- Guard cells
- Xylem cells
- Root hair cells
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Answer: B (1)
Question 2
Biology only (what this means)
Biology only: only in International GCSE Biology. Double Award students can skip it. What the labels meanCarbon dioxide moves from the air into the cells of a leaf during the day. Explain how this happens.
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- by diffusion (1)
- the concentration of carbon dioxide is higher in the air than in the leaf cells (which use it in photosynthesis) / down a concentration gradient (1)
Question 3
Carbon dioxide and oxygen are waste products of metabolism in a leaf. (a) Name the process in a leaf that produces carbon dioxide as a waste product. (b) Name the process that produces oxygen as a waste product. (c) State how these gases leave the leaf.
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- (a) respiration (1)
- (b) photosynthesis (1)
- (c) (by diffusion) through the stomata (1) allow through stomata without diffusion
Question 4
Biology only (what this means)
Biology only: only in International GCSE Biology. Double Award students can skip it. What the labels meanThe table shows the net exchange of carbon dioxide by a leaf at different light intensities. A negative value means carbon dioxide is released and a positive value means carbon dioxide is taken in. (a) Explain the value at light intensity 0. (b) Explain why the value is 0 at light intensity 4. (c) Explain the value at light intensity 8.
| Light intensity | Net carbon dioxide exchange |
|---|---|
| 0 | −4 |
| 2 | −2 |
| 4 | 0 |
| 6 | +3 |
| 8 | +5 |
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- (a) no light so no photosynthesis (1)
- only respiration happens so carbon dioxide is released (1)
- (b) rate of photosynthesis equals rate of respiration / carbon dioxide made by respiration is used in photosynthesis (1)
- (c) rate of photosynthesis is faster than respiration so there is a net uptake of carbon dioxide (1)
Question 5
Biology only (what this means)
Biology only: only in International GCSE Biology. Double Award students can skip it. What the labels meanA student puts the same size of leaf into three boiling tubes containing hydrogen-carbonate indicator. Tube A is in bright light, tube B is wrapped in foil and tube C has no leaf and is in bright light. Hydrogen-carbonate indicator is red/orange in normal air, yellow with more carbon dioxide and purple with less. (a) State the colour of the indicator in tube A and tube B after two hours. (b) Explain the result in tube B. (c) State the purpose of tube C.
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- (a) A purple (1)
- B yellow (1)
- (b) no light so no photosynthesis, but the leaf still respires and releases carbon dioxide (1)
- (c) control / to show the colour change is caused by the leaf (1)
Question 6
Biology only (what this means)
Biology only: only in International GCSE Biology. Double Award students can skip it. What the labels meanDescribe and explain the exchange of carbon dioxide and oxygen between a leaf and the air in bright light, at the compensation point and in darkness.
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| Level | Marks | What the answer does |
|---|---|---|
| 3 | 5–6 | Describes and explains the net exchange of both gases in all three conditions, linking each to the relative rates of respiration and photosynthesis. |
| 2 | 3–4 | Describes the net exchange in at least two conditions with some explanation, or explains one condition fully for both gases. |
| 1 | 1–2 | Gives simple statements about one or two gases or conditions, with little explanation. |
Indicative content
- respiration happens all the time and uses oxygen and releases carbon dioxide
- photosynthesis happens only in light and uses carbon dioxide and releases oxygen
- bright light: photosynthesis faster than respiration, so net uptake of carbon dioxide and net release of oxygen
- compensation point: photosynthesis equals respiration, so no net exchange of either gas
- darkness: only respiration, so net release of carbon dioxide and net uptake of oxygen
- gases diffuse through the stomata