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S5 Biology: Photosynthesis (Paper 3)
S5 Biology: Photosynthesis (Paper 3)
The third paper links photosynthesis to energy and farm practice, judging net gain over a full day and the meaning of the compensation point.
Direct answer Net photosynthesis is gross photosynthesis minus respiration over all 24 hours; extra carbon dioxide raises yield because it was the limiting factor.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 |
| Suggested time | 25 |
| Difficulty mix | Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1 |
| Chapter | Chapter: S5 Biology: Photosynthesis |
| Level | Four basic, three advanced, two challenge and one error-repair item |
02 Questions (10 items)
- FoundationInto which form of energy does photosynthesis convert light energy stored in glucose?
- FoundationWhere is chlorophyll located inside the chloroplast?
- FoundationCan a plant photosynthesise at night in complete darkness?
- FoundationWhy is the rate of photosynthesis usually lowest under green light?
- IntermediateWhat is meant by the compensation point of photosynthesis?
- IntermediateA grower raises greenhouse carbon dioxide to 0.1 per cent and yields rise. Why?
- IntermediateA leaf photosynthesises 18 units an hour over 12 lit hours and respires 3 units an hour all day. What is the net gain over 24 hours?
- ChallengeGlucose holds 15.6 kJ per gram. How much energy is stored in 12 g made by a plant in a day?
- ChallengeTwo leaf sets differ only in carbon dioxide: 20 units at 0.04 per cent and 50 units at 0.1 per cent. Both rise 2.5 times. What does this show?
- Error redoA student says the Calvin cycle needs no light and so can run indefinitely in the dark. What is wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | Chemical energy | Light energy ends up locked in the chemical bonds of glucose for plants and animals to use. |
| 2 | On the thylakoid membranes | Chlorophyll molecules sit in the thylakoid membranes next to the photosystems that capture photons. |
| 3 | No, because the light reaction cannot run without light | The light reaction needs photons, so at night only respiration continues. |
| 4 | Chlorophyll absorbs green light least, so most is reflected or transmitted | The absorption spectrum is lowest in the green region, so little energy is captured. |
| 5 | The light intensity at which photosynthesis equals respiration | Net photosynthesis is zero there, with no net exchange of oxygen with the air. |
| 6 | Carbon dioxide was limiting, so the Calvin cycle speeds up | Air holds little carbon dioxide, so adding it speeds the Calvin cycle and more organic matter builds up. |
| 7 | 144 units | Eighteen times twelve is 216, minus three times twenty-four, which is 72, leaving 144. |
| 8 | 187.2 kJ | Twelve grams times 15.6 kilojoules per gram gives 187.2 kilojoules of stored energy. |
| 9 | Rate is proportional to carbon dioxide here, so it is the limiting factor | Rate doubles with concentration, showing other factors are ample and carbon dioxide sets the pace. |
| 10 | It needs ATP and NADPH from the light reaction, which run out | The Calvin cycle needs ATP and NADPH from the light reaction, which run out in darkness. |
04 What to do after marking
- After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
- Repeat the same item two days later; move to the next paper only when both attempts are correct.
- Parents only need the answer and note columns; there is no need to explain every item.
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