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S5 Biology: Photosynthesis (Paper 4)
S5 Biology: Photosynthesis (Paper 4)
The fourth paper covers photolysis, NADPH and leaf structure, with net fixation and leaf-area calculations.
Direct answer Photolysis gives oxygen, hydrogen ions and electrons; NADPH supplies reducing power; net fixation subtracts respiration; the limiting factor is the one that lifts the rate when raised; 264 g of carbon dioxide maps to 180 g of glucose.
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)
- FoundationWhat three things are produced when water is split in the light reaction?
- FoundationWhat does NADPH do in the light-independent reaction?
- FoundationWhy do leaves turn yellow before they fall in autumn?
- FoundationHow do photosynthesis and respiration exchange gases together?
- IntermediateA leaf fixes 30 g of carbon dioxide an hour for ten hours and respires away 60 g in the day. What is the net fixation?
- IntermediateDoubling the light intensity changes nothing, but raising carbon dioxide from 0.04% to 0.1% does. What was the limiting factor?
- IntermediateWhich tissue has more chloroplasts, and why does it sit just under the upper epidermis?
- ChallengeA leaf area of 0.02 m² fixes 3 g of carbon dioxide per square metre per hour for eight hours. How much is fixed, and how much glucose can that make?
- ChallengeRaising carbon dioxide in a greenhouse helps yield, but leaf temperature above 40 °C reduces it. Explain.
- Error redoA student says all of photosynthesis happens in the stroma. What is wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | Oxygen, hydrogen ions and electrons | Water is split into oxygen, hydrogen ions and electrons, with oxygen released as a by-product. |
| 2 | It supplies the reducing power that turns fixed carbon dioxide into sugar | Without NADPH the fixed carbon cannot be reduced into carbohydrate. |
| 3 | Chlorophyll breaks down and the yellow carotenoids underneath become visible | Carotenoids are always there; the denser green chlorophyll simply hides them. |
| 4 | Photosynthesis takes in carbon dioxide and gives out oxygen, respiration does the reverse, and by day the leaf releases oxygen on balance | Both run at once, and the net exchange depends on which rate is higher. |
| 5 | 240 g | Thirty times ten is 300 g, and subtracting the 60 g lost to respiration leaves 240 g. |
| 6 | Carbon dioxide concentration | The limiting factor is whichever shortage comes first, so raising it lifts the rate again. |
| 7 | Palisade tissue, placed there to receive sunlight first | Closely packed columns right under the upper epidermis catch the most light. |
| 8 | About 0.48 g of carbon dioxide, giving roughly 0.33 g of glucose | 0.02 times 3 times 8 gives 0.48 g, and 264 g of carbon dioxide yields 180 g of glucose, so about 0.33 g. |
| 9 | Too much heat denatures enzymes and closes stomata, cancelling the benefit of extra carbon dioxide | Above the optimum the enzymes lose activity, so one gain can cancel out. |
| 10 | The light reaction runs on the thylakoid membrane; only the light-independent reaction happens in the stroma | Putting both stages in the stroma ignores the light reaction on the thylakoid membrane. |
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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