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S3 Science: Ecosystems and Energy Flow (Paper 3: challenge)
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S3 Science: Ecosystems and Energy Flow (Paper 3: challenge)
The third paper works on net production and efficiency comparisons, and separates one way energy flow from matter cycling.
Direct answers Two questions put the respiration subtraction before the multiplication, so the wrong order spoils the whole answer.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 |
| Suggested time | 20 |
| Difficulty mix | Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1 |
| Chapter | Chapter: S3 Science: Ecosystems and Energy Flow |
| Level | S3 Science |
02 Questions (10 items)
- FoundationDoes energy flow one way or cycle in an ecosystem?
- FoundationDo substances such as carbon and nitrogen cycle in an ecosystem?
- FoundationHow is net primary production calculated?
- IntermediateGross primary production is 50 000 kJ and the producers respire 20 000 kJ. What is net primary production?
- IntermediateFrom the same case, how much do herbivores get at ten per cent of net production?
- IntermediateA five level chain starts from 50 000 kJ. How much is left at the top level?
- ChallengeIf the transfer efficiency rises from ten to fifteen per cent, how much more does 20 000 kJ give the primary consumer?
- ChallengeWhy are top predators usually few in number?
- ChallengeBurning fossil fuels and decomposers both release carbon dioxide. What is the main difference?
- Error redoA student says an energy pyramid can be used to calculate how much energy cycles. What is wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | It flows one way and cannot cycle | Once energy leaves as heat it cannot be used again, so the flow is one way only. |
| 2 | Yes, they cycle and are used again and again | Matter passes from organisms to the environment and back, unlike the one way flow of energy. |
| 3 | Gross primary production minus the respiration of the producers | The total fixed by photosynthesis has to lose the part the plants themselves respire. |
| 4 | 30 000 kJ | Subtracting 20 000 from 50 000 leaves 30 000 kJ. |
| 5 | 3 000 kJ | Ten per cent of the net production of 30 000 kJ is 3 000 kJ. |
| 6 | 5 kJ | The chain runs 50 000 to 5 000 to 500 to 50 to 5 kJ, so four steps leave only 5 kJ. |
| 7 | 1 000 kJ more | Fifteen per cent gives 3 000 kJ against 2 000 kJ at ten per cent, a gain of 1 000 kJ. |
| 8 | Little energy is left, so only a small number of individuals can be supported | Very little energy survives several transfers, so one area can support only a few large predators. |
| 9 | Fossil fuels release carbon that was stored for a very long time, and do it quickly | Decomposers release recently fixed carbon at a biological pace, while burning returns carbon locked in rock layers to the air in a short time. |
| 10 | Energy flows one way and does not cycle | Energy leaves as heat at every step and never returns to the producers, so a pyramid shows flow, not a cycle. |
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.
The challenge paper compares net production and efficiency: subtract the respiration loss first, then apply the transfer efficiency, and remember energy flows one way while matter cycles.Tutor Registration →
Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.