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S4 Biology: Ecosystems and Energy Flow (Paper 3)
S4 Biology: Ecosystems and Energy Flow (Paper 3)
The third paper puts energy flow into food webs and human food production, covering efficiency, bioaccumulation and working backwards from a target energy.
Direct answer For reverse questions divide the target energy by the efficiency step by step; for bioaccumulation check fat solubility, as concentration magnifies up the chain.
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: S4 Biology: Ecosystems and Energy Flow |
| Level | Four basic, three advanced, two challenge and one error-repair item |
02 Questions (10 items)
- FoundationWhat happens first if a top predator is removed from a food web?
- FoundationBiomass falls from 200 g/m2 to 20 g/m2 at the next level. What is the transfer efficiency?
- FoundationThe concentration of the pesticide DDT rises at each level of a food chain. What is this called?
- FoundationHow do the movements of energy and matter in an ecosystem differ?
- IntermediateOf 2 000 000 kJ of sunlight, grass fixes 40 000 kJ, rabbits gain 4 000 kJ and foxes 400 kJ. What percentage of the sunlight do the foxes obtain?
- IntermediateWhy does producing meat need more land than producing plant food?
- IntermediateWhat is the main energy input to an ecosystem?
- ChallengeCattle take in 10 000 kJ, respire 6 000 kJ and lose 2 000 kJ as faeces, the rest going to growth. How much growth energy is needed for people to gain 200 kJ?
- ChallengeThree ecosystems transfer energy at 5, 12 and 20 per cent. Which is highest, and what follows?
- Error redoA student says energy returns to producers after top consumers die. What is wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | Its prey numbers rise in the short term and the levels below are then affected | Without predation the prey numbers rise first, then over-graze the level below and the web destabilises. |
| 2 | 10 per cent | 20 out of 200 is 10 per cent, in line with typical transfer values. |
| 3 | Bioaccumulation, or biological magnification | DDT dissolves in fat and is not easily excreted, so its concentration magnifies along the chain and peaks in top consumers. |
| 4 | Energy flows one way and is lost as heat, while matter cycles | Elements such as carbon and nitrogen return to the environment through decomposers, but energy cannot be recovered. |
| 5 | 0.02 per cent | 400 out of two million is 0.02 per cent, showing how sharply energy falls at each step. |
| 6 | Each transfer keeps only about one tenth, so animals need many times more plant production | Roughly ten units of plant energy are needed for one unit of animal energy, so far more land is required. |
| 7 | Sunlight captured by producers through photosynthesis | A few deep-sea systems use chemical energy, but almost all ecosystems run on sunlight. |
| 8 | 2 000 kJ | Working back at ten per cent, 200 divided by 0.10 gives 2 000 kJ of growth. |
| 9 | 20 per cent is highest, and since all fall far below 100 per cent the rest is lost as heat | Even the best system loses four fifths, so an energy pyramid must narrow upwards. |
| 10 | Energy is lost as heat and cannot be reused; what returns to producers is inorganic nutrients | Decomposers release matter such as nitrate, and producers take up matter, not the original energy. |
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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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.
