# A coral reef ecosystem contains the following food chain: phytoplankton → zooplankton → small reef fish → reef sharks. Scientists estimate that phytoplankton in this ecosystem fix 200,000 kJ of energy per year. Explain why reef sharks receive so little of the energy originally fixed by phytoplankton, and describe what happens to the energy that is not passed on at each trophic level.

> Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 3.2 Energy flow and ecosystems · Explain · 5 marks

> In a coral reef ecosystem, energy is transferred through a food chain: phytoplankton → zooplankton → small reef fish → reef sharks. Phytoplankton fix 200,000 kJ of energy per year through photosynthesis.

## Mark scheme (5 marks)

1. Only approximately 10% of energy is transferred from one trophic level to the next (energy transfer is inefficient)
2. Much of the energy is lost through respiration (used for movement, growth, maintaining body temperature etc.)
3. Energy is lost in faeces / urine / waste materials that are not consumed by the next organism
4. Because energy is lost at each of the three transfers (phytoplankton → zooplankton → small reef fish → reef sharks), the cumulative loss means reef sharks receive a very small fraction of the original energy
5. The energy not transferred enters the environment as heat / is used by decomposers breaking down dead organisms

## Key terms

- [trophic level](https://www.gradenine.co.uk/glossary/trophic-level)
- [food chain](https://www.gradenine.co.uk/glossary/food-chain)

## Related

- [Revision notes for Edexcel A-Level Biology A: Salters-Nuffield (9BN0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-coral-reef-ecosystem-contains-the-0a707adc) · Published by Druglandscape Ltd.