# Explain how dead organic matter in a terrestrial ecosystem is decomposed and its nutrients returned to the soil.

> IB DP Biology Standard Level (2023 syllabus) — C4.2 Transfers of energy and matter · Explain · 4 marks

## Mark scheme (4 marks)

1. Decomposers (bacteria and/or fungi) secrete extracellular enzymes onto dead organic matter
2. Enzymes break down complex organic molecules (e.g. proteins, lipids, carbohydrates/cellulose) into simpler soluble molecules
3. Decomposers absorb the soluble products and use them in cellular respiration and/or assimilation, releasing inorganic ions / mineral nutrients (e.g. nitrates, phosphates) into the soil
4. Released inorganic ions / nutrients are absorbed by plant roots, making them available for producers to synthesise new organic molecules, completing the nutrient cycle

## Related

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
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