Explain how dead organic matter in a terrestrial ecosystem is decomposed and its nutrients returned to the soil.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Decomposers such as bacteria and fungi secrete extracellular enzymes onto dead organic matter.
These enzymes hydrolyse complex organic molecules – proteins, lipids and carbohydrates such as cellulose – into simpler soluble molecules.
The decomposers absorb these soluble products and use them in cellular respiration or assimilation, releasing inorganic ions and mineral nutrients (for example nitrates and phosphates) into the soil.
These inorganic ions are then taken up by plant roots, allowing producers to synthesise new organic molecules and the nutrient cycle is completed.
These enzymes hydrolyse complex organic molecules – proteins, lipids and carbohydrates such as cellulose – into simpler soluble molecules.
The decomposers absorb these soluble products and use them in cellular respiration or assimilation, releasing inorganic ions and mineral nutrients (for example nitrates and phosphates) into the soil.
These inorganic ions are then taken up by plant roots, allowing producers to synthesise new organic molecules and the nutrient cycle is completed.
Examiner tips
- Use the word ‘decomposers’ and specify bacteria/fungi. Mention extracellular enzymes and the types of molecules they break down. Explain that the products are absorbed and lead to release of inorganic ions. Show the final step of plant uptake to complete the cycle.
Common mistakes
- Failing to name decomposers or the enzymes. Omitting the step where decomposers release inorganic ions. Not linking the released ions back to plant uptake.
Mark scheme (4 marks)
- Decomposers (bacteria and/or fungi) secrete extracellular enzymes onto dead organic matter
- Enzymes break down complex organic molecules (e.g. proteins, lipids, carbohydrates/cellulose) into simpler soluble molecules
- 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
- Released inorganic ions / nutrients are absorbed by plant roots, making them available for producers to synthesise new organic molecules, completing the nutrient cycle
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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