# Explain how the chemical composition of plant litter affects the rate of decomposition in a terrestrial ecosystem.

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

> In a temperate forest, oak leaves with a high lignin and tannin content decompose significantly more slowly than alder leaves, which have lower lignin levels and higher nitrogen content.

## Mark scheme (4 marks)

1. Lignin is a complex, recalcitrant organic polymer that is resistant to enzymatic breakdown by decomposers, slowing decomposition.
2. A high carbon-to-nitrogen (C:N) ratio in litter limits decomposer growth because nitrogen is a limiting nutrient required for synthesis of decomposer enzymes and proteins.
3. Tannins and other polyphenols inhibit decomposer enzyme activity by binding to extracellular enzymes or to the proteins in litter, reducing hydrolysis.
4. Litter with low lignin and high nitrogen (e.g. alder leaves) provides a favourable substrate for decomposer growth, increasing microbial / fungal biomass and enzyme secretion, thereby accelerating decomposition and nutrient cycling.

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-chemical-composition-of-8aff5c35) · Published by Druglandscape Ltd.