# Explain how the adaptations of a C4 plant allow it to maintain high rates of photosynthesis in hot, high-light environments where a C3 plant would be limited.

> IB DP Biology Higher Level (2023 syllabus) — B4.1 Adaptation to environment · Explain · 4 marks

## Mark scheme (4 marks)

1. In C4 plants, stomata can remain partially closed during the hottest part of the day, reducing water loss / limiting photorespiration by maintaining high CO2 concentrations in the bundle sheath cells.
2. CO2 is initially fixed in mesophyll cells by PEP carboxylase, which has a much higher affinity for CO2 than RuBisCO and does not react with O2.
3. The four-carbon compound (oxaloacetate / malate / aspartate) is transported from mesophyll cells to bundle sheath cells, where CO2 is released, concentrating CO2 around RuBisCO.
4. The high CO2 concentration around RuBisCO in bundle sheath cells suppresses photorespiration, so Calvin cycle / carbon fixation continues efficiently even at high temperatures when O2:CO2 ratios would otherwise favour photorespiration in C3 plants.

## Key terms

- [C4 plant](https://www.gradenine.co.uk/glossary/c4-plant)

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-adaptations-of-a-e447ac97) · Published by Druglandscape Ltd.