# Explain how elevated atmospheric CO₂ concentrations affect the rate of photosynthesis in C3 plants and discuss why this does not necessarily result in increased crop yields under projected climate change conditions.

> IB DP Biology Higher Level (2023 syllabus) — D4.3 Climate change · Explain / Discuss · 4 marks

## Mark scheme (4 marks)

1. Elevated CO₂ increases the rate of carbon fixation in C3 plants because RuBisCO is not CO₂-saturated under current atmospheric concentrations, so higher CO₂ reduces photorespiration and increases the carboxylation reaction.
2. Higher temperatures associated with climate change increase rates of photorespiration and enzyme denaturation, potentially offsetting any CO₂ fertilisation benefit.
3. Changes in precipitation patterns (drought / water stress) under climate change reduce stomatal opening, limiting CO₂ entry and water availability for photosynthesis, thereby reducing yield.
4. Increased frequency of extreme weather events, shifts in pest/pathogen ranges, or reduced nitrogen availability (dilution effect) under elevated CO₂ can further decrease crop yields despite enhanced photosynthetic rate.

## Key terms

- [C3 plants](https://www.gradenine.co.uk/glossary/c3-plants)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain / Discuss" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-elevated-atmospheric-co-concentrations-6d618d0b) · Published by Druglandscape Ltd.