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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Elevated CO₂ increases the rate of carbon fixation in C3 plants because RuBisCO is not CO₂‑saturated at current atmospheric concentrations; higher CO₂ reduces photorespiration and favours the carboxylation reaction.

However, projected climate change brings higher temperatures, which raise the rate of photorespiration and can denature key enzymes, offsetting the CO₂ fertilisation effect.

Altered precipitation patterns lead to drought or water‑stress; stomata close to conserve water, limiting CO₂ uptake and reducing photosynthetic capacity.

In addition, more frequent extreme weather, shifting pest/pathogen ranges and reduced nitrogen availability (the dilution effect) can further lower crop yields even though photosynthesis is initially stimulated.

Examiner tips

  • Use the term ‘CO₂ fertilisation’ and explain RuBisCO’s role; link higher CO₂ to reduced photorespiration. Mention temperature‑related photorespiration and enzyme denaturation. Include water‑stress via stomatal closure. Add a point on extreme events or nutrient dilution to show breadth.
  • common_mistakes
  • :
  • Assuming all CO₂ fertilisation leads to yield gains; ignoring temperature or water stress. Using vague terms like ‘more CO₂’ without explaining RuBisCO. Omitting the effect of extreme weather or nutrient dilution.

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 in this question

C3 plants

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