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 · View as Markdown

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

Model answer (4 marks)

In C4 plants the stomata can stay partially closed during the hottest part of the day, which reduces water loss and keeps a high CO₂ concentration in the bundle‑sheath cells.

CO₂ is first fixed in the mesophyll cells by PEP carboxylase, an enzyme with a much higher affinity for CO₂ than RuBisCO and which does not react with O₂.

The resulting four‑carbon compound (oxaloacetate, malate or aspartate) is transported to the bundle‑sheath cells, where it is decarboxylated and releases CO₂, concentrating it around RuBisCO.

This high CO₂ concentration around RuBisCO suppresses photorespiration, allowing the Calvin cycle to continue efficiently even at high temperatures where a C3 plant would suffer from increased photorespiration.

Examiner tips

  • Mention stomatal behaviour first to show water‑saving adaptation. Explain the role of PEP carboxylase before the transport step. Show the sequence: mesophyll → bundle‑sheath → CO₂ release. Highlight suppression of photorespiration as the key benefit.
  • common_mistakes
  • :
  • Confusing the order of the C4 pathway (e.g. saying CO₂ is fixed in bundle‑sheath first). Forgetting to mention that PEP carboxylase does not react with O₂. Over‑generalising the four‑carbon compound as a single molecule rather than listing the possibilities.

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 in this question

C4 plant

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