Photosynthesis

AP Biology· difficulty 4/5

C4 plants outperform C3 plants in hot, dry conditions primarily because they

  • A

    Concentrate CO₂ at Rubisco via PEP carboxylase, suppressing photorespiration

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  • B

    Use Rubisco in mesophyll cells instead of bundle-sheath cells

  • C

    Skip the Calvin cycle entirely, using only the light reactions

  • D

    Open stomata only at night to absorb CO₂

Explanation

PEP carboxylase fixes CO₂ in mesophyll, then 4-carbon shuttle delivers it to bundle-sheath at high concentration. High CO₂ near Rubisco favors carboxylation over oxygenation, minimizing photorespiration. Choice (D) is CAM, not C4.

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