Boundless Biology by Lukas Waller

3.3 - Calvin Cycle (LIR)

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Location: Takes place in the stroma of the chloroplast.

  • Phases of the Calvin Cycle:

    1. Carbon Fixation:
    2. Reduction Phase:
    3. Regeneration of RuBP:

    4. Key Enzyme:

      • RuBisCO is the primary enzyme, acting as both a carboxylase and an oxygenase.
      • Can bind to CO₂ (carboxylation) or O₂ (oxygenation), the latter leading to photorespiration, which reduces efficiency.

      • Energy Requirement:

        • For each full turn of the cycle (producing 1 G3P):
        • To produce one glucose molecule, the cycle must run twice, requiring:

        • Products:

          • G3P is the direct product, which can be used to form glucose and other carbohydrates.

          • Photorespiration Issue:

            • Occurs when RuBisCO fixes O₂ instead of CO₂, leading to wasteful energy loss.
            • More likely in hot, dry climates when stomata close to conserve water.

            • Adaptations:

              • C4 and CAM plants have evolved mechanisms to minimize photorespiration and improve efficiency, which will be explored further.
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                Boundless Biology by Lukas WallerBy Lukas Waller