3.3 - Calvin Cycle (LIR)

Location: Takes place in the stroma of the chloroplast. Phases of the Calvin Cycle: Carbon Fixation: Reduction Phase: Regeneration of RuBP: 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.