Ch 11 covers photosynthesis in detail — the light-dependent reactions (in thylakoids), the Calvin cycle (in stroma), C3 vs C4 pathways, photorespiration, and factors affecting the process.
In thylakoid membranes. Photosystem II (P680): absorbs light, splits water (photolysis: H₂O → 2H⁺ + ½O₂ + 2e⁻), passes electrons through ETC. Photosystem I (P700): absorbs light, reduces NADP⁺ to NADPH. Cyclic photophosphorylation: only PS I, produces ATP only. Non-cyclic: both PS, produces ATP + NADPH + O₂. ATP synthesis by chemiosmosis (CF₀-CF₁ complex).
Calvin cycle (C3 cycle, dark reactions, stroma): (1) Fixation: CO₂ + RuBP → 2 PGA (by RuBisCO), (2) Reduction: PGA → G3P using ATP + NADPH, (3) Regeneration: G3P → RuBP. Net: 3 CO₂ → 1 G3P. C4 pathway (Hatch-Slack): in tropical plants (sugarcane, maize). CO₂ fixed first by PEP carboxylase in mesophyll → OAA → malate → transported to bundle sheath → releases CO₂ for Calvin cycle. Avoids photorespiration.
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Photorespiration occurs when RuBisCO binds O₂ instead of CO₂ (especially at high temperature and low CO₂). It produces phosphoglycolate instead of PGA, which is then recovered through an energy-consuming pathway that releases CO₂ without making ATP or NADPH. It wastes fixed carbon and energy. C4 plants avoid this by concentrating CO₂ around RuBisCO in bundle sheath cells.
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