Photosynthesis

$6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

Two stages:

• Light-dependent reactions: thylakoid membranes
• Light-independent reactions (Calvin cycle): stroma

Occur on thylakoid membranes. Products: ATP, reduced NADP, O₂.

1. Light absorbed by chlorophyll → electrons excited
2. Photolysis of water: $2\text{H}_2\text{O} \rightarrow 4\text{H}^+ + 4e^- + \text{O}_2$
3. Electrons pass along electron transport chain (carrier proteins in thylakoid membrane)
4. Energy released → used to pump H⁺ into thylakoid space (chemiosmosis)
5. H⁺ flow through ATP synthase → ATP produced (photophosphorylation)
6. Electrons + H⁺ + NADP → reduced NADP (NADPH)

Non-cyclic vs Cyclic Photophosphorylation

• Non-cyclic: both photosystems (I and II); produces ATP + reduced NADP + O₂
• Cyclic: only photosystem I; electrons return to PSI; produces ATP only; no NADPH or O₂

Occur in stroma. Requires ATP and reduced NADP from light reactions.

1. Carbon fixation: CO₂ + RuBP (5C) → 2 × GP (3C) [catalysed by RuBisCO]
2. Reduction: GP → G3P (triose phosphate) using ATP and reduced NADP
3. Regeneration: Some G3P → RuBP (using ATP)
4. Some G3P → glucose, amino acids, fatty acids

For every 3 CO₂ fixed: 6 G3P made; 5 regenerate RuBP; 1 is net gain.

• Light intensity: affects rate of light-dependent reactions
• CO₂ concentration: affects carbon fixation rate
• Temperature: affects enzyme activity (especially RuBisCO)

• Light-dependent: thylakoids; photolysis, ETC, chemiosmosis → ATP + NADPH + O₂
• Calvin cycle: stroma; CO₂ + RuBP → GP → G3P → glucose; uses ATP + NADPH
• RuBisCO fixes CO₂; G3P is the first useful product
• Limiting factors: light, CO₂, temperature