Photosynthesis

Adaptations of Chloroplasts

• Grana has a large surface area for the attachment of chlorophyll, electrons and enzymes

• Granal membrane has ATP synthase channels and are selectively permeable so a proton gradient can be formed and ATP can be synthesised

• Chloroplasts contain DNA and ribosomes allowing them to synthesise proteins needed in the light dependent reaction

Light Dependent Reaction

1. Photoionisation: Light hits chlorophyll molecules in PSII, exciting electrons

2. Photolysis of water: H₂O → 2H⁺ + 2e^- + 0.5O₂

3. Electron transport chain (E.T.C.): Excited electrons move from chlorophyll in PSII to PSI

4. Active transport of H⁺ ions: H⁺ ions are actively transported from the stroma to the thylakoid membrane

5. Chemiosmosis: H⁺ ions diffuse back into the stroma via ATP synthase channels, forming ATP from ADP and P_i

6. NADP is reduced: Acting as the final electron acceptor of the E.T.C., NADP is reduced to NADPH (NADP + H⁺ + e^- → NADPH)

Light Independent Reaction (Calvin Cycle)

1. CO₂ is added to a 5C ribulose biphosphate (RuBP) using rubisco enzyme

2. RuBP splits into 2 3C glycerate-3-phosphate molecules (GP)

3. Each GP molecule is reduced to a 3C triose phosphate (TP) using energy from ATP hydrolysis and oxidation of NADPH

4. The 2 TP molecules re-form RuBP. One C is kept for forming organic molecules

5. Every 6 turns, 1 glucose molecule is formed.

Order of compounds in Calvin cycle: RuBP → GP → TP → RuBP