Photosynthesis in Higher Plants

The light reactions of photosynthesis happen on the thylakoid membranes of chloroplasts. Two photosystems (PS II and PS I) absorb light energy and use it to:

1. Split water (photolysis) → O₂ released
2. Pump H+ into the thylakoid lumen → ATP synthesis
3. Produce NADPH (reducing power for the Calvin cycle)

The flow (Z-scheme):
H₂O → PS II → ETC (cytochrome b6f) → PS I → NADP+ → NADPH

Key facts:
• PS II uses chlorophyll-a P680 (absorbs maximally at 680 nm).
• PS I uses chlorophyll-a P700 (absorbs at 700 nm).
• Oxygen is released by PS II — not PS I.
• ATP is made by chemiosmosis: H+ flows down its gradient through ATP synthase.

Two types of phosphorylation:
• Non-cyclic (PS II → PS I): produces both ATP and NADPH; releases O₂.
• Cyclic (PS I only): produces only ATP; no O₂; happens when the cell needs more ATP than NADPH.

Net products of light reactions per 2 H₂O split: 2 NADPH, ~3 ATP, 1 O₂.

The Calvin cycle (also called the Calvin-Benson cycle, dark reactions, or C3 pathway) takes place in the stroma of the chloroplast. Inputs: CO₂, ATP, NADPH (latter two from light reactions). Output: glucose precursors.

Three phases:

Phase 1: Carbon fixation.

• Enzyme: RuBisCO (Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase) — the most abundant enzyme on Earth.
• 6 CO₂ + 6 RuBP (5C) → 12 unstable 6C intermediates → 12 PGA (3-phosphoglycerate, 3C).

Phase 2: Reduction.

• 12 PGA + 12 ATP → 12 BPG (1,3-bisphosphoglycerate)
• 12 BPG + 12 NADPH → 12 G3P (glyceraldehyde-3-phosphate, 3C)
• 2 G3P leave the cycle to form 1 glucose (or stored as starch).

Phase 3: Regeneration.

• 10 G3P + 6 ATP → 6 RuBP (regenerates the starting material).

Net per glucose:

• 6 CO₂ consumed
• 18 ATP consumed (12 in reduction + 6 in regeneration)
• 12 NADPH consumed
• 1 glucose produced

Why C4 plants exist: RuBisCO has a side reaction with O₂ (photorespiration) that wastes energy. In hot, dry climates, stomata close to conserve water → CO₂ drops, O₂ rises → photorespiration accelerates.

C4 pathway (Hatch-Slack, e.g., maize, sugarcane):

• CO₂ first fixed in mesophyll cells by PEP carboxylase (no oxygenase activity).
• Forms 4-carbon oxaloacetate → malate, transported to bundle sheath cells.
• In bundle sheath, malate decarboxylated → high CO₂, low O₂. RuBisCO operates efficiently.
• Kranz anatomy: bundle sheath cells with chloroplasts surrounded by mesophyll.

CAM plants (cacti, pineapple): same biochemistry as C4 but separated by time. Stomata open at night to fix CO₂; closed during day to perform Calvin.

Key facts to remember:

• CO₂ acceptor in C3 = RuBP (5C)
• CO₂ acceptor in C4 = PEP (3C)
• First stable product in C3 = PGA (3C)
• First stable product in C4 = OAA (4C)