Photosynthesis and Respiration
Energy capture
The exam tests light reaction, photophosphorylation, Calvin cycle, C4 and CAM pathways, photorespiration, and the steps and ATP yield of respiration.
Core rules
- Overall photosynthesis: 6CO2 + 12H2O → C6H12O6 + 6O2 + 6H2O; oxygen comes from water.
- Light reaction occurs in thylakoids: PSII (P680) and PSI (P700) form the Z-scheme; non-cyclic photophosphorylation yields ATP, NADPH and O2.
- Calvin cycle: RuBP + CO2 is fixed by RuBisCO to 3-phosphoglycerate; reduction and regeneration consume ATP and NADPH.
- C4 plants use PEP carboxylase and Kranz anatomy; the first stable product is oxaloacetate. CAM plants fix CO2 at night and store malate.
- Photorespiration occurs when RuBisCO acts as an oxygenase; it wastes carbon and energy.
- Respiration: glycolysis in cytoplasm gives net 2 ATP and 2 NADH; Krebs cycle in the matrix gives 2 ATP, 6 NADH and 2 FADH2; the electron transport chain gives most ATP.
Worked example
Trace one glucose: glycolysis gives 2 ATP and 2 NADH; pyruvate oxidation gives 2 NADH; Krebs gives 2 ATP, 6 NADH and 2 FADH2. Oxidative phosphorylation uses 10 NADH and 2 FADH2. The commonly quoted total is 36–38 ATP, depending on the shuttle used.
Traps
O2 is not released from CO2. C4 does not mean the first product is a 3-carbon acid. CAM is not the same as C4 anatomy. Respiration occurs in light and dark. Photorespiration is not the same as dark respiration.