Calvin cycle carboxylation reaction:
$$CO_2 + RuBP \xrightarrow{\text{RuBisCO}} 2 \times \text{3-PGA}$$A: PEP carboxylase = C4 mesophyll fixation, not Calvin cycle
B: Phosphoglycerate kinase = reduction stage (PGA → 1,3-bisphosphoglycerate)
D: Phosphoribulokinase = regeneration stage (Ru5P → RuBP)
Answer: C — RuBisCO
Three stages: Carboxylation: CO2 + RuBP (C5) → 2 x 3-PGA (C3), by RuBisCO. Reduction: 3-PGA + ATP → 1,3-BPGA (by PGK = phosphoglycerate kinase). 1,3-BPGA + NADPH → G3P (by G3P dehydrogenase). Regeneration: G3P → DHAP → F6P, various intermediates → Ru5P → RuBP (by phosphoribulokinase + ATP). Per 3 CO2 fixed: uses 9 ATP + 6 NADPH. Produces 1 net G3P. 2 G3P → glucose (via gluconeogenesis-like steps).
RuBisCO (RUBISCO): most abundant enzyme/protein on Earth (~0.7 billion metric tonnes). Large subunit (LSU, 55 kDa): encoded by chloroplast genome. Small subunit (SSU, 15 kDa): encoded by nuclear genome. L8S8 hexadecamer (8 large + 8 small subunits) in Form I (most plants, algae, cyanobacteria). Molecular weight ~540 kDa. Carboxylase: CO2 + RuBP → 2 PGA. Oxygenase: O2 + RuBP → PGA + phosphoglycolate (photorespiration). Selectivity factor (Sc/o): ratio of carboxylase to oxygenase activity per unit substrate. Higher Sc/o = more selective for CO2. Plants ~80-100. Some bacteria ~10. RuBisCO requires CO2 for activation (carbamylation of Lys201 + Mg2+ binding). RuBisCO activase: removes inhibitory RuBP/sugar phosphates from RuBisCO active site.
Chlorophylls: Chl a: absorbs 430 nm (blue) and 662 nm (red). All photosynthetic organisms. Chl b: absorbs 453 nm and 642 nm. Higher plants and green algae. Antenna pigment, transfers energy to Chl a. Ratio Chl a:b ≈ 3:1. Carotenoids: carotenes (beta-carotene, orange) and xanthophylls (yellow). Absorb 400-500 nm (blue-green). Transfer energy to Chl a. Photoprotection: quench excess energy (non-photochemical quenching). Phycobilins: in cyanobacteria and red algae. Phycocyanin (blue), phycoerythrin (red). Extend absorption range. Action spectrum: wavelengths of light effective in photosynthesis (matches absorption spectrum of photosynthetic pigments). Absorption spectrum: wavelengths absorbed by pigments. Engelmann experiment (1883): used prism to illuminate Spirogyra filament, oxygen-seeking bacteria clustered at violet and red wavelengths.
Photosystem II: chlorophyll a reaction centre P680, absorbs 680 nm. Mn cluster (oxygen-evolving complex) oxidises water. PSII → PQ (plastoquinone) → cytochrome b6f complex → plastocyanin (PC) → PSI. Cytochrome b6f: proton pumping (like Complex III in mitochondria). Q cycle. Plastocyanin: copper-containing electron carrier. Photosystem I: P700, absorbs 700 nm. Electrons from PC. Ferredoxin (Fd) accepts electrons. FNR (Fd-NADP+ reductase) reduces NADP+. Cyclic electron flow: electrons from Fd back to PQ via cytochrome b6f. Produces only ATP (no NADPH, no O2). Used when ATP/NADPH ratio needs adjusting. Z-scheme: overall path from water to NADPH. Named for zigzag shape of energy diagram.
Proton gradient across thylakoid membrane: PS II water splitting releases H+ into lumen. PQ carries H+ from stroma to lumen. Net: H+ accumulates in thylakoid lumen. Lumen pH ~5, stroma pH ~8 (3 pH unit gradient). Membrane potential: inside positive, outside negative. Proton motive force (pmf) drives ATP synthase. CF0-CF1 ATP synthase: CF0 (membrane channel), CF1 (catalytic knob in stroma). H+ flows down gradient through CF0 → drives rotation of gamma subunit → conformational changes in beta subunits → ADP + Pi → ATP. Mitchells chemiosmosis hypothesis (Nobel 1978). ~3 H+ per ATP made. ~12 H+ pumped per O2 evolved. Provides ~4 ATP per 2 H2O oxidised plus light energy.
Light activation: many Calvin cycle enzymes activated by light (via thioredoxin/ferredoxin system): FBPase (fructose-1,6-bisphosphatase), SBPase, PRK (phosphoribulokinase), GAPDH. Inactive in dark (oxidised S-S bonds). Active in light (reduced -SH groups). Prevents futile cycling between photosynthesis and respiration. pH regulation: stroma pH increases in light (H+ pumped into lumen), activates several Calvin cycle enzymes. Mg2+ activation: Mg2+ released from lumen into stroma in light, activates RuBisCO and other enzymes. Feedback regulation: rising G3P/RuBP ratio signals photosynthesis rate. Stomatal regulation: CO2 supply regulated by guard cell opening/closing.
Photorespiration pathway: Chloroplast: RuBisCO oxygenase → phosphoglycolate. Phosphoglycolate phosphatase → glycolate. Glycolate exported to peroxisome. Peroxisome: glycolate oxidase → glyoxylate + H2O2. H2O2 destroyed by catalase. Aminotransferase: glyoxylate + glutamate → glycine + alpha-ketoglutarate. Glycine to mitochondria. Mitochondria: 2 glycine → serine + CO2 + NH3 (glycine decarboxylase complex). Serine back to peroxisome → 3-PGA back to chloroplast. Net cost: 1 RuBP lost per 4 turns of the cycle. CO2 and NH3 released (wasteful). But significance: may be unavoidable consequence of RuBisCO evolution in ancient high-CO2, low-O2 atmosphere. Photorespiration generates serine (amino acid). Provides carbon skeletons for nitrogen assimilation. Protects against excess light energy under conditions of CO2 limitation.
Plants must reduce N (NO3- or NH4+) to assimilate into organic compounds. NO3- reduction: Nitrate reductase (NR): NO3- → NO2-. NADH/NADPH dependent. Cytoplasm. Nitrite reductase (NiR): NO2- → NH4+. Ferredoxin-dependent. Chloroplast. Ammonium assimilation: GS-GOGAT pathway: GS (Glutamine synthetase): NH4+ + glutamate → glutamine. Requires ATP. GOGAT (Glutamate:2-oxoglutarate aminotransferase): glutamine + alpha-ketoglutarate → 2 glutamate. Requires NADH/NADPH (and ferredoxin in plastids). Net: 1 NH4+ + 1 alpha-ketoglutarate → 1 glutamate. Glutamate: central amino group donor for all other amino acids (transamination). Symbiotic N-fixation: Rhizobium in root nodules of legumes. Nitrogenase complex: N2 + 16 ATP + 8e- + 8H+ → 2 NH3 + H2 + 16 ADP + 16 Pi. Very energy-intensive. Requires anoxic conditions (O2 inactivates nitrogenase).