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Photosynthesis 6CO2 + 12H20 C6H12O6 + 6O2 + 6 H2O Overview • Light-dependent reactions of photosynthesis • Light-independent reactions of photosynthesis • C3 versus C4 metabolism • Write an essay on the light-dependent (light) reactions of photosynthesis • Write an essay on the light-independent (Dark) reactions of photosynthesis • Compare and contrast C3 and C4 metabolism Photosynthesis CO2 ADP Light reactions ATP H2O O2 NADP+ Calvin/Benson Cycle (“Dark Reactions”) NADPH Sucrose Photosynthesis occurs only in the chloroplasts Stroma Vacuole Grana Chloroplast Palisade cell Thylakoids Thylakoid space PSII PSI The light reactions of photosynthesis Plants absorb Light Chlorophyll a Chlorophyll b Carotenoids Abs 400 Blue 500 Green 600 Red Wavelength of light (nm) 700 Absorption of light leads to excited electrons • When light energy is absorbed by chlorophyll, an electron is promoted to a higher (more energetic) orbital. This energy can be lost by: – from one orbital to the next with loss of heat – back to the ground state with loss of light (fluorescence) and heat – transfer to another chlorophyll. Chlorophylls are organised into photosystems to trap light energy Light Special chlorophyll molecule - primary electron acceptor Transfer of light energy from one chlorophyll to another Chlorophylls are organised into photosystems Cyclic photosphorylation generates ATP Stromal side Fd PQ PQ PSII Thylakoid space side P700 PSI Non-Cyclic photosphorylation generates ATP and NADPH Stromal side NADP+ NADPH Q Fd PQ PQ P680 PSII H20 2H+ & 2e- & 1/2 O2 P700 PSI Thylakoid space side H+ Generation of ATP Stroma • H+ gradient generated by: ATP – PQ shuttle (H+) – Splitting of water (2H+) – NADPH production (2H+ removed from stroma) • ATP generated by an ATP synthase as H+ ions flow through it H+ ADP +Pi H+ H+ H+ H+ H+ Light-independent reactions The Calvin/Benson cycle Carbon Phosphorus Ribulose biphosphate CO2 3-phosphoglycerate ATP ADP ADP 1,3 diphosphoglycerate ATP NADPH Sugars G-3-P NADP+ Glyeraldehyde-3-phosphate C4 metabolism C3 and C4 metabolism • Inefficient as RUBISCO can act as both a carboxylase and as an oxygenase. • Oxygenase activity leads to loss of carbon that has already been fixed. • Some plants have an alternative pathway where CO2 is first fixed into C4 organic acids (C4 pathway) and then liberated later to undergo the Calvin/Benson cycle (C3 metabolism). C4 metabolism • 2 types • C3 and C4 metabolism are separated in space (different cells) e.g. sugar cane Characteristic aggregation of cells around the vascular bundles - Krantz morphology • C3 and C4 metabolism are separated in time (same cell) e.g. pineapple. Comparison of C3 & C4 leaves • C3 - note the lack of chloroplasts in the bundle sheath • C4 - note the extensive chloroplasts in the bundle sheath CO2 OAA (4C) PEP (3C) ADP ATP Separation in space Mesophyll cell surrounding bundle sheath Pyruvate (C3) CO2 C3 sugar Vascular tissue Bundle sheath cell