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Photosynthesis • Two-stage process • Light-dependent reaction – Occurs only during daylight • Light-independent reaction – May continue in the dark • Photosynth • https://www.youtube.com/watch?v=g78utc LQrJ4 • Respiration • https://www.youtube.com/watch?v=0IJMR sTcwcg Evidence for two stages • Temperature has a clear effect on the rate of photosynthesis – This suggests that although light levels initially limit the rate, there is a second stage controlled by temperature-sensitive enzymes • A plant given alternating periods of dark and light forms more carbohydrate than one in continuous light – Period of darkness ensures that all the product from first stage is converted into carbohydrate before it builds up Excitation of electrons Photon of light hits chlorophyll molecule Energy transferred to the electrons in the molecule Electrons raised to higher energy levels If they are raised to a high enough level they will dissociate Electrons picked up by an electron acceptor ATP formed as the electron is passed along an electron transport chain Electron transport chain electron acceptor ATP ADP + Pi final electron acceptor Light-dependent stage • Occurs in the thylakoids • Involves splitting of water by light – photolysis • ADP is converted to ATP by light – photophosphorylation – Cyclic photophosphorylation uses only PSI – Non-cyclic photophosphorylation uses both PSI and PSII Cyclic photophosphorylation electron acceptor eATP ADP + Pi e- Chlorophyll light Non-cyclic photophosphorylation electron acceptor e- 2H ATP electron acceptor eADP + Pi 2H+ e- Photosystem I e- Photosystem II light light H 2O ½O2 NADP NADP+ e- 2H NADPH + H+ 2H+ To the light independent reaction Photolysis of water 4 H2O → 4 H+ + 4 OHNADP+ 4 OH- - 4 e- → O2 + 2 H2O Photosystem II Splitting of water is catalysed by an enzyme – perhaps PSII! Light-dependent stage • Water is converted to protons, electrons and oxygen • Reduced NADP is generated • Occurs in the thylakoid membrane Non-cyclic photophosphorylation electron acceptor e- 2H ATP electron acceptor eADP + Pi 2H+ e- Photosystem I e- Photosystem II light light H 2O ½O2 NADP NADP+ e- 2H NADPH + H+ 2H+ To the light independent reaction Melvin Calvin • Member of the Radiation Laboratory at Berkeley, University of California • Studied using radioactively labelled carbon dioxide • 14CO2 fed to Chlorella algae and its path tracked Light-independent stage • Carbon dioxide is converted to carbohydrate • Occurs in the stroma of the chloroplast Match structure and function Double membrane chloroplast envelope As much light as possible can be absorbed Thylkakoids have large surface area Needed for the lightdependent reaction to take place ATP synthase molecules in thylakoid membrane Reactants kept close to reaction sites Stroma contains enzymes, sugars and organic acids Produce ATP in the lightdependent reaction Three phases 1. Carboxylation Carbon dioxide fixation with ribulose bisphosphate (RuBP) 2. Reduction Reduction of glycerate 3-phosphate (GP) to glyceraldehyde 3-phosphate (GALP) 3. Regeneration Re-formation of the CO2 acceptor molecules Chemicals Phase 1: Carboxylation Rubisco CO2 GP 6C molecule RuBP GP RUBISCO Phase 2: Reduction 2ATP 2ADP + 2Pi GP GALP GP GALP 2NADPH + H+ 2NADP+ Phase 3: Regeneration ATP ADP + Pi GALP RuBP GALP Synthesis of Organic Substances GALP GALP hexose GALP GP glycerol fatty acids lipids GALP amino acids The Maths • 2 molecules of GALP are made each turn • 5 out of 6 molecules of GALP are used to regenerate RuBP • 2 molecules of GALP are needed to make a hexose sugar • How many turns are needed to make 1 molecule of hexose sugar? • How many molecules of ATP and reduced NADP are needed?
