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Photosynthesis Two-stage process Light-dependent reaction
Occurs only during daylight Light-independent reaction May continue in the dark
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Photosynth Respiration
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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
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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
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Electron transport chain
electron acceptor ATP ADP + Pi final electron acceptor
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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
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Cyclic photophosphorylation
electron acceptor e- ATP ADP + Pi Chlorophyll e- light
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Non-cyclic photophosphorylation
electron acceptor e- 2H ATP electron acceptor e- ADP + Pi 2H+ e- Photosystem I Photosystem II H2O e- light light ½O2
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To the light independent reaction
NADP NADP+ e- NADPH + H+ 2H To the light independent reaction 2H+
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Splitting of water is catalysed by an enzyme – perhaps PSII!
Photolysis of water 4 H2O → 4 H+ + 4 OH- 4 OH- - 4 e- → O2 + 2 H2O Splitting of water is catalysed by an enzyme – perhaps PSII! NADP+ Photosystem II
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Light-dependent stage
Water is converted to protons, electrons and oxygen Reduced NADP is generated Occurs in the thylakoid membrane
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Non-cyclic photophosphorylation
electron acceptor e- 2H ATP electron acceptor e- ADP + Pi 2H+ e- Photosystem I Photosystem II H2O e- light light ½O2
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To the light independent reaction
NADP NADP+ e- NADPH + H+ 2H To the light independent reaction 2H+
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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
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Light-independent stage
Carbon dioxide is converted to carbohydrate Occurs in the stroma of the chloroplast
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Match structure and function
Double membrane chloroplast envelope Thylkakoids have large surface area ATP synthase molecules in thylakoid membrane Stroma contains enzymes, sugars and organic acids As much light as possible can be absorbed Needed for the light-dependent reaction to take place Reactants kept close to reaction sites Produce ATP in the light-dependent reaction
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Three phases Carboxylation Reduction Regeneration
Carbon dioxide fixation with ribulose bisphosphate (RuBP) Reduction Reduction of glycerate 3-phosphate (GP) to glyceraldehyde 3-phosphate (GALP) Regeneration Re-formation of the CO2 acceptor molecules
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Chemicals
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Phase 1: Carboxylation Rubisco CO2 GP 6C molecule RuBP GP
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RUBISCO
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Phase 2: Reduction 2ATP 2ADP + 2Pi GP GALP GP GALP 2NADPH + H+ 2NADP+
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Phase 3: Regeneration ATP ADP + Pi GALP RuBP GALP
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Synthesis of Organic Substances
GALP GALP GALP GP GALP fatty acids glycerol lipids amino acids hexose
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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?
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