BCHM 307 1nd Edition Lecture 40 Outline of Last Lecture I Photosynthetic Energy and Light II Light versus Dark Reactions III Electron Transport Chain IV Photosystems Outline of Current Lecture I Light Reaction A Steps of Light Reactions II Dark Reactions A RUBISCO B Steps of Dark Reactions III Gluconeogenesis Current Lecture This is the last lecture and finishes the topic of photosynthesis We will start by going through the general steps of photosynthesis The following steps are applicable to the light reactions only The first step occurs when photosystem II absorbs a photon of light Photosystem II s redox potential now become very negative making it a good reductant Photosystem II is able to reduce plastoquinone and itself become oxidized When photosystem II is oxidized it has a very high redox potential It can oxidize water which liberates molecular oxygen and H The following steps start after plastoquinone has been reduced and complete the Z scheme 1 Plastoquinone reduces the cytochrome b6f complex Hydrogen ions are then pumped into the thylakoid lumen 2 The cytochrome b6f complex then reduces plastocyanin 3 Plastocyanin shuttles electrons to photosystem I 4 Photosystem I becomes reduced and absorbs a photon of light 5 Photosystem I reduces ferredoxin and becomes oxidized 6 Ferredoxin has a negative enough redox potential and can reduce NADP This whole process also generates a proton gradient This gradient is used to make ATP in the process called photophosphorylation The cytochrome b6f complex pumps protons into the thylakoid lumen The splitting of water also does this NADPH is synthesized in the stroma by using 2 hydrogen ions The ATP synthase complex uses the proton gradient to make ATP The dark reactions go through a different process than the above These dark reactions occur in the stroma of the chloroplast The purpose of the dark reactions are to use ATP and NADPH to fix carbon dioxide into an organic form These dark reactions are dependent upon the light reactions The main enzyme that does the initial fixation is ribulose 1 5 bisphosphate carboxylase oxygenase This enzyme is known as RUBISCO RUBISCO is found in the chloroplasts stroma The type of carbon fixation in plants is known as C3 photosynthesis because the first stable product is a 3 carbon sugar These dark reactions are known as the Calvin cycle The steps in this process are below with the enzyme for each step listed 1 Carbon dioxide ribulose 1 5 bisphosphate H2O yields 2 3 phosphoglycerate The enzyme used is RUBISCO 2 3 phosphoglycerate ATP yields reversibly 1 3 bisphosphoglycerate ADP This uses phosphoglycerate kinase as the enzyme 3 1 3 bisphosphoglycerate NADPH H yields glyceraldehyde 3 phosphate NADP Pi The enzyme used is NADP glyceraldehyde 3 phosphate dehydrogenase For every carbon dioxide molecule that is fixed 2 NADPH and 3 ATP must be put in The glyceraldehyde 3 phosphate has two different fates 6 of these molecules are formed 5 of the 6 go back into the Calvin cycle to regenerate RUBP The remaining molecule goes into the gluconeogenesis pathway This pathway is used to synthesize sugar starch and cellulose This pathway employs most of the same enzyme as glycolysis It also resynthesizes glucose from glycolytic intermediates It does employ 4 new steps to replace the ones that are irreversible
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