Photosynthesis PDF

 Photosynthesis Takes Place in Complexes Containing Light-Harvesting Antennas and Photochemical Reaction Centers The majority of the pigments serve as an antenna complex, collecting light and transferring the energy to the reaction center c...

 Photosynthesis Takes Place in Complexes Containing Light-Harvesting Antennas and Photochemical Reaction Centers The majority of the pigments serve as an antenna complex, collecting light and transferring the energy to the reaction center complex  In all eukaryotic photosynthetic organisms that contain both chlorophyll a and chlorophyll b, the most abundant antenna proteins are members of a large family of structurally related proteins. Some of these proteins are associated primarily with photosystem II and are called light-harvesting complex II (LHCII) proteins; others are associated with photosystem I and are called LHCI proteins. These antenna complexes are also known as chlorophyll a/b antenna proteins. The protein contains three α-helical regions and binds about 15 chlorophyll a and b molecules, as well as a few carotenoids. Light absorbed by carotenoids or chlorophyll b in the LHC proteins is rapidly transferred to chlorophyll a and then to other antenna pigments that are intimately associated with the reaction center. The LHCII complex is also involved in regulatory processes, which are discussed later in the chapter shows a current version of the Z scheme, in which all the electron carriers known to function in electron flow from H2O to NADP+ are arranged vertically at their midpoint redox potentials All the chemical processes that make up the light reactions of photosynthesis are carried out by four major protein complexes: photosystem II, the cytochrome b6 f complex, photosystem I, and the ATP synthase. These four integral membrane complexes are vectorially oriented in the thylakoid membrane to function as follows. Non cyclic flow Photosystem II oxidizes water to O2 in the thylakoid lumen and in the process releases protons into the lumen. Cytochrome b6 f receives electrons from PSII and delivers them to PSI. It also transports additional protons into the lumen from the stroma. Photosystem I reduces NADP+ to NADPH in the stroma by the action of ferredoxin (Fd) and the flavoprotein ferredoxin–NADP reductase (FNR). ATP synthase produces ATP as protons diffuse back through it from the lumen into the stroma. Cyclic Electron Flow Generates ATP but no NADPH Some of the cytochrome b6 f complexes are found in the stroma region of the membrane, where photosystem I is located. Under certain conditions cyclic electron flow from the reducing side of photosystem I, through the b6 f complex and back to P700, is known to occur. This cyclic electron flow is coupled to proton pumping into the lumen, which can be utilized for ATP synthesis but does not oxidize water or reduce NADP+. Cyclic electron flow is especially important as an ATP source in the bundle sheath chloroplasts of some plants that carry out C4 carbon fixation.  Carbon Reactions These stroma reactions were long thought to be independent of light and, as a consequence, were referred to as dark reactions. However, because these stroma- localized reactions depend on the products of the photochemical processes, and are also directly regulated by light, they are more properly referred to as the carbon reactions of photosynthesis.  THE CALVIN CYCLE The Calvin Cycle Has Three Stages: Carboxylation, Reduction, and Regeneration The Calvin cycle proceeds in three stages: 1. Carboxylation of the CO2 acceptor ribulose-1,5- bisphosphate, with the help of enzyme Rubisco forming two molecules of 3-phosphoglycerate, the first stable intermediate of the Calvin cycle. 2. Reduction of 3-phosphoglycerate, forming gyceraldehyde-3-phosphate, a carbohydrate 3. Regeneration of the CO2 acceptor ribulose-1,5- bisphosphate from glyceraldehyde-3-phosphate  THE C4 CARBON CYCLE There are differences in leaf anatomy between plants that have a C4 carbon cycle (called C4 plants) and those that photosynthesize solely via the Calvin photosynthetic cycle (C3 plants). A cross-section of a typical C3 leaf reveals one major cell type that has chloroplasts, the mesophyll. In contrast, a typical C4 leaf has two distinct chloroplast-containing cell types: mesophyll and bundle sheath. No mesophyll cell of a C4 plant is more than two or three cells away from the nearest bundle sheath cell. In addition, an extensive network of plasmodesmata connects mesophyll and bundle sheath cells, thus providing a pathway for the flow of metabolites between the cell types. The C4 Cycle Concentrates CO2 in Bundle Sheath Cells The basic C4 cycle consists of four stages: 1. Fixation of CO2 by the carboxylation of phosphoenolpyruvate in the mesophyll cells to form a C4 acid (malate and/or aspartate). 2. Transport of the C4 acids to the bundle sheath cells. 3. Decarboxylation of the C4 acids within the bundle sheath cells and generation of CO2, which is then reduced to carbohydrates via the Calvin cycle. 4. Transport of the C3 acid (pyruvate or alanine) that is formed by the decarboxylation step back to the mesophyll cell and regeneration of the CO2 acceptor phosphoenolpyruvate  CRASSULACEAN ACID METABOLISM (CAM pathway) A third mechanism for concentrating CO2 at the site of rubisco is found in crassulacean acid metabolism (CAM). Despite its name, CAM is not restricted to the family Crassulaceae (Crassula, Kalanchoe, Sedum); it is found in numerous angiosperm families. Cacti and euphorbias are CAM plants, as well as pineapple, vanilla, and agave. The Stomata of CAM Plants Open at Night and Close during the Day CAM plants such as cacti achieve their high water use efficiency by opening their stomata during the cool, desert Nights and closing them during the hot, dry days. Closing the stomata during the day minimizes water loss, but because H2O and CO2 share the same diffusion pathway, CO2 must then be taken up at night. During the night CO2 is incorporated via carboxylation of phosphoenolpyruvate to oxaloacetate, which is then reduced to malate. The malate accumulates and is stored in the large vacuoles. During the day With the onset of day, the stomata close, preventing loss of water and further uptake of CO2. The leaf cells deacidify as the reserves of vacuolar malic acid are consumed. Decarboxylation is usually achieved by the action of NADP-malic enzyme on malate. Because the stomata are closed, the internally released CO2 cannot escape from the leaf and instead is fixed and converted to carbohydrate by the Calvin cycle.