Chapter 7 Notes
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This 3 page Class Notes was uploaded by Min-Young Kim on Tuesday April 5, 2016. The Class Notes belongs to BIOL 3040 at Clemson University taught by Christina Wells in Spring 2016. Since its upload, it has received 15 views. For similar materials see Biology of Plants in Biology at Clemson University.
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Date Created: 04/05/16
Chapter 7 Notes -‐ In photosynthesis, light energy is converted to chemical energy and carbon is “fixed” into organic compounds 3CO 2+ 6H2O light à 3 C6H 3 + 2 3O 2 3H O o First step: absorption of light energy by pigment molecules § Eukaryotic photosynthesis: chlorophylls and carotenoids, packed in thylakoids of chloroplasts as photosynthetic units called photosystems § Light absorbed by pigment molecules boosts electrons to higher energy level § Arrangement of pigment molecules allows transfer of energy to a pair of special chlorophyll a molecules at reaction centers o Photosystem I & Photosystem II work together simultaneously and continuously § Photosystem I: can carry out photosynthesis independently of Photosystem II • No external donor (water) and thus no production of NADPH o Cyclic phosphorylation only results in proton gradients used for ATP production o Major reactions of photosynthesis divided into light reactions and carbon-‐fixation reactions -‐ In light reactions, electrons flow from water to Photosystem II, down Electron Transport Chain to Photosystem I, and finally to NADP+ o Light energy enters Photosystem II, trapped by pigment molecules, passed to P680 chlorophyll molecules of reaction center o Energized electrons transferred from P 680 to electron acceptor o As electrons removed from P 680, replaced by low-‐energy electrons from water molecules and oxygen is produced (water photolysis) o Pairs of electrons pass downhill to Photosystem I along ETC; passage generates proton gradient that drives synthesis of ATP from ADP and phosphate (photophosphorylation) o Light energy absorbed in Photosystem I is passed to P 700 chlorophyll molecules of Photosystem I reaction center o Energized electrons ultimately accepted by coenzyme molecules NADP+, and electrons removed from P 700 are replaced by electrons from Photosystem II o Energy yield from light-‐dependent reactions stored in molecules of NADPH and in ATP formed by photophosphorylation o Photophosphorylation also occurs in cyclic electron flow; does not require Photosystem II o Only product of cyclic electron flow is ATP o Extra ATP is required by Calvin cycle, which uses ATP and NADPH in a 3:2 ratio -‐ In electron transport chain, electron flow is coupled to proton pumping and ATP synthesis by a chemiosmotic mechanism o Like oxidative phosphorylation in mitochondria, photosphorylation in chloroplasts is a chemiosmotic process. o As electrons flow down ETC from Photosystem II to Photosystem I, protons pumped from stroma into thylakoid lumen, creating gradient of potential energy o As protons flow down gradient from thylakoid lumen back into stroma, pass through ATP synthase, generating ATP -‐ In Calvin cycle, C2 is fixed via a 3-‐Carbon pathway o In carbon-‐fixation reactions, which take place in stroma of chloroplast, NADPH and ATP produced in light reactions are used to reduce carbon dioxide to organic carbon o Calvin cycle responsible for initial fixation of CO 2 and subsequent reduction of newly fixed carbon o In Calvin cycle, molecule of CO 2 combines with starting compound: 5-‐ carbon sugar called ribulose 1,5-‐biphosphate (RuBP), to form two molecules of 3-‐carbon compound 3-‐phosphoglycerate (PGA) o PGA reduced to 3-‐carbon molecule glyceraldehyde 3-‐phosphate (PGAL), with electrons provided by NADPH and energy provided by ATP hydrolysis o At each turn of Calvin cycle, one carbon atom enters cycle. Three turns of cycle produce one molecule of glyceraldehyde 3-‐phosphate. o At each turn of cycle, RuBP is generated. Most of fixed carbon is converted to either sucrose or starch -‐ Carbon-‐fixation pathway in C 4lants is a solution to problem of photorespiration o C 3lants: Plants in which Calvin cycle is only carbon-‐fixation pathway, and in which first detectable product of C2 fixation is 3-‐carbon compound 3-‐phosphoglycerate (PGA) o C 4lants: CO 2initially fixed to phosphoenolpyruvate (PEP) to yield oxaloacetate, a four carbon compound § Reaction occurs in mesophyll cells of leaf § Oxaloacetate rapidly converted to malate, which moves from mesophyll cells to bundle-‐sheath cells § Malate decarboxylated and CO ent2rs Calvin cycle by reacting with ribulose 1,5-‐biphosphate (RuBP) to form PGA § C 4 pathway takes place in mesophyll cells, but Calvin cycle occurs in bundle-‐sheath cells o C 4lants more efficient utilizers of CO 2 than C3 plants § PEP carboxylase is not inhibited by O2. § C 4 plants can attain same photosynthetic rate as C3 plants, but with smaller stomatal openings and less water loss § C plants more competitive than C plants at high temperatures 4 3 -‐ CAM plants can fix CO2 in the dark o Crassulacean acid metabolism (CAM) occurs in many succulent plants o Fixation of CO 2 to phosphoenolpyruvate (PEP) to form oxaloacetate occurs at night when stomata open o Oxaloacetate rapidly converted to malate, which is stored overnight in vacuole as malic acid o During daytime, when stomata are closed, malic acid recovered from vacuole and fixed CO2 is transferred to ribulose 1,5-‐biphosphate (RuBP) of Calvin cycle o C 4pathway and Calvin cycle occur within same cells in CAM plants; hence, two pathways which are spatially separated in C4 plants, are temporally separated in CAM plants
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