BCH3004 Principles of Biochemistry - Photosynthesis PDF

PRINCIPLES OF BIOCHEMISTRY (BCH3004) PHOTOSYNTHESIS  What you should know…… Photosynthesis ▪ Light reaction: function of chlorophyll and pigments in photosynthesis, effect of Emerson addition, energy and photosynthetic phosphorylation ▪ Dark reaction: Calvin cycle and car...

PRINCIPLES OF BIOCHEMISTRY (BCH3004) PHOTOSYNTHESIS  What you should know…… Photosynthesis ▪ Light reaction: function of chlorophyll and pigments in photosynthesis, effect of Emerson addition, energy and photosynthetic phosphorylation ▪ Dark reaction: Calvin cycle and carbon reduction: C3, C4 and CAM cycles ▪ Photorespiration Photosynthesis Photosynthesis Scan me!! https://www.youtube.com/watch?v=NqCmzk4Z9LA ▪ The energy content of light depends on its wavelength Chl a = 430 nm (blue) = 662 nm (red) Chl b = 450 nm (blue) = 640 nm (red) ▪ The ratio chl-a to chl-b is about three to one Chlorophyll CHO in chlorophyll b (ring of four pyrroles) CH3 in chlorophyll a formyl methyl I II I II IV III IV III Anchors the Chl molecule to an inner membrane within chloroplast Chlorophyll a Chlorophyll b 6 Distribution of different chlorophylls in nature 7  Stages of photosynthesis a) Light dependent reaction (Light reaction) - the photo part of photosynthesis - occurs in thylakoids b) Light independent reaction (Calvin cycle) - the synthesis part of photosynthesis - occurs in stroma 8 Light Reaction 9 Light Reaction 10  Light Reaction ▪ Two cooperating photosystems: i. Photosystem I (PSI) Reaction center = P700 Absorption peak = 700 nm ii. Photosystem II (PSII) Reaction centre = P680 Absorption peak = 680 nm ▪ Takes place on the thylakoid membranes within chloroplast ▪ Each photosystem is a complex of several hundred chlorophyll (known as light harvesting complex/ light harvesting antenna) Light harvesting complexes (LHC) When light strikes a pigment molecule in either photosystem, the energy is funnelled into ▪ Function: absorb light energy reaction centre ▪ LHC consist i. Reaction center (P680 or P700) ii. Antenna ▪ Antenna - a protein-bound pigment molecule - absorb photons and transfer their energy to reaction center Excitation of Chlorophyll by Light 1. Photon absorption and electron excitation When a chlorophyll molecule absorbs a photon of light, an electron within the molecule is energized and moves to a higher energy orbital. 2. Transition to an Excited State This transition raises the electron from its ground state to an excited state. 3. The Transient Excited State The excited state is inherently unstable and does 5. Energy release not persist for an extended period. During this transition, the excess 4. Return to Ground State energy is released as heat, contributing to the stability of the Consequently, the excited electron rapidly reverts to chlorophyll molecule and facilitating its ground state. further photosynthetic processes.  Enzymes Coenzymes Thylakoid membrane- bound electron carriers Photophosphorylation OEC - oxygen-evolving complex ▪ Oxidation of water (water photolysis) 2H2O → O2 + 4H+ ▪ The hydrogen ions contribute to the transmembrane chemiosmotic potential that leads to ATP synthesis (photophosphorylation)  Photophosphorylation ▪ Proton motive force is generated across the thylakoid membrane of chloroplast ▪ Two types of photophosphorylation 1. Non cyclic photophosphorylation 2. Cyclic photophosphorylation Non cyclic photophosphorylation Cyclic photophosphorylation ▪ Use Photosystem I ▪ Electron is cycle back from ferredoxin (Fd) to Cytochrome complex, then via a plastocyanin molecule (Pc) to the P700 chlorophyll ▪ No production of NADPH and no release of oxygen ▪ Generate ATP What is the function of Cyclic photophosphorylation? Function of Cyclic photophosphorylation? ▪ Recall: What are products from Non cyclic cycles? ▪ Calvin cycle consumes more ATP than NADPH. ▪ The [NADPH] in the chloroplast helps to regulate which pathway, non cyclic vs cyclic, electrons take through light reactions. ▪ If the chloroplast runs low on ATP for the Calvin cycle, NADPH begins to accumulates as the Calvin cycle slows down. ▪ Rise in NADPH stimulates a temporary shift from non cyclic to cyclic electron flow until ATP supply catches up with demand. ❑ 3 compounds are generated form light reaction i. Reduced nicotinamide adenine dinucleotide phosphate (NADPH) – reducing agent ii. Adenosine triphosphate (ATP) iii. Oxygen Light Independent Reaction 6CO2 + 12H2O -> C6H12O6 + 6H2O + 6O2 3 phases: i. Carbon fixation ii. Reduction iii. Regeneration of the CO2 acceptor ribulose bisphosphate (RuBP) Phase 1: Carbon fixation ▪ CO2 (entering one at a time) attach to 3 molecules of RuBP (5C sugar). Catalyzed by RuBP carboxylase (rubisco). ▪ Produce unstable six-carbon compound that immediately split into half, forming two molecules of 3 phosphoglycerate (3C molecules) for each CO2 fixed. Phase 2: Reduction ▪ Each molecule of 3-phosphoglycerate receives an additional phosphate group from ATP forming a molecules of 1,3-bisphosphoglycerate. ▪ NADPH reduces 1,3- bisphophoglycerate forming Glyceraldehyde 3- phosphate (G3P)/ PGAL. - 6 NADPH reduces 6 molecules of 1,3-bisphophoglycerate - Forming 6 molecules of Glyceraldehyde 3-phosphate (G3P)/ PGAL ▪ From 6 molecules of G3P (3C) formed only 1 molecule of G3P (3C) will be used to synthesis glucose (6C). Phase 3: Regeneration of the CO2 acceptor ribulose bisphosphate (RuBP) ▪ Other 5 molecules (G3P) are rearranged to regenerate 3 molecules of RuBP (5C). ▪ Phase 3 spends 3 molecules of ATP. ▪ For the net synthesis of 1 G3P (3C), the Calvin cycle consumed: - 9 molecules of ATP - 6 molecules of NADPH ▪ To form 1 molecule of sugar (glucose), the cycle needs 6 molecules of CO2. ▪ The complete, cycle must occur TWICE. Recall: C3 plants → Common plants → Optimal photosynthesis in cool and wet climates → E.g., rice, wheat, soybean  What happens to C3 plants when stomata close? ▪ Reduced CO2 levels in the Phosphoglycolate leaf deprive the Calvin phosphatase Cycle ▪ Rubisco uses O2 as substrate instead of CO2 Glycolate Why photorespiration Catalase oxidase lowers photosynthetic output? Glutamate-glyoxylate Serine-glyoxylate aminotransferase aminotransferase ▪ Release CO2 ▪ Use ATP ▪ Produce no sugar Glycine decarboxylase-serine hydroxymethyltransferase complex ** Occurs only in C3 plants  Alternative mechanisms of carbon fixation ▪ In some plant species, alternate modes of carbon fixation have evolved, minimizing photorespiration and optimizing the Calvin cycle even in hot, arid climates. ▪ Two photosynthetic adaptation: 1. Hatch-Slack pathway (C4 pathway) 2. Crassulacean acid metabolism (CAM) pathway  C4 pathway ▪ C4 plants are named because they initiate the Calvin cycle with an alternative mode of Sugarcane carbon fixation, resulting in the formation of a four-carbon compound as the initial product. Corn Leaf anatomy of C4 pathway ▪ In C4 plants, there are two types of photosynthetic cells: 1. Bundle sheath cells: arrange tightly packed sheath around the veins of leaf. 2. Mesophyll cells: loosely arranged between bundle sheath and leaf surface ▪ This arrangement is called Kranz anatomy C4 pathway https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcSofHjClJ9viok02-ywooKDjuSndyZ1o8-HOX2wtdnLBHECsZSjy1VsPw Malate C4 pathway Carbonic anhydrase Malic enzyme Phosphoenolpyruvate Carboxylase Pyruvate- phosphate dikinase  ▪ Malic enzyme: 1. NADP-malic enzyme type 2. NAD-malic enzyme type ▪ NADP-malic enzyme: release of CO2 occurs in the bundle sheath chloroplasts and oxidation of malate to pyruvate is coupled with the reduction of NADP+. (E.g., maize and sugarcane) ▪ NAD-malic enzyme: Decarboxylation takes place in the mitochondria of the bundle sheath cells and is accompanied by the reduction of NAD+. C4 Rice Project Crassulacean acid metabolism (CAM) ▪ This mode of carbon fixation is called Crassulacean Acid Metabolism or CAM after the plant family Crassulaceae, the succulents in which the process was first discovered. ▪ Solve the problem of water loss during photosynthesis by opening their stomata only during the night, when it is cool and air humidity is comparatively high. ▪ E.g., pineapple, cacti, succulent ornamental plant ▪ The mesophyll cells of CAM plants store the organic acids they make during the night in their vacuoles until morning. ▪ During the day, when the light reactions can supply ATP and NADPH for the Calvin cycle, CO2 is released from the organic acid to become incorporated into sugar in the chloroplast. ▪ CAM plants open their stomata during the night and close them during the day to minimize photorespiration. ▪ Closing stomata during the day helps desert plants to conserve water, but this prevents CO2 from entering the leaves. Azzeme et al., 2016 Azzeme et al., 2016 Azzeme et al., 2017