Photosynthesis PDF

Photosynthesis Learning Outcomes To relate the structure of chloroplasts to the events in photosynthesis To describe light-dependent reactions and carbon fixation reactions To identify the reactants and products of the photosynthetic reactions Photosynthesis A biological process carried out...

Photosynthesis Learning Outcomes To relate the structure of chloroplasts to the events in photosynthesis To describe light-dependent reactions and carbon fixation reactions To identify the reactants and products of the photosynthetic reactions Photosynthesis A biological process carried out by photolithoautotrophs, where light energy is converted to chemical energy  The Light-capturing Pigments Chlorophyll a Porphyrin ring Hydrocarbon tail Chlorophyll b Beta-carotene Reactions of Photosynthesis  Light-dependent Reactions Reactants Products Light adenosine Oxygen triphosphate Water ADP ATP nicotinamide NADP+ NADPH adenine dinucleotide Involves three phases: Photochemical reactions phosphate Electron transport Chemiosmosis  Photosystems Highly organized clusters of proteins, chlorophyll and accessory pigments PS I is discovered first, but it is PS II that transports excited electrons to “activate” PS I The Electron Transport Chain 1 Photochemical reaction Light is absorbed by chlorophyll in PSII Excites the electrons in the reaction centre The Electron Transport Chain 8 Photolysis Splitting of water to release O2, H+ ions and electrons The Electron Transport Chain 2&3 Energized electrons leave the reaction centre Move into electron transport chain next to PSII The Electron Transport Chain 4 Chemiosmosis or “photophosphorylation” H+ ions diffuse across thylakoid membrane, releasing energy Energy is used to synthesize ATP The Electron Transport Chain 5&6 Energy-depleted electrons enter PSI reaction centre Light is absorbed to re-energize electrons The Electron Transport Chain 7 Electrons leave reaction centre and move into electron transport chain Use energy to synthesize NADPH The Electron Transport Chain 9 Energized electrons from PSI return to PSI again to produce ATP Overview – Light reactions of photosynthesis  Light-independent Reaction Also known as C3 cycle / Calvin cycle / Dark reaction Reactants Products CO2 Glucose RuBP ADP ribulose ATP bisphosphate NADP+ NADPH H2O Involves three phases: Carbon fixation Carbon reduction Regeneration of RuBP Calvin Cycle RuBP 1 Carbon Fixation (RuBP carboxylase/oxygenase) Rubisco combines carbon from CO2 to the 5-C sugar RuBP The unstable 6-C sugar formed will split in half to form 3-C PGA. Calvin Cycle RuBP 2 Carbon Reduction Energy from ATP and NADPH is used to convert PGA to G3P. Calvin Cycle RuBP 3 RuBP Regeneration G3P molecules will combine to form a glucose molecule. The remaining G3P molecules will undergo a series of enzyme- catalysed reactions requiring energy from ATP to regenerate RuBP Overview – Calvin cycle of photosynthesis Differences between Reactions  Photorespiration Stomata closes during hot weather to - Prevent water loss - Reduce CO2 intake - Accumulation of O2 in chloroplast  Photorespiration Competitive Inhibition Rubisco binds with O2 Intermediates are degraded to CO2 and H2O Unlike photosynthesis, no glucose is produced; Unlike respiration, no energy is produced. Rate of photosynthesis is reduced  C4 Cycle A two-stage carbon-fixation pathway to avoid photorespiration and conserve water, especially for plants thriving in deserts and dry environment, e.g. corn, sugarcane  C4 Cycle Stage 1 - CO2 enters a mesophyll cell, phosphoenolpyruvate (PEP) carboxylase binds with three-carbon PEP to form 4-C oxaloacetate. - This substance then turns into an organic acid called malate.  C4 Cycle Stage 2 - Malate is conveyed to the bundle sheath cells where the oxygen concentration is low. - The malate is disintegrated, releasing CO2 molecules and enter the Calvin cycle or C3 pathway for production of sugar.  Crassulacean acid metabolism (CAM) pathway Similar to C4 pathway. Carried out by plants such as pineapple and snake plant. Stomata open at night for carbon fixation. Malate decarboxylation and sugar production during the day. Both events occur in the same cell only – mesophyll cell. End of slides