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Photosynthesis Photosynthesis in nature:-Plants and other autotrophs are producers of biosphere-Photoautotrophs: use light E to make organic molecules(Photoautotrophs include: plants, protists, cyanobacteria)-Heterotrophs: consume organic molecules from other organisms for E and carbon Photosynthesis – converts light energy to chemical energy of food. Sites of photosynthesis:-chloroplasts are the site of photosynthesis in plants-chloroplasts are located in the mesophyll cells of the leaf-stomata = pores in the leaf that allow gas exchange (CO2 in/O2 out)-chlorophyll: green pigment in thylakoid membranes of chloroplasts Equation for photosynthesis:6CO2 + 6H2O + Light Energy à C6H12O6 + 6O2 redox reaction: water is split à electron transferred with H+ to CO2 à sugarremember OILRIG… oxidation lose electron / reduction gain electron Tracking atoms through photosynthesis-evidence that chloroplasts split water molecules enabled researchers to track atoms through photosynthesis (C.B. van Neil) Photosynthesis = Light Reactions + Calvin Cycle photo synthesis Light reactions: convert solar E to chemical E of ATP and NADPH Nature of Sunlight:-light = energy = electromagnetic radiation-shorter wavelength – higher E-visible light detected by human eye-light can be reflected, transmitted, or absorbed Interaction of light with chloroplasts: via photosynthetic pigments-pigments absorb different wavelengths of light-chlorophyll: absorbs violet-blue/red, reflects green --chlorophyll a (blue-green): light reaction, converts solar E to chemical E --chlorophyll b (yellow-green): conveys E to chlorophyll a--carotenoids (yellow, orange): photoprotection, broaden color spectrum for photosynthesis Absorption spectrum: determines effectiveness of different wavelengths of photosynthesis-technique: white light is refracted to pass light of a specific wavelength through a slit à specific wavelength passes through a chlorophyll solution à exits and passes to a photoelectric tube à transmittance is measured with a galvanometer.-high transmittance = low absorption. Chlorophyll absorbs v little green light.-low transmittance = high absorption. Chlorophyll absorbs most blue light. Action spectrum: plots rate of photosynthesis vs. wavelength (absorption of chlorophylls a, b, and carotenoids combined) Engelmann: used bacteria to measure rate of photosynthesis in algae; established action spectrum THE LIGHT REACTIONS-photosystem = reaction center + light-harvesting complexes (pigment+protein)-electrons in chlorophyll (pigment) molecules are excited by the absorption of light -two routes for electron flow: linear (noncyclic) and cyclic-Light Reaction (linear electron flow – uses PS II and PS I)1. chlorophyll excited by light absorption2. E passed to reaction center of Photosystem II (protein+chlorophyll a)3. electron captured by primary electron acceptor -redox reaction à electron transfer -electron prevented from losing E (dropping to ground state)4. H2O is split to replace electron à O2 is formed5. electron passed to Photosystem I via ETC6. E transfer pumps H+ to thylakoid space creating proton gradient7. ATP produced by photophosphorylation8. electron moves from PS I’s primary electron acceptor to 2nd ETC9. NADP+ reduced to NADPH Main idea: use solar E to generate ATP and NADPH to provide E for the Calvin cycle Cyclic electron flow: uses PS I only; produces ATP for Calvin cycle (no O2 or NADPH produced) Both respiration and photosynthesis use chemiosmosis to generate ATP-proton motive force generated by: 1. H+ from water 2. H+ pumped across by cytochrome 3. removal of H+ from stroma when NADP+ is reduced-thylakiod space = high H+ concentration-stroma = low H+ concentration THE CALVIN CYCLE-uses ATP, NADPH, CO2-uses ATP and NADPH to convert CO2 to sugar-produces 3-C sugar G3P (glyceraldehyde-3-phosphate)-has 3 phases: 1. carbon fixation 2. reduction 3. regeneration of RuBP (CO2 acceptor) Phase 1:3 CO2 + RuBP (5-C sugar ribulose bisphosphate). Catalyzed by the enzyme rubisco Phase 2:Use 6 ATP and 6 NADPH to produce 1 net G3P Phase 3:Use 3 ATP to regenerate RuBP Alternative mechanisms of carbon fixation have evolved in hot, arid climates-Photorespiration: metabolic pathway which uses O2 and produces CO2. Uses ATP. No sugar production (rubisco binds O2 à breakdown of RuBP). Occurs on hot, dry, bright days when stomata close to conserve water (why? Early atmosphere was low O2, high CO2) Evolutionary AdaptationsProblem with C3 plants:-CO2 fixed to 3-C in Calvin cycle-ex: rice, wheat, soybeans-hot,dry days: partially close stomata, decrease in CO2, photorespiration, no sugars produced C4 Plants:-CO2 fixed to 4-C compounds-ex: corn, sugarcane, grass-hot,dry days à stomata close -2 cell types = mesophyll and bundle sheath cells -mesophyll: PEP carboxylase fixes CO2 (4-C), pump CO2 to bundle sheath -bundle sheath: CO2 used in calvin cycle-decreases photorespiration and increases sugar production-spatial separation of steps CAM Plants:-Crassulacean acid metabolism (CAM)-Night: stomata open à CO2 enters à converts to organic acid, stored in mesophyll cells-Day: stomata closed à light reactions supply ATP, NADPH; CO2 released form organic acids for calvin cycle-ex: cacti, pineapples, succulent (H2O-storing) plants-why? Advantage in arid conditions-temporal separation of steps COMPARISONC3 – C fixation and Calvin together, RubiscoC4 – C fixation and Calvin in different cells, PEP carboxylaseCAM – C fixation and Calvin at different times, organic acid Importance of photosynthesis:Plant – glucose for respiration, celluloseGlobal – O2 production, food source Respiration vs. Photosynthesis ComparisonRespiration:-plants and animals-needs O2 and food-produces CO2, H2O and ATP, NADH-occurs in mitochondria membrane and matrix-oxidative phosphorylation-proton gradient across membrane Photosynthesis :-plants-needs CO2, H2O, sunlight-produces glucose, O2 and ATP, NADPH-occurs in chloroplast thylakoid membrane and stroma-photorespiration-proton gradient across membrane
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