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Chapter 10: Photosynthesis

Chapter 10: Photosynthesis. PCA:. Define: Photosynthesis Autotrophs vs. Heterotrophs Producers vs. Consumers. Photosynthesis in nature. Autotrophs: self feeders/ producers photoautotrophs/ chemoautotrophs obtains organic food without eating other organisms

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Chapter 10: Photosynthesis

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  1. Chapter 10: Photosynthesis

  2. PCA: • Define: • Photosynthesis • Autotrophs vs. Heterotrophs • Producers vs. Consumers

  3. Photosynthesis in nature • Autotrophs: self feeders/ producers • photoautotrophs/ chemoautotrophs • obtains organic food without eating other organisms • Heterotrophs: consumers • obtains organic food by eating other organisms or their by-products (includes decomposers)

  4. The chloroplast • Sites of photosynthesis • Pigment: chlorophyll • Plant cell: mesophyll • Gas exchange: stomata • Chloroplast/Double membrane • Thylakoids, grana, stroma

  5. Photosynthesis: an overview • Redox process • Compared to cellular respiration, it reverses the flow of electrons • Photons split water, excite electrons • Electrons are then transferred to CO2 • Reducing it to a sugar • Electrons increase in potential energy

  6. Two stage process: • 1. Light reactions • Converts solar energy to potential energy • Produces ATP (photophosphorylation) • Reduces NADP+ to NADPH

  7. Two Stage Process: • 2. Calvin Cycle • Incorporates CO2 from the air to organic molecules • Requires ATP • Occurs during the day time because needs light to reduce NADP+ • Light independent reaction(dark reaction)

  8. Stage One: Light Reactions • Converts solar energy to chemical energy (ATP and NADPH) • Uses photosynthetic pigments • Chlorophyll a and b • Carotenoids

  9. Photosystems • Light harvesting units of the thylakoid membrane • Composed mainly of protein and pigment antenna complexes • Antenna pigment molecules are struck by photons • Energy is passed to reaction centers (redox location) • Excited e- from chlorophyll is trapped by a primary e- acceptor

  10. Noncyclic electron flow • Photosystem II (P680): • photons excite electrons of chlorophyll to an acceptor • electrons are replaced by splitting of H2O (release of O2) • electrons travel to Photosystem I down an electron transport chain (PqcytochromesPc) • as electrons fall, ADP ATP (noncyclic photophosphorylation)

  11. Photosystem I (P700): • Fallen electrons are excited by a photon • Electrons enter second electron transport chain • Ferrodoxin (Fp) • Electrons from Fp reduce NADP+ to NADPH

  12. Cyclic electron flow • Alternative cycle when ATP is deficient • Photosystem I used but not II; produces ATP but no NADPH • Why? The Calvin cycle consumes more ATP than NADPH……. • Cyclic photophosphorylation

  13. The Calvin Cycle: • Uses ATP and NADPH from light reactions to convert CO2 to sugar • Anabolic • Endergonic

  14. The Calvin Cycle: • Glucose is not produced directly • 3 carbon sugar molecule (G3P) • Glyceraldehyde 3- phosphate

  15. Three phases of the Calvin Cycle: • 1. Carbon Fixation: • Each molecule of CO2 is fixed to ribose biphosphate(RuBP) • 5 carbon sugar • Catalyzed by Rubisco • RuBP carboxylase

  16. Three phases of the Calvin Cycle: • 1. The product of step one is a short lived 6 carbon intermediate molecule • Forms two molecules of 3- phosphoglycerate

  17. Three phases of the Calvin Cycle: • 2. Reduction: • Each 3- phosphoglycerate is phosphorylate by ATP • Produces 1,3 bisphophoglycerate • Each 1,3 biphosphoglycerate gets reduced to G3P by NADPH • 1 of the 6 G3P produced will go on to produce glucose

  18. Three phases of the Calvin Cycle: • 3. Regeneration of RuBP: • The other 5 G3P molecules will be rearranged into 3 molecules of RuBP

  19. Input vs. Output • 3 CO2 molecules enter one at a time • 3 short lived 6 carbon molecules are produced • 6, 3-phosphoglycerate molecules result

  20. Input vs. Output • 6, 3 –phosphoglycerate are phosphorylated into 6, 1,3 biphosphoglycerate • 6 ATP

  21. Input vs. Output • 6, 1,3 biphosphoglycerate are reduced to 6 G3P by NADPH • 1 of the 6 G3P will go onto form glucose • The other 5 of the G3P will go onto regenerate RuBP • Phosphorylated by 3 ATP

  22. Input vs. Output • 9 ATP consumed • 6 NADPH oxidized

  23. How do plants fix carbon in hot/arid climates? • Plants close their stomata to prevent dehydration during hot and dry days. • How does this affect photosynthesis?

  24. Alternative carbon fixation methods: • Photorespiration: • Plant adds O2 to the Calvin cycle when CO2 is unavailable • No benefit to the plant • No ATP • Not sugar produced

  25. C4 Pathway • C4 plants: alternative mode of carbon fixation • 2 photosynthetic cells, bundle-sheath & mesophyll • PEP carboxylase (instead of rubisco) fixes CO2 in mesophyll • new 4C molecule releases CO2 to Calvin cycle (grasses, corn, sugar cane)

  26. Alternative carbon fixation methods, II • CAM plants: (crassulacean acid metabolism) • open stomata during night, close during day • Cacti and other succulents(Jade plant) pineapples • CO2 is taken in at night and fixed into different organic acids • During the day, when stomata are close but light reaction is active the organic acids deliver CO2 to the Calvin cycle

  27. A review of photosynthesis

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