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CHAPTER 8 Photosynthesis

CHAPTER 8 Photosynthesis. Photosynthesis . Energy within light is captured and used to synthesize carbohydrates CO 2 + H 2 O + light energy → C 6 H 12 O 6 + O 2 + H 2 O Notice that this rxn is the opposite of aerobic respiration CO 2 is reduced H 2 O is oxidized

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CHAPTER 8 Photosynthesis

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  1. CHAPTER 8 Photosynthesis

  2. Photosynthesis • Energy within light is captured and used to synthesize carbohydrates CO2 + H2O + light energy → C6H12O6 + O2 + H2O • Notice that this rxn is the opposite of aerobic respiration • CO2 is reduced • H2O is oxidized • Energy from light drives this endergonic rxn

  3. The Story of Photosynthesis

  4. Evidence of the Great Oxidation Event

  5. Photosynthesis Powers the Biosphere • Biosphere = regions on the surface of the Earth and in the atmosphere where living organisms exist • Cycle where cells use organic molecules for energy and plants replenish those molecules using photosynthesis • Plants also release oxygen

  6. Patterns of Metabolism • Autotroph • Make organic molecules from inorganic sources • Photoautotroph • Use light as a source of energy • Green plants, algae, cyanobacteria • Heterotroph • Must eat food, organic molecules from their environment, to sustain life • All animals, fungi, most bacteria

  7. Leaf cross-section • Majority of photosynthesis occurs in cells in mesophyll Chloroplast cross-section

  8. Chloroplast • Organelles in plants and algae that carry out photosynthesis • Chlorophyll a and b = green, light absorbing pigment

  9. Chloroplast anatomy • Outer and inner membrane • Intermembrane space • 3rd membrane - thylakoid membrane contains pigment molecules • Forms thylakoids • Granum = stack of thylakoids • Stroma = fluid filled region between thylakoid membrane and inner membrane

  10. 2 Stages of Photosynthesis • 1st Stage = Light Reactions • 2 kinds of light rxns: • Cyclic • Noncyclic • 2nd Stage = Calvin Cycle

  11. 2 Stages of Photosynthesis • First Stage = Light reactions • Uses light energy = (must have light source) • Take place on thylakoid membranes in chloroplasts • Produce ATP, NADPH and O2 • 2 pathways: Cyclic and Noncyclic

  12. About Light energy • Type of electromagnetic radiation • Travels as waves • Short to long wavelengths • Also behaves as particles- photons • Shorter wavelengths have more energy

  13. After an electron absorbs energy, it is an excited state and usually unstable • Releases energy as • Heat • Light • Excited electrons in pigments can be transferred to another molecule or “captured”

  14. About Pigments • Pigments absorb some light energy and reflect others • Photosynthetic pigments – and therefore leaves - are green because they reflect green wavelengths • Absorption boosts electrons to higher energy levels • Wavelength of light that a pigment absorbs depends on the amount of energy needed to boost an electron to a higher orbital

  15. Pigments Chlorophyll a & Carotenoids Chlorophyll b

  16. Absorption spectrum Wavelengths that are absorbed by different pigments in the plant Action spectrum Rate of photosynthesis by whole plant at specific wavelengths Absorption vs. action spectrum

  17. The Photosystems Absorb Light Energy • About the Photosystems: • Captured light energy can be transferred to other molecules to ultimately produce energy intermediates for cellular work • Thylakoid membranes of chloroplast contain… • Photosystem I (PSI) • Photosystem II (PSII)

  18. Cyclic Photophosphorylation

  19. Non-cyclic Photophosphorylation

  20. ATP synthesis • Chemiosmotic mechanism • Driven by flow of H+ from thylakoid lumen into stroma via ATP synthase • H+ gradient generated by • ↑H+ in thylakoid lumen by splitting of water • ↑H+ by ETC pumping H+ into lumen • ↓H + from formation of NADPH in stroma

  21. Light excites pigment molecules in PSII and PSI • PSII - excited electrons travel to PSI • Water is oxidized- generates O atoms, electrons and H+ ions • Electrons are passed to electron transport chain • H+ ions are pumped across membrane • Oxygen is released as a “waste” product • PSII – primary role is to make NADPH • Addition of H+ to NADP contributes to H+ electrochemical gradient

  22. Cyclic and noncyclic electron flow • Noncyclic photophosphorylation • Electrons begin at PSII and eventually transfer to NADPH • Linear process produces ATP and NADPH in equal amounts • Cyclic photophosphorylation • Electron cycling releases energy to transport H+ into lumen driving synthesis of ATP • PSI electrons excited, release energy and eventually return to PSI

  23. Photosytem II (PSII) • 2 main components • Light-harvesting complex or antenna complex • Directly absorbs photons • Energy transferred via resonance energy transfer • Reaction center • P680 →P680* • Relatively unstable • Transferred to primary electron acceptor • Removes electrons from water to replace oxidized P680 • Oxidation of water yields oxygen gas

  24. Z scheme • Zigzag shape of energy curve • Events involve increases and decreases in the energy of an electron as it moves from PSII through PSI to NADPH • Electron of nonexcited pigment molecule has lowest energy in PSII • Highest energy level from being boosted by PSI

  25. Summary • O2 is released into the thylakoid lumen by oxidation of H2O by PSII • 2 electrons from water are transferred to PSII • ATP is produced by PSI and PSII • NADPH is produced when both PSI and PSII are used (non-cyclic photophosphorylation) • NADPH is NOT produced when only PS I is used (cyclic photophosphorylation)

  26. The cytochrome complexes of mitochondria and chloroplasts contain evolutionarily related proteins • Recurring theme is descent with modification • Homologous genes are similar because they are derived from a common ancestor • Comparing the electron transport chains of mitochondria and chloroplasts reveals homologous genes • Family of cytochrome b-type proteins plays similar but specialized roles

  27. 2nd stage = Calvin cycle • Occurs in stroma • Uses ATP and NADPH made in light rxns to reduce CO2 sugars • While Calvin cycle does not directly use light energy, it must occur in the light so that light rxns can form ATP and NADPH that is needed in the Calvin cycle

  28. Calvin cycle • Also called Calvin-Benson cycle • ATP and NADPH used to make carbohydrates • CO2 incorporated into carbohydrates • Precursors to ALL organic molecules • Energy storage

  29. CO2 Incorporation • Requires massive input of energy • For every 6 CO2 incorporated, 18 ATP and 12 NADPH used • Product is glyceraldehyde-3-phosphate (G3P) • Glucose is not directly made

  30. 3 phases • Carbon fixation • CO2 incorporated in RuBP using rubisco • 6 carbon intermediate splits into 2 3PG • Reduction and carbohydrate production • ATP is used to convert 3PG into 1,3-bisphosphoglycerate • NADPH electrons reduce it to G3P • 6 CO2→ 12 G3P • 2 for carbohydrates • 10 for regeneration of RuBP • Regeneration of RuBP • 10 G3P converted into 6 RuBP using 6 ATP

  31. A reminder of glycolysis. Find the molecules that are mentioned on last slide!

  32. Photorespiration can be a big problem for plants! • RuBP + CO2→ 2 3PG • Rubisco (Ribulose bisphosphate carboxylase/oxygenase) • functions as a carboxylase • C3 plants make 3PG • Rubisco can also be an oxygenase • Adds O2 to RuBP eventually releasing CO2 • Runs Calvin cycle backwards = Photorespiration • Using O2 and liberating CO2 is wasteful • Mostly occurs in hot and dry environments • Favored when CO2 low and O2 high

  33. Variations in photosynthesis • Certain environmental conditions can influence both the efficiency and way the Calvin cycle works • Light intensity • Temperature (heat can create problems) • Water availability (lack of water can create problems)

  34. C3 Plants- • Live where temperatures are moderate • Stomata remain open and release O2 to the environment • O2 conc. in mesophyll cells stays low • Since stomata stay open, CO2 conc. stay high

  35. C4 Plants • When temperatures go too high, stomata in mesophyll cells close • This allows O2 to build up in cell • Which favors photorespiration • Now Calvin cycle works backwards!

  36. How C4 plants Overcome the Problem of Photorespiration • C4 plants make oxaloacetate (4 carbon compound) in the first step of carbon fixation • Leaves have 2-cell layer organization • Mesophyll cells • CO2 enters via stomata and 4 carbon compound formed (PEP carboxylase does not promote photorespiration) • Bundle-sheath cells • 4 carbon compound transferred that releases steady supply CO2

  37. In C4 Plants, the mesophyll layer protects the Bundle sheath cell from high levels of O2. See mechanics below:

  38. Which is better – C3 or C4? • In warm dry climates C4 plants have the advantage in preventing photorespiration • Ex: sugarcane, corn • In cooler climates, C3 plants use less energy to fix CO2 • 90% of plants are C3

  39. CAM plants • Some C4 plants separate processes across day and night • Crassulacean Acid Metabolism • CAM plants open their stomata at night to allow diffusion of CO2 into the cell • At night, temperatures are lower • CO2 enters cell and is joined to PEP to form malate • Stomata close during the day to conserve water • Malate is broken down, releases CO2 to drive Calvin cycle forward during the day

  40. CAM Plant C4 Plant

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