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Chapter 5. Photosynthesis. http://www.youtube.com/watch?v=sQK3Yr4Sc_k. Photosynthesis. Converting light energy into chemical energy to assemble organic molecules Two stages Light-dependant reactions Absorption of photons of light PI and PII Light-independent reactions (Calvin Cycle)

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  1. Chapter 5 Photosynthesis http://www.youtube.com/watch?v=sQK3Yr4Sc_k

  2. Photosynthesis • Converting light energy into chemical energy to assemble organic molecules • Two stages • Light-dependant reactions • Absorption of photons of light • PI and PII • Light-independent reactions (Calvin Cycle) • Does not require light 6CO2 + 6H2O → C6H12O6 + 6O2

  3. Photosynthesis • Photosynthesis takes place in the green portions of plants • Leaf of flowering plant contains mesophyll tissue • Cells containing chloroplasts • Specialized to carry on photosynthesis • CO2 enters leaf through stomata • Diffuses into chloroplasts in mesophyll cells • In stroma, CO2 fixed to C6H12O6 (sugar) • Energy supplied by light

  4. Chloroplasts • Photosynthesis takes place • Both stages • Consists of • Stroma • Aqueous environment • Houses enzymes used for reactions • Thylakoid membranes • Form stacks of flattened disks called grana • Contains chlorophyll and other pigments

  5. Photosynthetic Stages • Light-dependant reactions • Occur in the thylakoid membranes • capture energy from sunlight • make ATP and reduce NADP+ to NADPH • Carbon fixation reactions (light-independent reactions) • Occurs in the stroma • use ATP and NADPH to synthesize organic molecules from CO2

  6. Photosynthesis

  7. Capturing Light Energy • Pigments • Absorb photon • Excited electron moves to a high energy state • Electron is transferred to an electron accepting molecule – primary electron acceptor

  8. Pigments A pigment molecule does not absorb all wavelengths of light • Chlorophyll a • Donates electron to PEA • Accessory pigments • Chlorophyll b • Carotenoids • Known as antenna complex • Transfers light energy to chlorophyll a

  9. Pigments • Photosynthesis depends on the absorption of light by chlorophylls and carotenoids

  10. Pigments and Photosystems • Chlorophylls and carotenoids do not float freely within thylakoid • Bound by proteins • Proteins are organized into photosystems • Two types • Photosystem I • Photosystem II

  11. Photosystem I and II • Composed of • Large antenna complex • 250-400 pigment molecules surrounding reaction centre • Reaction Centre • Small number of proteins bound to chlorophyll a moelcules and PEA • PI - Contains p700 • PII - Contains p680

  12. How Photosystem I and II Work • Trap photons of light • Energy trapped used to energize chlorophyll a molecule in reaction centre • Chlorophyll a is oxidized (loses electrons) • Transfers electrons to PEA • Water molecule donates electron chlorophyll a lost • Transported through electron transport chain • High energy electrons are used to drive ATP synthesis and assemble glucose molecules • http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120072/bio13.swf::Photosynthetic%20Electron%20Transport%20and%20ATP%20Synthesis

  13. How Photosystem I and II Work

  14. Light Dependant Reactions Photosystem II

  15. Linear Electron Transport and ATP Synthesis

  16. The Role of Light Energy • Z scheme • Two photons of light needed for production of NADPH

  17. Oxygen • How many photons of light are needed to produce a single molecule of oxygen? • 2 H₂O → 4 H⁺ + 4 e⁻ + O₂

  18. Cyclic Electron Transport • PI can function independently from PII • Produces additional ATP molecules • Reduction of CO₂ requires ATP

  19. Light-Independent Reactions • Carbon Fixation • Series of 11 enzyme-catalyzed reactions • NADPH reduces CO₂ into sugars • Overall process is endergonic • ATP is hydrolyzed to supply energy of reactions • Divided into three phases • Fixation • Reduction • Regeneration

  20. Calvin Cycle: Fixation: C₃ Metabolism • CO2 is attached to 5-carbon RuBPmolecule • Result in a 6-carbon molecule • This splits into two 3-carbon molecules (3PG) • Reaction accelerated by RuBPCarboxylase (Rubisco) • CO2 now “fixed” because it is part of a carbohydrate

  21. Calvin Cycle: Reduction • Two 3PG is phosphorylated • ATP is used • Molecule is reduced by NADPH • Two G3P are produced

  22. Calvin Cycle: Regeneration • RuBP is regenerated for cycle to continue • Takes 3 cycles • Produces 3 RuBP molecules • Process (3 turns of cycle) • 3CO₂ combine with 3 molecules of RuBP • 6 molecules of 3PG are formed • 6 3PG converted to 6 G3P • 5 G3P used to regenerate 3 RuBP molecules • 1 G3P left over

  23. G3P • Ultimate goal of photosynthesis • Raw material used to synthesize all other organic plant compounds (glucose, sucrose, starch, cellulose) • What is required to make 1 molecule of G3P? • 9 ATP • 6 NADPH • What is required to make 1 molecule of glucose? • 18 ATP • 12 NADPH • 2 G3P

  24. Alternate Mechanisms of Carbon Fixation • Problems with photosynthesis • Not enough CO₂- 0.04% of atmosphere • Rubisco • can also catalyze O₂ • Slows Calvin Cycle, consumes ATP, releases carbon – photorespiration • Decrease carbon fixation up to 50% • Stomata • Hot dry climates • Low levels of CO₂

  25. C₄ Metabolism

  26. C₄ Plants • Minimize photorespiration • Calvin Cycle • Performed by bundle-sheath cells • Separates exposure of Rubisco to O₂ • C₄ Cycle • CO₂ combines with PEP (3 carbon molecule) • Produces oxaloacetate (4 carbon molecule) • Oxaloacetate reduced to malate • Malate diffuses into bundle-sheath cells and enters chloroplast • Malate oxidized to pyruvate releasing CO₂

  27. C₄ vs C₃ • C₄ • Can open stomata less • Require 1/3 to 1/6 as much rubisco • Lower nitrogen demand • Run Calvin cycle and C₄ cycles simultaneously

  28. CAM Plants • CrassulaceanAcid Metabolism • Run Calvin Cycle and C₄ at different time of the day • C₄ - night • Calvin Cycle – day

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