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Photosynthesis - PowerPoint PPT Presentation

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Photosynthesis

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  1. Photosynthesis • Conversion of light energy from the sun into stored chemical energy in the form of glucose and other organic molecules

  2. Site of Photosynthesis • Photosynthesis takes place in mesophyll tissue • Cells containing chloroplasts • Specialized to carry out photosynthesis • CO2 enters leaf through stomata (pore) • Exchange of gases occurs here • Controlled by guard cells (opening/closing) • CO2 diffuses into chloroplasts • CO2 fixed to C6H12O6 (sugar) • Energy supplied by light

  3. Chloroplasts • Site of Photosynthesis • Consists of • Stroma • Aqueous environment • Houses enzymes used for reactions • Thylakoid membranes • Form stacks of flattened disks called grana • Contains chlorophyll and other pigments

  4. Photosynthesis • 2 stages • Light-dependant reactions • Photosystem II and I • Occurs in the thylakoid membrane of chloroplasts • capture energy from sunlight • make ATP and reduce NADP+ to NADPH 2. Calvin Cycle (light-independent reactions) • Occurs in stroma of chloroplast • use ATP and NADPH to synthesize organic molecules from CO2

  5. Capturing Light Energy • Pigments • Absorb photon (wave of light) • Excited electron moves to a high energy state • Electron is transferred to an electron accepting molecule (primary electron acceptor) • Chloryphyll a • donates electrons to PEA

  6. Accessory Pigments • Chlorophyll b and carotenoids • Known as antenna complex • Transfers light energy to chlorophyll a • Chloryphyll donates electrons to PEA • A pigment molecule does not absorb all wavelengths of light

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

  8. 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

  9. 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 molecules and PEA • PI - Contains p700 • PII - Contains p680

  10. Photosystem II • Oxidation of p680 • Photon absorbed excites p680 • Transfers e⁻ to PEA • e⁻ supplied by splitting of a water molecule inside lumen • Oxidation-reduction of platiquinone • PEA transfers e⁻ to plastiquinone • Plastiquinone • shuttles electrons between PII and cytochrome complex • responsible for increase proton concentration in thylakoid lumen 3. Electron transfer to PI • Cytochrome complex transfers e⁻ to plastocyanin • Plastocyanin • Shuttles electrons from cytochrome complex to PI

  11. Photosystem I • Oxidation-reduction of p700 • Photon absorbed excites p700 • p700 transfers electron to PEA • P700⁺ forms ready to accept another e⁻ from plastocyanin • Electron transfer to NADP⁺ by ferredoxin • PEA transfer e⁻ to ferredoxin • Ferredoxin • Iron-sulfur protein • Oxidation of ferredoxin reduces NADP⁺ to NADP • Formation of NADPH • Ferredoxin transfers second e⁻ and H⁺ • NADP⁺ reductase reduces NADP to NADPH

  12. Linear Electron Transport and ATP Synthesis

  13. The Role of Light Energy • Z scheme • Two photons of light needed for production of NADPH • p700 molecule too electronegative to give up e⁻ • Second photon needed to move e⁻ further away from nucleus of p700 so it can transfer to NADP⁺

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

  15. Chemiosmosis and ATP Synthesis • Proton gradient inside lumen increases • e⁻ transfer by plastoquinone between PII and cytochrome complex • Water molecule splitting inside lumen • Removal of H⁺ from stroma for each NADPH molecule produced • Proton-motive force created inside thylakoid lumen • ATP synthase uses proton-motive force to synthesize ATP molecule

  16. Cyclic Electron Transport • PI can function independently from PII • Ferredoxin does not transfer e⁻ to NADP⁺ • Ferredoxin transfers e⁻ back to plastoquinone • Plastoquinone continually moves protons into thylakoid lumen • Splitting of water molecule not needed • Produces additional ATP molecules (photophosphorylation) • Reduction of CO₂ requires ATP

  17. 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

  18. Calvin Cycle: Fixation • CO₂ is attached to 5C RuBP molecule • 6C molecule is produced • 6C splits into 2 3C molecules (3PG) • RuBisco • RuBPcarboxylase • Most abundant protein on earth • Involvd in first major step of carbon fixation • CO₂ is now fixed • Becomes part of carbohydrate

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

  20. 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

  21. Glyceraldehyde-3-phosphate (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

  22. 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% • Wasteful to cell • Costs 1 ATP and 1 NADPH • Stomata • Hot dry climates – closes to prevent water loss • Low levels of CO₂

  23. C₄ Cycle • 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

  24. Benefits of C4 Plants • Can open stomata less • Require 1/3 to 1/6 as much rubisco • Lower nitrogen demand • Run C3 and C4 cycles simultaneously • Corn

  25. CAM Plants • Crassulacean Acid Metabolism • Run Calvin Cycle and C4at different time of the day • C4 - night • Calvin Cycle – day • Cactus