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Where It Starts: Photosynthesis

Where It Starts: Photosynthesis. Introduction. Before photosynthesis evolved, Earth’s atmosphere had little free oxygen Oxygen released during photosynthesis changed the atmosphere Favored evolution of new metabolic pathways, including aerobic respiration. Electromagnetic Spectrum.

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Where It Starts: Photosynthesis

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  1. Where It Starts:Photosynthesis

  2. Introduction • Before photosynthesis evolved, Earth’s atmosphere had little free oxygen • Oxygen released during photosynthesis changed the atmosphere • Favored evolution of new metabolic pathways, including aerobic respiration

  3. Electromagnetic Spectrum

  4. Overview of Photosynthesis • Photosynthesis proceeds in two stages • Light-dependent reactions • Light-independent reactions Summary equation: 6H2O + 6CO2 6O2 + C6H12O6

  5. sunlight Light- Dependent Reactions H2O O2 NADPH ADP + Pi NADP+ ATP Light- Independent Reactions Calvin-Benson cycle H2O CO2 phosphorylated glucose end products (e.g., sucrose, starch, cellulose) Fig. 6.13, p.104

  6. Sites of Photosynthesis: Chloroplasts • Light-dependent reactions occur at a much-folded thylakoid membrane • Forms a single, continuous compartment inside the stroma (chloroplast’s semifluid interior) • Light-independent reactions occur in the stroma

  7. Sites of Photosynthesis

  8. Sites of Photosynthesis

  9. Sites of Photosynthesis

  10. Products of Light-Dependent Reactions • Typically, sunlight energy drives the formation of ATP and NADPH • Oxygen is released from the chloroplast (and the cell)

  11. electron transfer chain light energy electron transfer chain light energy NADPH Photosystem II Photosystem I THYLAKOID COMPARTMENT THYLAKOID MEMBRANE oxygen (diffuses away) STROMA Fig. 6.8b, p.99

  12. ATP Formation • In both pathways, electron flow through electron transfer chains causes H+ to accumulate in the thylakoid compartment • A hydrogen ion gradient builds up across the thylakoid membrane • H+ flows back across the membrane through ATP synthases • Results in formation of ATP in the stroma

  13. Light Independent Reactions:The Sugar Factory • Light-independent reactions proceed in the stroma • Carbon fixation: Enzyme rubisco attaches carbon from CO2 to RuBP to start the Calvin–Benson cycle

  14. Calvin–Benson Cycle • Cyclic pathway makes phosphorylated glucose • Uses energy from ATP, carbon and oxygen from CO2, and hydrogen and electrons from NADPH • Reactions use glucose to form photosynthetic products (sucrose, starch, cellulose) • Six turns of Calvin–Benson cycle fix six carbons required to build a glucose molecule from CO2

  15. Light-Independent Reactions

  16. Adaptations: Different Carbon-Fixing Pathways • Environments differ • Plants have different details of sugar production in light-independent reactions • On dry days, plants conserve water by closing their stomata • O2 from photosynthesis cannot escape

  17. A Burning Concern • Photoautotrophs remove CO2 from atmosphere; metabolic activity of organisms puts it back • Human activities disrupt the carbon cycle • Add more CO2 to the atmosphere than photoautotrophs can remove • Imbalance contributes to global warming

  18. Fossil Fuel Emissions

  19. How Cells Release Chemical Energy

  20. Overview of Carbohydrate Breakdown Pathways • All organisms (including photoautotrophs) convert chemical energy of organic compounds to chemical energy of ATP • ATP is a common energy currency that drives metabolic reactions in cells

  21. Pathways of Carbohydrate Breakdown • Start with glycolysis in the cytoplasm • Convert glucose and other sugars to pyruvate • Fermentation pathways • End in cytoplasm, do not use oxygen, yield 2 ATP per molecule of glucose • Aerobic respiration • Ends in mitochondria, uses oxygen, yields up to 36 ATP per glucose molecule

  22. Pathways of Carbohydrate Breakdown

  23. Overview of Aerobic Respiration • Three main stages of aerobic respiration: 1. Glycolysis 2. Krebs cycle 3. Electron transfer chain Summary equation: C6H12O6 + 6O2 → 6CO2 + 6 H2O

  24. Overview of Aerobic Respiration

  25. Glycolysis – Glucose Breakdown Starts • Enzymes of glycolysis use two ATP to convert one molecule of glucose to two molecules of three-carbon pyruvate • Reactions transfer electrons and hydrogen atoms to two NAD+ (reduces to NADH) • 4 ATP form by substrate-level phosphorylation

  26. Products of Glycolysis • Net yield of glycolysis: • 2 pyruvate, 2 ATP, and 2 NADH per glucose • Pyruvate may: • Enter fermentation pathways in cytoplasm • Enter mitochondria and be broken down further in aerobic respiration

  27. Second Stage of Aerobic Respiration • The second stage of aerobic respiration takes place in the inner compartment of mitochondria • It starts with acetyl-CoA formation and proceeds through the Krebs cycle

  28. Acetyl-CoA Formation • Two pyruvates from glycolysis are converted to two acetyl-CoA • Two CO2 leave the cell • Acetyl-CoA enters the Krebs cycle

  29. Krebs Cycle • Each turn of the Krebs cycle, one acetyl-CoA is converted to two molecules of CO2 • After two cycles • Two pyruvates are dismantled • Glucose molecule that entered glycolysis is fully broken down

  30. Energy Products • Reactions transfer electrons and hydrogen atoms to NAD+ and FAD • Reduced to NADH and FADH2 • ATP forms by substrate-level phosphorylation • Direct transfer of a phosphate group from a reaction intermediate to ADP

  31. Fig. 7.6a, p.113

  32. Third Stage:Aerobic Respiration’s Big Energy Payoff • Coenzymes deliver electrons and hydrogen ions to electron transfer chains in the inner mitochondrial membrane • Energy released by electrons flowing through the transfer chains moves H+ from the inner to the outer compartment

  33. Hydrogen Ions and Phosphorylation • H+ ions accumulate in the outer compartment, forming a gradient across the inner membrane • H+ ions flow by concentration gradient back to the inner compartment through ATP synthases (transport proteins that drive ATP synthesis)

  34. The Aerobic Part of Aerobic Respiration • Oxygen combines with electrons and H+ at the end of the transfer chains, forming water • Overall, aerobic respiration yields up to 36 ATP for each glucose molecule

  35. Electron Transfer Chain

  36. Summary: Aerobic Respiration

  37. Anaerobic Pathways • Lactic acid fermentation • End product: Lactic acid (lactate) • Alcoholic fermentation • End product: Ethyl alcohol (or ethanol) • Both pathways have a net yield of 2 ATP per glucose (from glycolysis)

  38. Alcoholic Fermentation

  39. Muscles and Lactate Fermentation

  40. Life’s Unity • Photosynthesis and aerobic respiration are interconnected on a global scale • In its organization, diversity, and continuity through generations, life shows unity at the bioenergetic and molecular levels

  41. Energy, Photosynthesis, and Aerobic Respiration

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