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Harvesting Chemical Energy: Cellular Respiration

Harvesting Chemical Energy: Cellular Respiration. Chapter 8. Cellular Respiration: THE BIG PICTURE. Cellular respiration is the process by which organisms can get energy (ATP) from their food (glucose) Cellular respiration is CRITICAL for life Occurs in BOTH plants and animals

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Harvesting Chemical Energy: Cellular Respiration

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  1. Harvesting Chemical Energy: Cellular Respiration Chapter 8

  2. Cellular Respiration:THE BIG PICTURE • Cellular respiration is the process by which organisms can get energy (ATP) from their food (glucose) • Cellular respiration is CRITICAL for life • Occurs in BOTH plants and animals • Two main mechanisms • Aerobic cellular Respiration – Requires Oxygen • Anaerobic cellular Respiration –Does not require Oxygen

  3. Aerobic Respiration Evolved later Require oxygen Start with glycolysis in cytoplasm Completed in mitochondria Anaerobic Respiration Evolved first Don’t require oxygen Start with glycolysis in cytoplasm Completed in cytoplasm Main Types of Cellular Respiration Pathways

  4. Aerobic Respiration • Overall Equation: C6H1206 + 6O2 6CO2 + 6H20 glucose oxygen carbon water dioxide • Several steps occur in the middle (intermediates) • Each step (rxn) catalyzed by enzymes

  5. Aerobic respiration Overview Stage One: • Glycolysis (cytoplasm) Stage Two: • Preparation for Krebs (mitochondrial matrix) • Krebs Cycle (matrix) Stage Three: • Electron Transfer Chain (across inner membrane of mitochondria)

  6. The Role of Coenzymes • Several oxidation-reduction rxns take place in aerobic respiration (Glucose gets oxidized to carbon dioxide) • In order to aid in the redox rxns, enzymes use coenzymes NAD+ and FAD to carry electrons from glucose derivatives to the electron transfer chain • NAD+ and FAD accept electrons and hydrogen to become NADH and FADH2during the first two stages of aerobic respiration (Glycolysis, Krebs) and deliver electrons and hydrogen to the Electron Transfer Chain to make ATP

  7. Coenzymes: NAD+ & FAD Nicotinamide adenine dinucleotide (NAD+)

  8. Stage One: Glycolysis • Glucose (6-carbon) is broken down into 2 molecules of pyruvate (3-carbon) • Yields 2 ATP by substrate level phosphorylation

  9. Glycolysis: Overall Reaction 2 ATP 2 ADP G3P (3C) Pyruvate (3C) 1 NADH 1 NAD+ 2 ADP 2 ATP Glucose (6C) Fructose Bisphosphate (6C) 1 NADH 1 NAD+ G3P (3C) Pyruvate (3C) 2 ATP 2 ADP

  10. Aerobic Respiration:1. Glycolysis

  11. Glycolysis: Net Yield Energy requiring steps: 2 ATP invested Energy releasing steps: 2 NADH formed 4 ATP formed Net yield: 2 ATP + 2 NADH + 2 molecules of Pyruvate

  12. What happens next? • Depends on the organism and the presence of oxygen • If oxygen is around: Aerobic respiration, proceed to Krebscycle • If no oxygen: Anaerobic respiration, Proceed to Fermentation

  13. Second Stage: Krebs cycle • Preparatory reactions: Oxidation of pyruvate • Pyruvate is oxidized into two-carbon acetyl units and carbon dioxide • NAD+ is reduced • Krebs cycle • The acetyl units are oxidized to carbon dioxide • NAD+and FAD are reduced

  14. Oxidation of Pyruvate

  15. Results of the Second Stage • All of the carbon atoms in pyruvate end up in carbon dioxide • Coenzymes are reduced (they pick up electrons and hydrogen) • One molecule of ATP forms • Four-carbon oxaloacetate regenerates

  16. Coenzyme Reductions during First Two Stages • Glycolysis 2 NADH • Preparatory reactions 2 NADH • Krebs cycle 2 FADH2 + 6 NADH • Total 2 FADH2 + 10 NADH

  17. Electron Transfer Chain • Occurs in the mitochondria • Coenzymes deliver electrons to electron transfer chains • Electron transfer sets up H+ ion gradients • Flow of H+ down gradients powers ATP formation

  18. Importance of Oxygen Why does aerobic respiration require oxygen? • Oxygen acts as the final electron (and hydrogen ion) acceptor in ETC • Binds to leftover e- and H+ to form water • “Clean up crew”

  19. ATP Actual Yield

  20. Energy Harvest Varies • NADH formed in cytoplasm cannot enter mitochondrion • It delivers electrons to mitochondrial membrane • Membrane proteins shuttle electrons to NAD+ or FAD inside mitochondrion • Electrons given to FAD yield less ATP than those given to NAD+

  21. Efficiency of Aerobic Respiration • 686 kcal of energy are released • 7.5 kcal are conserved in each ATP • When 32 ATP form, 240 kcal (32 X 7.5) are captured in ATP • Efficiency is 240 / 686 X 100 = 35% • Most energy is lost as heat

  22. Anaerobic Pathways • Do not use oxygen • Produce less ATP than aerobic pathways • Two types • Fermentation pathways • Anaerobic electron transport

  23. Fermentation Pathways • Begin with glycolysis • Do not break glucose down completely to carbon dioxide and water • Yield only the 2 ATP from glycolysis • Steps that follow glycolysis serve only to regenerate NAD+

  24. Evolution of Metabolic Pathways • When life originated, atmosphere had little oxygen • Earliest organisms used anaerobic pathways • Later, noncyclic pathway of photosynthesis increased atmospheric oxygen • Cells arose that used oxygen as final acceptor in electron transport

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