How Cells Release Stored Energy

1. How Cells Release Stored Energy Chapter 8

2. Aerobic pathways Evolved later Require oxygen Start with glycolysis in cytoplasm Completed in mitochondria Anaerobic pathways Evolved first Don’t require oxygen Start with glycolysis in cytoplasm Completed in cytoplasm 8.1 Main Types of Energy-Releasing Pathways

3. Summary Equation for Aerobic Respiration C6H1206 + 6O2 6CO2 + 6H20 glucose oxygen carbon water dioxide

4. CYTOPLASM glucose ATP 4 Overview of Aerobic Respiration 2 ATP Glycolysis e- + H+ (2 ATP net) 2 pyruvate 2 NADH e- + H+ 2 CO2 2 NADH e- + H+ 4 CO2 8 NADH KrebsCycle e- + H+ 2 ATP 2 FADH2 e- Electron Transfer Phosphorylation 32 ATP H+ water e- +oxygen Typical Energy Yield: 36 ATP Figure 8.3Page 135

5. The Role of Coenzymes • NAD+ and FAD accept electrons and hydrogen • Become NADH and FADH2 • Deliver electrons and hydrogen to the electron transfer chain

6. Glucose 8.2 GLYCOLYSIS • A simple sugar (C6H12O6) • Atoms held together by covalent bonds In-text figurePage 136

7. Glycolysis Occurs in Two Stages • Energy-requiring steps • ATP energy activates glucose and its six-carbon derivatives • Energy-releasing steps • The products of the first part are split into three-carbon pyruvate molecules • ATP and NADH form

8. Energy-Requiring Steps of Glycolysis 2 ATP invested glucose ADP ATP ATP P P P P P P glucose-6-phosphate fructose-6-phosphate ADP fructose1,6-bisphosphate PGAL PGAL Energy-Requiring Steps Figure 8.4(2)Page 137

9. ATP ATP ATP ATP P P P P P P P P P P P P PGAL PGAL NAD+ NAD+ Energy-Releasing Steps NADH NADH Pi Pi 1,3-bisphosphoglycerate 1,3-bisphosphoglycerate ADP ADP 3-phosphoglycerate 3-phosphoglycerate 2-phosphoglycerate 2-phosphoglycerate H2O H2O PEP PEP ADP ADP pyruvate pyruvate Figure 8.4 Page 137

10. Glycolysis: Net Energy Yield Energy requiring steps: 2 ATP invested Energy releasing steps: 2 NADH formed 4 ATP formed Net yield is 2 ATP and 2 NADH

11. 8.3 Second Stage Reactions • Preparatory reactions • 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

12. O O Preparatory Reactions pyruvate coenzyme A (CoA) NAD+ carbon dioxide NADH CoA acetyl-CoA

13. ATP O O =CoA acetyl-CoA Krebs Cycle CoA oxaloacetate citrate H2O NADH NAD+ H2O malate isocitrate NAD+ H2O O O NADH fumarate FADH2 a-ketoglutarate FAD NAD+ CoA NADH succinate succinyl-CoA ADP + phosphate group Figure 8.6Page 139

14. Overall Products Coenzyme A 2 CO2 3 NADH FADH2 ATP Overall Reactants Acetyl-CoA 3 NAD+ FAD ADP and Pi The Krebs Cycle

15. Results of the Second Stage • All of the carbon molecules 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. 8.4 Electron Transfer Phosphorylation • 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. Creating an H+ Gradient OUTER COMPARTMENT NADH INNER COMPARTMENT

19. Making ATP: Chemiosmotic Model ATP INNER COMPARTMENT ADP+Pi

20. Importance of Oxygen • Electron transport phosphorylation requires the presence of oxygen • Oxygen withdraws spent electrons from the electron transfer chain, then combines with H+ to form water

21. Summary of Energy Harvest(per molecule of glucose) • Glycolysis • 2 ATP formed by substrate-level phosphorylation • Krebs cycle and preparatory reactions • 2 ATP formed by substrate-level phosphorylation • Electron transport phosphorylation • 32 ATP formed

22. 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+

23. Efficiency of Aerobic Respiration • 686 kcal of energy are released • 7.5 kcal are conserved in each ATP • When 36 ATP form, 270 kcal (36 X 7.5) are captured in ATP • Efficiency is 270 / 686 X 100 = 39 percent • Most energy is lost as heat

24. 8.5 Anaerobic Pathways • Do not use oxygen • Produce less ATP than aerobic pathways • Two types • Fermentation pathways • Anaerobic electron transport

25. 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+

26. Lactate Fermentation GLYCOLYSIS C6H12O6 ATP 2 energy input 2 NAD+ 2 ADP NADH 2 ATP 4 2 pyruvate energy output 2 ATP net LACTATE FORMATION electrons, hydrogen from NADH 2 lactate

27. GLYCOLYSIS C6H12O6 Alcoholic Fermentation 2 ATP 2 NAD+ energy input 2 ADP NADH 2 ATP 4 2 pyruvate energy output 2 ATP net ETHANOL FORMATION 2 H2O 2 CO2 2 acetaldehyde electrons, hydrogen from NADH 2 ethanol

28. Anaerobic Electron Transport • Carried out by certain bacteria • Electron transfer chain is in bacterial plasma membrane • Final electron acceptor is compound from environment (such as nitrate), not oxygen • ATP yield is low

29. FOOD fats glycogen complex carbohydrates proteins simple sugars fatty acids glycerol amino acids glucose-6-phosphate NH3 carbon backbones GLYCOLYSIS urea PGAL pyruvate acetyl-CoA 8.6 ALTERNATIVE ENERGY SOURCES KREBS CYCLE Figure 8.11Page 145

30. 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

31. 8.7 Processes Are Linked sunlight energy PHOTOSYNTHESIS water + carbondioxide sugarmolecules oxygen AEROBICRESPIRATION In-text figurePage 146