Energy Transfer in Cellular Processes: Redox and ATP Synthesis

1. LE 9-2 Light energy ECOSYSTEM Photosynthesis in chloroplasts Organic molecules CO2 + H2O + O2 Cellular respiration in mitochondria ATP powers most cellular work Heat energy

2. Redox Reactions: Oxidation and Reduction • The transfer of electrons during chemical reactions releases energy stored in organic molecules • This released energy is ultimately used to synthesize ATP

3. becomes oxidized(loses electron) Xe- + Y X + Ye- becomes reduced(gains electron) The Principle of Redox • Chemical reactions that transfer electrons between reactants are called oxidation-reduction reactions, or redox reactions • In oxidation, a substance loses electrons, or is oxidized • In reduction, a substance gains electrons, or is reduced (the amount of positive charge is reduced)

4. The electron donor is called the reducing agent • The electron receptor is called the oxidizing agent

5. becomes oxidized C6H12O6 + 6O2 6CO2 + 6H2O + Energy becomes reduced Oxidation of Organic Fuel Molecules During Cellular Respiration • During cellular respiration, the fuel (such as glucose) is oxidized and oxygen is reduced:

6. Stepwise Energy Harvest via NAD+ and the Electron Transport Chain • In cellular respiration, glucose and other organic molecules are broken down in a series of steps • Electrons from organic compounds are usually first transferred to NAD+, a coenzyme • As an electron acceptor, NAD+ functions as an oxidizing agent during cellular respiration • Each NADH (the reduced form of NAD+) represents stored energy that is tapped to synthesize ATP

7. LE 9-4 2e–+ 2H+ 2e–+ H+ H+ NADH NAD+ Dehydrogenase + 2[H] (from food) H+ + Nicotinamide (reduced form) Nicotinamide (oxidized form)

8. LE 9-6_1 Glycolysis Pyruvate Glucose Cytosol Mitochondrion ATP Substrate-level phosphorylation

9. LE 9-6_2 Glycolysis Citric acid cycle Pyruvate Glucose Cytosol Mitochondrion ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation

10. LE 9-6_3 Electrons carried via NADH and FADH2 Electrons carried via NADH Oxidative phosphorylation: electron transport and chemiosmosis Glycolysis Citric acid cycle Pyruvate Glucose Cytosol Mitochondrion ATP ATP ATP Substrate-level phosphorylation Oxidative phosphorylation Substrate-level phosphorylation

11. LE 9-8 Energy investment phase Glucose 2 ATP 2 ADP + 2 P used Citric acid cycle Glycolysis Oxidative phosphorylation Energy payoff phase formed ATP ATP ATP 4 ADP + 4 P 4 ATP 2 NADH + 2 H+ 2 NAD+ + 4 e– + 4 H+ 2 Pyruvate + 2 H2O Net 2 Pyruvate + 2 H2O Glucose 4 ATP formed – 2 ATP used 2 ATP 2 NAD+ + 4 e– + 4 H+ 2 NADH + 2 H+

12. Concept 9.2: Glycolysis harvests energy by oxidizing glucose to pyruvate • Glycolysis (“splitting of sugar”) breaks down glucose into two molecules of pyruvate • Glycolysis occurs in the cytoplasm and has two major phases: • Energy investment phase • Energy payoff phase

13. LE 9-9a_1 Citric acid cycle Glycolysis Oxidation phosphorylation ATP ATP ATP Glucose ATP Hexokinase ADP Glucose-6-phosphate

14. LE 9-9a_2 Citric acid cycle Glycolysis Oxidation phosphorylation ATP ATP ATP Glucose ATP Hexokinase ADP Glucose-6-phosphate Phosphoglucoisomerase Fructose-6-phosphate ATP Phosphofructokinase ADP Fructose- 1, 6-bisphosphate Aldolase Isomerase Dihydroxyacetone phosphate Glyceraldehyde- 3-phosphate

15. LE 9-9b_1 2 NAD+ Triose phosphate dehydrogenase NADH 2 + 2 H+ 1, 3-Bisphosphoglycerate 2 ADP Phosphoglycerokinase 2 ATP 3-Phosphoglycerate Phosphoglyceromutase 2-Phosphoglycerate

16. LE 9-9b_2 2 NAD+ Triose phosphate dehydrogenase NADH 2 + 2 H+ 1, 3-Bisphosphoglycerate 2 ADP Phosphoglycerokinase 2 ATP 3-Phosphoglycerate Phosphoglyceromutase 2-Phosphoglycerate Enolase 2 H2O Phosphoenolpyruvate 2 ADP Pyruvate kinase 2 ATP Pyruvate

17. Concept 9.3: The citric acid cycle completes the energy-yielding oxidation of organic molecules • Before the citric acid cycle can begin, pyruvate must be converted to acetyl CoA, which links the cycle to glycolysis

18. LE 9-10 MITOCHONDRION CYTOSOL NAD+ NADH + H+ Acetyl Co A CO2 Coenzyme A Pyruvate Transport protein

19. The citric acid cycle, also called the Krebs cycle, takes place within the mitochondrial matrix • The cycle oxidizes organic fuel derived from pyruvate, generating one ATP, 3 NADH, and 1 FADH2 per turn

20. LE 9-11 Pyruvate (from glycolysis, 2 molecules per glucose) Citric acid cycle Glycolysis Oxidation phosphorylation CO2 NAD+ CoA NADH ATP ATP ATP + H+ Acetyl CoA CoA CoA Citric acid cycle 2 CO2 FADH2 3 NAD+ 3 NADH FAD + 3 H+ ADP + P i ATP

21. LE 9-12_1 Citric acid cycle Glycolysis Oxidation phosphorylation ATP ATP ATP Acetyl CoA H2O Oxaloacetate Citrate Isocitrate Citric acid cycle

22. LE 9-12_2 Citric acid cycle Glycolysis Oxidation phosphorylation ATP ATP ATP Acetyl CoA H2O Oxaloacetate Citrate Isocitrate CO2 Citric acid cycle NAD+ NADH + H+ a-Ketoglutarate CO2 NAD+ NADH Succinyl CoA + H+

23. LE 9-12_3 Citric acid cycle Glycolysis Oxidation phosphorylation ATP ATP ATP Acetyl CoA H2O Oxaloacetate Citrate Isocitrate CO2 Citric acid cycle NAD+ NADH + H+ Fumarate a-Ketoglutarate FADH2 CO2 NAD+ FAD Succinate NADH P i Succinyl CoA + H+ GDP GTP ADP ATP

24. LE 9-12_4 Citric acid cycle Glycolysis Oxidation phosphorylation ATP ATP ATP Acetyl CoA NADH H2O + H+ NAD+ Oxaloacetate Malate Citrate Isocitrate CO2 Citric acid cycle NAD+ H2O NADH + H+ Fumarate a-Ketoglutarate FADH2 CO2 NAD+ FAD Succinate NADH P i Succinyl CoA + H+ GDP GTP ADP ATP

25. Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis • Following glycolysis and the citric acid cycle, NADH and FADH2 account for most of the energy extracted from food • These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation

26. The Pathway of Electron Transport • The electron transport chain is in the cristae of the mitochondrion • Most of the chain’s components are proteins, which exist in multiprotein complexes • The carriers alternate reduced and oxidized states as they accept and donate electrons • Electrons drop in free energy as they go down the chain and are finally passed to O2, forming water

27. LE 9-13 NADH 50 FADH2 Multiprotein complexes I FAD 40 FMN II Fe•S Fe•S Q III Cyt b Oxidative phosphorylation: electron transport and chemiosmosis Citric acid cycle Glycolysis Fe•S 30 Cyt c1 IV Free energy (G) relative to O2 (kcal/mol) Cyt c ATP ATP ATP Cyt a Cyt a3 20 10 2 H+ + 1/2 O2 0 H2O

28. The electron transport chain generates no ATP • The chain’s function is to break the large free-energy drop from food to O2 into smaller steps that release energy in manageable amounts

29. Chemiosmosis: The Energy-Coupling Mechanism • Electron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space • H+ then moves back across the membrane, passing through channels in ATP synthase • ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP • This is an example of chemiosmosis, the use of energy in a H+ gradient to drive cellular work

30. The energy stored in a H+ gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis • The H+ gradient is referred to as a proton-motive force, emphasizing its capacity to do work

31. LE 9-15 Inner mitochondrial membrane Oxidative phosphorylation: electron transport and chemiosmosis Citric acid cycle Glycolysis ATP ATP ATP H+ H+ H+ H+ Cyt c Protein complex of electron carriers Intermembrane space Q IV III I ATP synthase II Inner mitochondrial membrane H2O 2H+ + 1/2 O2 FADH2 FAD NAD+ NADH + H+ ATP ADP + P i (carrying electrons from food) H+ Mitochondrial matrix Electron transport chain Electron transport and pumping of protons (H+), Which create an H+ gradient across the membrane Chemiosmosis ATP synthesis powered by the flow of H+ back across the membrane Oxidative phosphorylation

32. LE 9-16 Electron shuttles span membrane MITOCHONDRION CYTOSOL 2 NADH or 2 FADH2 2 NADH 2 NADH 6 NADH 2 FADH2 Oxidative phosphorylation: electron transport and chemiosmosis Glycolysis 2 Acetyl CoA Citric acid cycle 2 Pyruvate Glucose + 2 ATP + 2 ATP + about 32 or 34 ATP by substrate-level phosphorylation by substrate-level phosphorylation by oxidation phosphorylation, depending on which shuttle transports electrons form NADH in cytosol About 36 or 38 ATP Maximum per glucose:

33. Concept 9.5: Fermentation enables some cells to produce ATP without the use of oxygen • Cellular respiration requires O2 to produce ATP • Glycolysis can produce ATP with or without O2 (in aerobic or anaerobic conditions) • In the absence of O2, glycolysis couples with fermentation to produce ATP

34. Types of Fermentation • Fermentation consists of glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysis • Two common types are alcohol fermentation and lactic acid fermentation

35. In alcohol fermentation, pyruvate is converted to ethanol in two steps, with the first releasing CO2 • Alcohol fermentation by yeast is used in brewing, winemaking, and baking

36. LE 9-17a P 2 ADP + 2 2 ATP i Glycolysis Glucose 2 Pyruvate 2 NAD+ 2 NADH CO2 2 + 2 H+ 2 Acetaldehyde 2 Ethanol Alcohol fermentation

37. In lactic acid fermentation, pyruvate is reduced to NADH, forming lactate as an end product, with no release of CO2 • Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt • Human muscle cells use lactic acid fermentation to generate ATP when O2 is scarce

38. LE 9-17b P 2 ADP + 2 2 ATP i Glycolysis Glucose 2 NAD+ 2 NADH CO2 2 + 2 H+ 2 Pyruvate 2 Lactate Lactic acid fermentation

39. The Evolutionary Significance of Glycolysis • Glycolysis occurs in nearly all organisms • Glycolysis probably evolved in ancient prokaryotes before there was oxygen in the atmosphere