1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chapter 9: Cellular Respiration: Harvesting Chemical Energy - Life Is Work Living cells require energy from outside sources Some animals obtain energy by eating plants; others feed on organisms that eat plants Energy flows into an ecosystem as sunlight and leaves as heat Photosynthesis generates oxygen and organic molecules, which are used in cellular respiration Cells use chemical energy stored in organic molecules to regenerate ATP, which powers work

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Outline Cell metabolism is driven by redox reactions – electron transfers Overview of glucose breakdown and NAD+ as an electron carrier for electrons into the ‘electron transport chain’ Steps of cellular respiration – All along the way, we gain electrons that enter into the electron transport chain – Glycolysis – The Citric Acid (TCA) Cycle – Oxidative Phosphorylation – where the electrons that we gain enter into the electron transport chain to drive ATP synthesis Fermentation

3. ECOSYSTEM Light energy Photosynthesis in chloroplasts Cellular respiration in mitochondria Organic molecules + O 2 CO 2 + H 2 O ATP powers most cellular work Heat energy The flow of energy in an ecosystem

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Catabolic pathways yield energy by oxidizing organic fuels to produce ATP Several processes are central to cellular respiration and related pathways The breakdown of organic molecules is exergonic Fermentation is a partial degradation of sugars that occurs without oxygen Cellular respiration consumes oxygen and organic molecules and yields ATP Although carbohydrates, fats, and proteins are all consumed as fuel, it is helpful to trace cellular respiration with the sugar glucose: C 6 H 12 O 6 + 6O 2  6CO 2 + 6H 2 O + Energy (ATP + heat)

5. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 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) The electron donor is called the reducing agent The electron receptor is called the oxidizing agent Xe - + Y X + Ye - becomes oxidized (loses electron) becomes reduced (gains electron)

6. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Oxidation of Organic Fuel Molecules During Cellular Respiration Some redox reactions do not transfer electrons but change the electron sharing in covalent bonds – and thus, there is no change in the charge of the molecules During cellular respiration, the fuel (such as glucose) is oxidized and oxygen is reduced: C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O + Energy becomes oxidized becomes reduced

7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 NAD + Nicotinamide (oxidized form) Dehydrogenase 2 e – + 2 H + 2 e – + H + NADH H+H+ H+H+ Nicotinamide (reduced form) + 2[H] (from food) + 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

8. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings NADH passes the electrons to the electron transport chain Unlike an uncontrolled reaction, the electron transport chain passes electrons in a series of steps instead of one explosive reaction Oxygen pulls electrons down the chain in an energy-yielding tumble The energy yielded by the flow of energy down the electron transport chain is used to regenerate ATP Stepwise Energy Harvest via NAD+ and the Electron Transport Chain

9. 2 H + + 2 e – 2 H (from food via NADH) Controlled release of energy for synthesis of ATP 2 H + 2 e – H2OH2O + 1 / 2 O 2 H2H2 + H2OH2O Explosive release of heat and light energy Cellular respiration Uncontrolled reaction Free energy, G Electron transport chain The Electron Transport Chain

10. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Stages of Cellular Respiration: A Preview Cellular respiration has three stages: – Glycolysis (breaks down glucose into two molecules of pyruvate) – The citric acid cycle (TCA Cycle) (completes the breakdown of glucose) It is also known as The Krebs cycle – Oxidative phosphorylation – energy produced by the flow of electrons down the electron transport chain drives this process (accounts for most of the ATP synthesis) The process that generates most of the ATP is called oxidative phosphorylation because it is powered by redox reactions of the electron transport chain

11. Mitochondrion Glycolysis Pyruvate Glucose Cytosol ATP Substrate-level phosphorylation Glycolysis occurs in the cytoplasm

12. Mitochondrion Glycolysis Pyruvate Glucose Cytosol ATP Substrate-level phosphorylation ATP Substrate-level phosphorylation Citric acid cycle The Citric acid cycle occurs in the mitochondria

13. Mitochondrion Glycolysis Pyruvate Glucose Cytosol ATP Substrate-level phosphorylation ATP Substrate-level phosphorylation Citric acid cycle ATP Oxidative phosphorylation Oxidative phosphorylation: electron transport and chemiosmosis Electrons carried via NADH Electrons carried via NADH and FADH 2 Oxidative phosphorylation occurs in the mitochondria

14. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respiration A small amount of ATP is formed in glycolysis and the citric acid cycle by substrate-level phosphorylation Enzyme ADP P Substrate Product Enzyme ATP + Substrate level phosphorylation Where do you get ATP?

15. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

16. Energy investment phase Glucose 2 ATP used 2 ADP + 2 P 4 ADP + 4 P 4 ATP formed 2 NAD + + 4 e – + 4 H + Energy payoff phase + 2 H + 2 NADH 2 Pyruvate + 2 H 2 O 2 ATP 2 NADH + 2 H + Glucose 4 ATP formed – 2 ATP used 2 NAD+ + 4 e – + 4 H + Net Glycolysis Citric acid cycle Oxidative phosphorylation ATP Glycolysis harvests energy by oxidizing glucose to pyruvate

17. Glucose ATP ADP Hexokinase ATP Glycolysis Oxidation phosphorylation Citric acid cycle Glucose-6-phosphate Phosphoglucoisomerase Phosphofructokinase Fructose-6-phosphate ATP ADP Fructose- 1, 6-bisphosphate Aldolase Isomerase Dihydroxyacetone phosphate Glyceraldehyde- 3-phosphate Glycolysis: Steps 1-5 THE ENERGY INVESTMENT PHASE The breakdown of glucose results in the production of two three carbon molecules. There is a net input of 2 molecules of ATP for substrate level phosphorylation to break down glucose.

18. 2 NAD + Triose phosphate dehydrogenase + 2 H + NADH 2 1, 3-Bisphosphoglycerate 2 ADP 2 ATP Phosphoglycerokinase Phosphoglyceromutase 2-Phosphoglycerate 3-Phosphoglycerate 2 ADP 2 ATP Pyruvate kinase 2 H 2 O Enolase Phosphoenolpyruvate Pyruvate Glycolysis: Steps 6-10 THE ENERGY PAYOFF PHASE The breakdown of glucose results in the production of two molecules of pyruvate, which is then able to enter into the TCA Cycle. There is a net production of 4 molecules of ATP by substrate level phosphorylation of ADP. Therefore, with one molecule of glucose, you have a net production of 2 molecules of ATP. Also, you have produced two high energy electrons stored in NADH.

19. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 This produces two more high energy electrons stored in NADH from one glucose molecule CYTOSOL Pyruvate NAD + MITOCHONDRION Transport protein NADH + H + Coenzyme ACO 2 Acetyl Co A

20. Pyruvate (from glycolysis, 2 molecules per glucose) ATP Glycolysis Oxidation phosphorylation Citric acid cycle NAD + NADH + H + CO 2 CoA Acetyl CoA CoA Citric acid cycle CO 2 2 3 NAD + + 3 H + NADH3 ATP ADP + P i FADH 2 FAD The citric acid cycle, also called the Krebs cycle, takes place within the mitochondrial matrix. The cycle oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH 2 per turn. Thus, the TCA cycle goes Around 2X from one molecule Of glucose, making 2 ATP, 6 NADH and 2 FADH 2. Citric acid cycle

21. ATP Glycolysis Oxidation phosphorylation Citric acid cycle Citric acid cycle Citrate Isocitrate Oxaloacetate Acetyl CoA H2OH2O CO2CO2 NAD + NADH + H +  -Ketoglutarate CO2CO2 NAD + NADH + H + Succinyl CoA Succinate GTP GDP ADP ATP FAD FADH 2 P i Fumarate H2OH2O Malate NAD + NADH + H + The citric acid cycle has eight steps, each catalyzed by a specific enzyme. The acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate The next seven steps decompose the citrate back to oxaloacetate, making the process a cycle The NADH and FADH 2 produced by the cycle relay electrons extracted from food to the electron transport chain Citric acid cycle

22. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings During oxidative phosphorylation, electron transport is coupled to ATP synthesis Following glycolysis and the citric acid cycle, NADH and FADH 2 account for most of the energy extracted from food as stored high energy electrons These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation The electron transport chain happens in the cristae of the mitochondrion Most of the chain’s components are proteins, which exist in multi- protein 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 O 2, forming water

23. ATP Glycolysis Oxidative phosphorylation: electron transport and chemiosmosis Citric acid cycle NADH 50 FADH 2 40 FMN FeS I FAD FeS II III Q FeS Cyt b 30 20 Cyt c Cyt c 1 Cyt a Cyt a 3 IV 10 0 Multiprotein complexes Free energy (G) relative to O2 (kcal/mol) H2OH2O O2O2 2 H + + 1 / 2 The pathway of electron transport in the cristae of the mitochondria

24. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The electron transport chain itself generates no ATP The chain’s function is to break the large free-energy drop from food to O 2 into smaller steps that release energy in manageable amounts Electron transfer in the electron transport chain causes proteins to pump H + from the mitochondrial matrix to the intermembrane space – THIS IS H + /PROTON PUMPING!!! H + then moves back across the membrane, DOWN its concentration gradient, passing through channels in the ATP synthase protein ATP synthase uses the exergonic flow of H + to drive phosphorylation of ADP with P i (inorganic phosphate) to form ATP! This is an example of chemiosmosis, the use of energy in a H + gradient by its movement across a membrane to drive cellular work Chemiosmosis: The Energy-Coupling Mechanism

25. INTERMEMBRANE SPACE H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ ATP MITOCHONDRAL MATRIX ADP + P i A rotor within the membrane spins as shown when H + flows past it down the H + gradient. A stator anchored in the membrane holds the knob stationary. A rod (or “stalk”) extending into the knob also spins, activating catalytic sites in the knob. Three catalytic sites in the stationary knob join inorganic phosphate to ADP to make ATP. 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 ATP synthase

26. Protein complex of electron carriers H+H+ ATP Glycolysis Oxidative phosphorylation: electron transport and chemiosmosis Citric acid cycle H+H+ Q III I II FAD FADH 2 + H + NADH NAD + (carrying electrons from food) Inner mitochondrial membrane Inner mitochondrial membrane Mitochondrial matrix Intermembrane space H+H+ H+H+ Cyt c IV 2H + + 1 / 2 O 2 H2OH2O ADP + H+H+ ATP synthase Electron transport chain Electron transport and pumping of protons (H + ), Which create an H + gradient across the membrane P i Chemiosmosis ATP synthesis powered by the flow of H + back across the membrane Oxidative phosphorylation How the electron transport chain drives oxidative phosphorylation to produce copious amounts of ATP

27. CYTOSOL Electron shuttles span membrane 2 NADH or 2 FADH 2 MITOCHONDRION Oxidative phosphorylation: electron transport and chemiosmosis 2 FADH 2 2 NADH6 NADH Citric acid cycle 2 Acetyl CoA 2 NADH Glycolysis Glucose 2 Pyruvate + 2 ATP by substrate-level phosphorylation + 2 ATP by substrate-level phosphorylation + about 32 or 34 ATP by oxidation phosphorylation, depending on which shuttle transports electrons form NADH in cytosol About 36 or 38 ATP Maximum per glucose: ATP Production by Cellular Respiration During cellular respiration, most energy flows in this sequence: glucose  NADH  electron transport chain  proton-motive force  ATP About 40% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 38 ATP

28. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Spatial separation of metabolic processes inside of the mitochondria Glycolysis in Cytoplasm: Pyruvate enters mitochondria to the krebs cycle!

29. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fermentation enables some cells to produce ATP without the use of oxygen Cellular respiration requires O 2 to produce ATP Glycolysis can produce ATP with or without O 2 (in aerobic or anaerobic conditions) – in anaerobic conditions, you have a net gain 2 molecules of ATP total – what you have from glycolysis only (no TCA cycle or oxidative phosphorylation takes place). In the absence of O 2, glycolysis couples with fermentation to produce ATP Fermentation consists of glycolysis plus reactions that regenerate NAD +, which can be reused by glycolysis – In alcohol fermentation, pyruvate is converted to ethanol in two steps, with the first releasing CO 2 – Alcohol fermentation by yeast is used in brewing, winemaking, and baking

30. CO 2 + 2 H + 2 NADH2 NAD + 2 Acetaldehyde 2 ATP 2 ADP + 2 P i 2 Pyruvate 2 2 Ethanol Alcohol fermentation Glucose Glycolysis

31. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In lactic acid fermentation, pyruvate is reduced to NADH, forming lactate as an end product, with no release of CO 2 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 O 2 is scarce Lactic acid fermentation

32. CO 2 + 2 H + 2 NADH2 NAD + 2 ATP 2 ADP + 2 P i 2 Pyruvate 2 2 Lactate Lactic acid fermentation Glucose Glycolysis

33. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fermentation and Cellular Respiration Compared Both processes use glycolysis to oxidize glucose and other organic fuels to pyruvate The processes have different final electron acceptors: an organic molecule (such as pyruvate) in fermentation and O 2 in cellular respiration Cellular respiration produces MUCH more ATP Yeast and many bacteria are facultative anaerobes, meaning that they can survive using either fermentation or cellular respiration – They usually prefer to live without oxygen, but can survive in its presence In a facultative anaerobe, pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes

34. Pyruvate Glucose CYTOSOL No O 2 present Fermentation Ethanol or lactate Acetyl CoA MITOCHONDRION O 2 present Cellular respiration Citric acid cycle Facultative anaerobes can survive using either fermentation or respiration

35. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Glycolysis and the citric acid cycle connect to many other metabolic pathways Glycolysis occurs in nearly all organisms Glycolysis probably appeared in ancient prokaryotes before there was oxygen in the atmosphere Gycolysis and the citric acid cycle are major intersections to various catabolic and anabolic pathways Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration Glycolysis accepts a wide range of carbohydrates Proteins must be digested to amino acids; amino groups can feed glycolysis or the citric acid cycle Fats are digested to glycerol (used in glycolysis) and fatty acids (used in generating acetyl CoA) An oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbohydrate

36. Citric acid cycle Oxidative phosphorylation Proteins NH 3 Amino acids Sugars Carbohydrates Glycolysis Glucose Glyceraldehyde-3- P Pyruvate Acetyl CoA Fatty acids Glycerol Fats

37. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

38. Biosynthesis (Anabolic Pathways) The body uses small molecules to build other substances These small molecules may come directly from food, from glycolysis, or from the citric acid cycle

39. Citric acid cycle Oxidative phosphorylation Glycolysis Glucose Pyruvate Acetyl CoA Fructose-6-phosphate Phosphofructokinase Fructose-1,6-bisphosphate – Inhibits ATP Citrate Inhibits Stimulates AMP + – Regulation of Cellular Respiration via Feedback Mechanisms Feedback inhibition is the most common mechanism for control If ATP concentration begins to drop, respiration speeds up; when there is plenty of ATP, respiration slows down Control of catabolism is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway