Cellular Respiration: Harvesting Chemical Energy

1. Cellular Respiration: Harvesting Chemical Energy

2. Light energy ECOSYSTEM Photosynthesis in chloroplasts Organic molecules CO2 + H2O + O2 Cellular respiration in mitochondria ATP powers most cellular work Heat energy Cellular Respiration • Energy flows into an ecosystem as sunlight and leaves as heat • Photosynthesis generates oxygen and organic molecules, which are used in cellular respiration • All cells can harvest energy from organic molecules to power work • To do this, they break down the organic molecules and use the energy that is released to make ATP from ADP and phosphate

3. Catabolic Pathways and Production of ATP • Heterotrophs live off the energy produced by autotrophs - extracting energy from food via digestion and catabolism • There are different catabolic pathways used in ATP production: • Fermentation - the partial degradation of sugars in the absence of oxygen. • Cellular respiration - A more efficient and widespread catabolic process that consumes oxygen as a reactant to complete the breakdown of a variety of organic molecules.

4. Catabolic Pathways and Production of ATP • Although carbohydrates, fats, and proteins are all consumed as fuel, it is helpful to trace cellular respiration with the sugar glucose: • The catabolism of glucose is exergonic with a G of −686 kcal per mole of glucose. • Some of this energy is used to produce ATP, which can perform cellular work C6H12O6 + 6O2  6CO2 + 6H2O + Energy (ATP + heat)

5. becomes oxidized(loses electron) Na + Cl Na+ + Cl– becomes reduced(gains electron) Redox Reactions • Catabolic pathways yield energy through the transfer electrons from one reactant to another by oxidation and reduction • Redox reactions • In oxidation - A substance loses electrons, or is oxidized • In reduction - A substance gains electrons, or is reduced

6. becomes oxidized C6H12O6 + 6O2 6CO2 + 6H2O + Energy ~686kcal/mole becomes reduced Oxidation of Organic Fuel Molecules During Cellular Respiration • Cellular respiration provides the energy for the cell using the exergonic reaction: • During cellular respiration glucose is oxidized and oxygen is reduced • Glucose oxidation is accomplished in a series of steps

7. H2 + 1/2 O2 Explosiverelease ofheat and lightenergy (a) Uncontrolled reaction Free energy, G Figure 9.5 A H2O Glucose Oxidation • If electron transfer is not stepwise • A large release of energy occurs • As in the reaction of hydrogen and oxygen to form water

8. 2 e– + 2 H+ 2 e– + H+ NADH NAD+ Dehydrogenase H O O H H Reduction of NAD+ + + 2[H] C NH2 NH2 C (from food) Oxidation of NADH N N+ Nicotinamide(reduced form) Nicotinamide(oxidized form) CH2 O O O O– P O H H O OH O– HO P NH2 HO CH2 O N N H N H N O H H HO OH Figure 9.4 Glucose Catabolism • Glucose catabolism is a series of redox reactions that release energy by repositioning electrons closer to oxygen atoms. • The high energy electrons are stripped from glucose and picked up by NAD+ and FAD. H

9. NADH 50 FADH2 2 H + 1/2 O2 Multiprotein complexes I FAD 40 FMN II (from food via NADH) Fe•S Fe•S Q III Controlled release of energy for synthesis ofATP Cyt b 2 H+ + 2 e– Fe•S 30 Cyt c1 IV Free energy (G) relative to O2 (kcal/mol) ATP Cyt c Cyt a ATP Cyt a3 Free energy, G 20 Electron transport chain ATP 10 2 e– 1/2 O2 2 H+ 2 H+ + 1/2 O2 0 H2O H2O The Electron Transport Chain • Passes electrons in a series of steps instead of in one explosive reaction • Uses the energy from the electron transfer to form ATP • Eventually, the electrons, along with H+, are passed to a final acceptor.

10. Glucose Catabolism • If molecular oxygen (O2) is the final electron acceptor, the process is called aerobic respiration. • If some other inorganic molecule is the final electron acceptor, the process is called anaerobic respiration. • If an organic molecule is the final electron acceptor, the process is called fermentation.

11. The Stages of Cellular Respiration • Respiration is a cumulative function of three metabolic stages • Glycolysis - breaks down glucose into two molecules of pyruvate • The Citric Acid Cycle (Kreb’s) - completes the breakdown of glucose • Oxidative phosphorylation - driven by the electron transport chain and Generates ATP

12. Electrons carried via NADH and FADH2 Electrons carried via NADH Oxidativephosphorylation:electron transport andchemiosmosis Citric acid cycle Glycolsis Pyruvate Glucose Cytosol Mitochondrion ATP ATP ATP Substrate-level phosphorylation Oxidative phosphorylation Substrate-level phosphorylation Cellular Respiration

13. Enzyme Enzyme ADP Substrate P + Product ATP PEP Pyruvate P ATP P P P Enzyme ADP P P Adenosine Adenosine Substrate Phosphorylation • Both glycolysis and the citric acid cycle can generate ATP by substrate-level phosphorylation

14. Glycolysis • Glycolysis harvests energy by oxidizing glucose to pyruvate • Glycolysis • Means “splitting of sugar” • Breaks down glucose into pyruvate • Occurs in the cytoplasm of the cell

15. Citric acid cycle Glycolysis Oxidative phosphorylation ATP ATP ATP Glycolysis • Occurs in the cytoplasm of the cell • Results in the partial breakdown of glucose • Anaerobic – no oxygen is used during glycolysis • For each molecule of glucose that passes through glycolysis, the cell nets two ATP molecules.

16. Glycolysis ATP/NADH Ledger - 2 ATP The energy investment phase carbons Energy coupling ATP  ADP + P: exergonic Glu  Glu-6-P :endergonic

17. Glycolysis ATP/NADH Ledger -2ATP +2 NADH +2ATP The energy payoff phase Redox reactions Energy coupling

18. Glycolysis ATP/NADH Ledger -2ATP +2 NADH +2ATP +2ATP More energy coupling End-products of glycolysis are 2 pyruvate molecules

19. Energy investment phase Glucose 2 ADP + 2 P 2 ATP used Energy payoff phase formed 4 ATP 4 ADP + 4 P 2 NAD+ + 4 e– + 4 H+ 2 NADH + 2 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+ Glycolysis Summary • Occurs in the cytoplasm • Glucose converted to two 3-C chains • Anaerobic - no oxygen • 2 ATP used, 4ATP produced • Inefficient - net yield only 2 ATPs • Not discarded by evolution but used as starting point for energy production • If no O2 - Fermentation occurs • End products: • 2 ATP • Pyruvate (3 C) • 2 x NADH

20. The Citric Acid (Krebs) Cycle The Krebs cycle is named after Hans Krebs and is a metabolic event that follows glycolysis. This process occurs in the fluid matrix of the mitochondrion, uses the pyruvic acid from glycolysis and is aerobic. To begin the Krebs cycle, pyruvic acid is converted to acetyl CoA.

21. MITOCHONDRION CYTOSOL NAD+ NADH + H+ Acetyl Co A CO2 Coenzyme A Pyruvate Transport protein Oxidation of Pyruvate • More energy can be extracted if oxygen is present • Within mitochondria, pyruvate is decarboxylated, yielding acetyl-CoA, NADH, and CO2

22. The Citric Acid (Krebs) Cycle • Occurs in the mitochondrial matrix • Aerobic – although O2 is not used directly in this pathway, it will not occur unless enough is present in the cell. • Main catabolic pathway • Acetyl-CoA is oxidized in a series of nine reactions

23. Citric acid cycle Glycolysis Oxidation phosphorylation ATP ATP ATP Acetyl CoA H2O Oxaloacetate Citrate Isocitrate Citric acid cycle Krebs Cycle • AcetylCoA reacts with oxaloacetate using an enzyme called citrate synthase producing citric acid. • Because of this, the Krebs cycle is sometimes called the citric acid cycle.

24. Krebs Cycle Citric acid cycle Glycolysis Oxidation phosphorylation ATP ATP ATP • The next 7 steps decompose the citrate back to oxaloacetate, • Citric acid is systematically decarboxylated and dehyrogenated in order to use up the acetyl groups that were attached to the oxaloacetate. • This allows oxaloacetate and CoA to be used in the next cycle. 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

25. 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 Krebs Cycle • The NADH and FADH2 produced by the cycle relay electrons extracted from food to the electron transport chain ATP/NADH Ledger + 2 ATP + 6 NADH + 2 FADH2

26. Krebs Cycle

27. ETC and Oxidative Phosphorylation • Occurs along the inner mitochondrial membrane (IMM) in the cristae of the mitochondrion • NADH/FADH2 molecules carry electrons from glycolysis and the citric acid cycle to the inner mitochondrial membrane, where they transfer electrons to a series of membrane-associated proteins.

28. NADH 50 FADH2 Multiprotein complexes I FAD 40 FMN II Fe•S Fe•S Q III Cyt b Fe•S 30 Cyt c1 IV Free energy (G) relative to O2 (kcal/mol) Cyt c Cyt a Cyt a3 20 10 2 H+ + 1/2 O2 0 H2O The Pathway of Electron Transport • 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

29. NADH 50 FADH2 Multiprotein complexes I FAD 40 FMN II Fe•S Fe•S Q III Cyt b Fe•S 30 Cyt c1 IV Free energy (G) relative to O2 (kcal/mol) Cyt c Cyt a Cyt a3 20 10 2 H+ + 1/2 O2 0 H2O The Pathway of Electron Transport • 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

30. Electron Transport Phosphorylation • Electron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space • The ETC uses energy from electrons to pump H+ across a membrane against their concentration gradient - potential energy. • 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

31. Inner mitochondrial membrane Oxidative phosphorylation: electron transport and chemiosmosis Citric acid cycle Glycolysis ATP ATP ATP H+ LE 9-15 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. Intermembrane space H+ H+ H+ H+ H+ H+ H+ H+ Rotor Rod Catalytic head ADP + Pi ATP H+ Mitochondrial matrix 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 • Most of the ATP produced in cells is made by the enzyme ATP synthase • The enzyme is embedded in the membrane and provides a channel through which protons can cross the membrane down their concentration gradient • The energy released causes the rotor and the rod structures to rotate. This mechanical energy is converted to chemical energy with the formation of ATP

33. INTERMEMBRANE SPACE A rotor within the membrane spins as shown when H+ flows past it down the H+ gradient. H+ H+ H+ H+ H+ H+ H+ A stator anchored in the membrane holds the knob stationary. LE 9-14 A rod (or “stalk”) extending into the knob also spins, activating catalytic sites in the knob. H+ Three catalytic sites in the stationary knob join inorganic phosphate to ADP to make ATP. ADP + ATP P i MITOCHONDRAL MATRIX

34. Summary of Glucose Catabolism

35. Theoretical ATP Yield of Aerobic Respiration

36. Catabolism of Proteins and Fats • Proteins are utilized by deaminating their amino acids, and then metabolizing the product. • Fats are utilized by beta-oxidation.

37. Regulating Aerobic Respiration • Control of glucose catabolism occurs at two key points in the catabolic pathway. • Glycolysis - phosphofructokinase • Pyruvate Oxidation – pyruvate decarboxylase

38. Recycling NADH • As long as food molecules are available to be converted into glucose, a cell can produce ATP. • Continual production creates NADH accumulation and NAD+ depletion. • NADH must be recycled into NAD+. • Aerobic respiration - oxygen as electron acceptor • Fermentation - organic molecule

39. Anaerobic Respiration • Final electron acceptor is an inorganic molecule other than oxygen • Some use NO3 -: E. coli • Some use SO42- • Important in nitrogen and sulfur cycles • ATP varies, less than 38 • Only part of Krebs cycle & ETC used

40. Fermentation • In some cases, the high energy electrons picked up by NAD+ during glycolysis are not donated to an ETC. • Instead, NADH donates its extra electrons and H+ directly to an organic molecule, which serves as the final electron acceptor.

41. Fermentation • Pyruvate converted to organic product • NAD+ regenerated • Doesn’t require oxygen • Does not use Krebs cycle or ETC • Organic molecule is final electron acceptor • Produces 2 ATP max

42. 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 Alcohol Fermentation • Occurs in single-celled fungi called yeast • A terminal CO2 is removed from the pyruvic acid (3C) produced during glycolysis, producing acetaldehyde (2C) • Acetaldehyde accepts 2 e- and a H+ from NADH, producing ethanol and NAD+ Glucose H G L Y C O L Y S I S H C OH 2 ADP CH3 2 Ethanol 2 NAD+ 2 ATP 2 NADH H H O C C C O O CO2 CH3 O 2 Acetaldehyde CH3 2 Pyruvic Acid

43. P 2 ADP + 2 2 ATP i Glycolysis Glucose 2 NAD+ 2 NADH CO2 2 + 2 H+ 2 Pyruvate 2 Lactate Lactic acid fermentation Lactic Acid Fermentation • Used by most animal cells when O2 is not available • NADH donates 2 e- and a H+ directly to the pyruvate (3C) produced during glycolysis, producing lactate (3C) and NAD+ Glucose O– G L Y C O L Y S I S C O 2 ADP H C OH CH3 2 ATP 2 Lactate 2 NAD+ O - 2 NADH C O C O CH3 2 Pyruvate

44. Fermentation Alcoholic fermentation and lactic acid fermentation each generate 2 ATP / glucose molecule compared to the theoretical maximum of 36 ATP per glucose during aerobic respiration.