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Cellular Respiration: Energy Production and Transfer

This text discusses the process of cellular respiration, which is the most prevalent and efficient catabolic pathway in biology. It explores how organic molecules are oxidized to release energy and produce ATP through stepwise electron transfer. The electron transport chain and its role in ATP synthesis are also explained.

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Cellular Respiration: Energy Production and Transfer

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  1. BSC 2010 - Exam I Lectures and Text Pages • I. Intro to Biology (2-29) • II. Chemistry of Life • Chemistry review (30-46) • Water (47-57) • Carbon (58-67) • Macromolecules (68-91) • III. Cells and Membranes • Cell structure (92-123) • Membranes (124-140) • IV. Introductory Biochemistry • Energy and Metabolism (141-159) • Cellular Respiration (160-180) • Photosynthesis (181-200)

  2. Cellular Respiration • ALL energy ultimately comes from the SUN • Catabolic pathways  Yield energy by oxidizing organic fuels • All the primary organic molecules can be consumed as fuel • We’ll only examine the most common fuel = sugar (C6H12O6) • Exergonic rxn: ∆G = -686 kcal/mol of Glucose (the energy will be used to generate ATP)

  3. Light energy ECOSYSTEM Photosynthesisin chloroplasts Organicmolecules CO2 + H2O + O2 Cellular respirationin mitochondria ATP powers most cellular work Heatenergy Figure 9.2 Energy ultimately comes from the Sun • Energy • Flows into an ecosystem as sunlight and leaves as heat

  4. Catabolic Pathways and Production of ATP • The breakdown of organic molecules is exergonic (releases energy) • Catabolic pathways yield energy by oxidizing organic fuels

  5. Catabolic Pathways • One catabolic process, fermentation • Is a partial degradation of sugars that occurs without oxygen • Involves Glycolysis • Yields 2 ATP/Glucose molecule

  6. Catabolic Pathways • Cellular respiration • Is the most prevalent and efficient catabolic pathway • Consumes oxygen and organic molecules such as glucose • Involves Glycolysis • Yields up to 38 ATP/Glucose molecule • To keep working • Cells must regenerate ATP

  7. Cellular Respiration Redox rxns = oxidation-reduction rxns • Transfer of electrons (e-) releases energy stored in organic molecules  this energy is ultimately used to generate ATP • Oxidation = loss of e- from one substance • Reduction = addition of e- to another substance • Na + Cl  Na+ + Cl- • Na is the reducing agent (donates an e- to CL) • Cl is the oxidizing agent (removes an e- from Na)

  8. Cellular Respiration Respiration is a redox rxn: • By oxidizing glucose, energy stored in glucose is liberated to make ATP • Happens in a series of enzyme-catalyzed steps • Coenzyme (NAD+) acts as e- shuttle • Electron transport chains (ETC) - breaks the energetic fall of e- into several energy-releasing steps (not one big explosive rxn), fig 9.5 • Consists of mostly proteins embedded in the inner mitochondrial membrane • Overview of Respiration: (fig 9.6)

  9. Redox Reactions: Oxidation and Reduction • Catabolic pathways yield energy • Due to the transfer of electrons

  10. The Principle of Redox • Redox reactions • Transfer electrons from one reactant to another by oxidation and reduction • In oxidation • A substance loses electrons, or is oxidized • In reduction • A substance gains electrons, or is reduced

  11. becomes oxidized(loses electron) Na + Cl Na+ + Cl– becomes reduced(gains electron) Examples of redox reactions • Examples of redox reactions

  12. Products Reactants becomes oxidized + + + Energy 2O2 CO2 2 H2O CH4 becomes reduced H C C O O O O H O H H H H Oxygen(oxidizingagent) Methane(reducingagent) Carbon dioxide Water Figure 9.3 Some redox reactions • Do not completely exchange electrons • Change the degree of electron sharing in covalent bonds

  13. becomes oxidized C6H12O6 + 6O2 6CO2 + 6H2O + Energy becomes reduced Oxidation of Organic Fuel Molecules During Cellular Respiration • During cellular respiration • Glucose is oxidized and oxygen is reduced

  14. Stepwise Energy Harvest via NAD+ and the Electron Transport Chain • Cellular respiration • Oxidizes glucose in a series of steps • Allows the cell to use the energy harvested from sugar to power work rather than losing it in one explosive reaction.

  15. 2 e– + 2 H+ 2 e– + H+ NAD+ NADH H Dehydrogenase 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 OH O O– HO P NH2 HO CH2 O N N H N H N O H H HO OH Figure 9.4 Electrons from organic compounds • Are usually first transferred to NAD+, a coenzyme

  16. NADH, the reduced form of NAD+ • Passes the electrons to the electron transport chain • So it is an electron shuttle and moves electrons to the ETC from both glycolysis and from the citric acid cycle.

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

  18. The electron transport chain (ETC) • Passes electrons in a series of steps instead of in one explosive reaction • Uses the energy from the electron transfer to form ATP

  19. 2 H + 1/2 O2 (from food via NADH) Controlled release of energy for synthesis ofATP 2 H+ + 2 e– ATP ATP Free energy, G Electron transport chain ATP 2 e– 1/2 O2 2 H+ H2O Figure 9.5 B (b) Cellular respiration Electron Transport Chain

  20. 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 Figure 9.6 An overview of cellular respiration

  21. Three Stages of Cellular Respiration: A Preview • Respiration is a cumulative function of three metabolic stages • Glycolysis • The citric acid cycle (Kreb’s Cycle) • Oxidative phosphorylation (driven by the ETC)

  22. Stages of Cellular Respiration 1. Glycolysis • Breaks down glucose into two molecules of pyruvate • Produces net 2 ATP and 2 NADH Conversion of pyruvate to acetyl CoA yields 2NADH 2. The citric acid cycle • Completes the breakdown of glucose • Produces net 2 ATP, 6 NADH and 2 FADH2 from 2 Acetyl CoA

  23. Stages of Cellular Respiration 3. Oxidative phosphorylation • Is driven by the electron transport chain (receives electrons from NADH and FADH2) • Generates 32 – 34 ATP

  24. 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 Figure 9.6 An overview of cellular respiration

  25. Cellular Respiration • Glycolysis & Citric Acid Cycle = catabolic pathways that breakdown glucose • Glycolysis  pyruvate + coenzymes + ATP • CAC  coenzymes + ATP • ATP formed by substrate-level phosphorylation (fig 9.7) = enzyme transfers a phosphate group from an organic substrate to ADP to make ATP • Oxidative Phosphorylation = ATP synthesis powered by ETC. Makes 90% of the 38 ATPs

  26. Enzyme Enzyme ADP P Substrate + ATP Product Figure 9.7 Both glycolysis and the citric acid cycle • Can generate ATP by substrate-level phosphorylation

  27. Glycolysis • Glycolysis harvests chemical E by oxidizing glucose to pyruvate • Glucose  Two 3-C sugars  oxidized & rearranged  Two pyruvates • Two Major Phases of Glycolysis • 1. E-investment phase (fig 9.9) • Rearrange glucose + add phosphate groups (uses 2 ATP) • Split 6-C sugar  two 3-C sugar isomers • Glyceraldehyde-3-phosphate form  next phase • 2. E-payoff phase (fig 9.9) • 2 NAD+  2 NADH & a phosphate group added to each of 2 3-C sugars • 4 ATP produced by substrate-level phosphorylation • Rearrangement of remaining phosphate group and the 3-C substrate Final Products from 1 Glucose = 2 ATP + 2 pyruvate + 2NADH

  28. 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

  29. Glycolysis Oxidativephosphorylation Citricacidcycle ATP ATP ATP Energy investment phase Glucose P 2 ADP + 2 used 2 ATP Energy payoff phase formed P 4 ATP 4 ADP + 4 + 2 H+ 2 NADH 2NAD+ + 4 e- + 4H + 2 Pyruvate + 2 H2O Glucose 2 Pyruvate + 2 H2O 4 ATP formed – 2 ATP used 2 ATP 2NADH + 2H+ 2NAD+ + 4e– + 4H + Figure 9.8 Glycolysis • Glycolysis consists of two major phases • Energy investment phase • Energy payoff phase

  30. A closer look at the energy investment phase CH2OH Citric acid cycle H H Oxidative phosphorylation H Glycolysis H HO HO OH H OH Glucose 4 3 5 1 2 ATP Hexokinase ADP CH2OH P O H H H H OH HO H OH Glucose-6-phosphate Phosphoglucoisomerase CH2O P O CH2OH H HO HO H H HO Fructose-6-phosphate ATP Phosphofructokinase ADP CH2 O O CH2 P P O HO H OH H HO Fructose- 1, 6-bisphosphate Aldolase H O CH2 P Isomerase C O O C CHOH CH2OH O CH2 P Dihydroxyacetone phosphate Glyceraldehyde- 3-phosphate Figure 9.9 A Uses 2 ATP. Produces 2 Glyceraldehyde-3-phosphates to feed into energy payoff phase.

  31. A closer look at the energy payoff phase 2 NAD+ Triose phosphate dehydrogenase P i 2 2 NADH + 2 H+ 10 7 9 8 6 2 O C O P CHOH P CH2 O 1, 3-Bisphosphoglycerate 2 ADP Phosphoglycerokinase 2 ATP O– 2 C CHOH O P CH2 3-Phosphoglycerate Phosphoglyceromutase O– 2 C O P C H O CH2OH 2-Phosphoglycerate Enolase 2 H2O O– 2 C O P C O CH2 Phosphoenolpyruvate 2 ADP Pyruvate kinase 2 ATP O– 2 C O C O CH3 Figure 9.8 B Pyruvate Produces 4 ATP, 2 NADH (for ETC), and 2 pyruvates to be converted to Acetyl-CoA and fed into Citric Acid Cycle. So, net of glycolysis is 2 ATP, 2 NADH, and 2 pyruvate.

  32. Citric acid cycle • Citric acid cycle completes the E-yielding oxidation of organic molecules • Pyruvate enters mitochondrion via active transport  converted to acetyl coenzyme A (acetyl CoA) • Happens in 3 rxns catalyzed by a multienzyme complex • Citric acid cycle (also = Krebs cycle) • Citrate (ionized form of citric acid) = 1st molecule produced • Acetyl CoA brings two C atoms to cycle  recycles oxaloacetate  C atoms leave cycle as CO2 (completely oxidized) • Ultimately get CO2, NADH, FADH2, and ATP from the CAC.

  33. CYTOSOL MITOCHONDRION + H+ NAD+ NADH O– CoA S 2 C O C O C O CH3 1 3 CH3 Acetyle CoA Pyruvate CO2 Coenzyme A Transport protein Figure 9.10 Before the citric acid cycle can begin • Pyruvate must first be converted to acetyl CoA, which links the citric acid cycle to glycolysis • Happens in 3 rxns catalyzed by a multienzyme complex. Process yields 2 NADH (for ETC) from 2 pyruvate

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