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How Cells Release Chemical Energy Chapter 7 Biology Concepts and Applications, Eight Edition, by Starr, Evers, Starr. Brooks/Cole, Cengage Learning 2011. 7.1 Overview of Carbohydrate Breakdown Pathways All organisms (including photoautotrophs) convert chemical energy of organic compounds to chemical energy of ATP ATP is a common energy currency that drives metabolic reactions in cells Pathways of Carbohydrate Breakdown Photoautotrophs Photosynthetic autotrophs • Produce sugar Fermentation pathways anaerobic pathway • End in cytoplasm, do not use oxygen, yield 2 ATP per molecule of glucose Aerobic respiration oxygen-requiring pathway that breaks down carbohydrates to produce ATP • Ends in mitochondria, uses oxygen, yields up to 36 ATP per glucose molecule • **Occurs in the presence of OXYGEN** Pathways of Carbohydrate Breakdown Overview of Aerobic Respiration Three main stages of aerobic respiration: 1. Glycolysis in the cytoplasm • Convert glucose and other sugars to (2) pyruvate and (2) ATP • Pyruvate is 3-carbon end product of glycolysis 2. Krebs cycle 3. Electron transfer phosphorylation Summary equation: C6H12O6 + 6O2 → 6CO2 + 6 H2O Overview of Aerobic Respiration Key Concepts: ENERGY FROM CARBOHYDRATE BREAKDOWN All organisms produce ATP by degradative pathways that extract chemical energy from glucose and other organic compounds Aerobic respiration yields the most ATP from each glucose molecule In eukaryotes, aerobic respiration is completed inside mitochondria 7.3 Glycolysis – Glucose Breakdown Starts Enzymes of glycolysis use two ATP to convert one molecule of glucose to two molecules of three-carbon pyruvate Reactions transfer electrons and hydrogen atoms to two NAD+ (reduces to NADH) 4 ATP form by substrate-level phosphorylation • Transfers a phosphate group directly from a substrate to ADP to form ATP Products of Glycolysis Net yield of glycolysis: • 2 pyruvate, 2 ATP, and 2 NADH per glucose Pyruvate may: • Enter fermentation pathways in cytoplasm • Enter mitochondria and be broken down further in aerobic respiration Glycolysis Glycolysis ENERGY REQUIRING PHASE Glycolysis glucose ATP ADP glucose–6–phosphate ATP ADP P P fructose–1,6–bisphosphate DHAP Fig. 7.4c1, p.111 ENERGY PRODUCING 2 NAD + 2 P PHASE + 2 PGAL i NADH 2 reduced coenzymes 2 PGA 2 ADP ATP 2 ATP produced by substrate-level phosphorylation 2 PEP 2 ADP ATP 2 pyruvate 2 ATP produced by substrate-level phosphorylation to second stage Net 2 ATP + 2 NADH Fig. 7.4c2, p.111 Key Concepts: GLYCOLYSIS Glycolysis is the first stage of aerobic respiration and of anaerobic routes (fermentation pathways) As enzymes break down glucose to pyruvate, the coenzyme NAD+ picks up electrons and hydrogen atoms Net energy yield is two ATP 7.4 Second Stage of Aerobic Respiration The second stage of aerobic respiration takes place in the inner compartment of mitochondria It starts with acetyl-CoA formation and proceeds through the Krebs cycle • Kreb cycle cyclic pathway that, along with acetyl-CoA formation, breaks down pyruvate to carbon dioxide Second Stage of Aerobic Respiration Acetyl-CoA Formation Two pyruvates from glycolysis are converted to two acetyl-CoA Two CO2 leave the cell Acetyl-CoA enters the Krebs cycle Krebs Cycle Each turn of the Krebs cycle, one acetyl-CoA is converted to two molecules of CO2 After two cycles • Two pyruvates are dismantled • Glucose molecule that entered glycolysis is fully broken down Energy Products Reactions transfer electrons and hydrogen atoms to NAD+ and FAD • Reduced to NADH and FADH2 ATP forms by substrate-level phosphorylation • Direct transfer of a phosphate group from a reaction intermediate to ADP Net Results Second stage of aerobic respiration results in • Six CO2, two ATP, eight NADH, and two FADH2 for every two pyruvates Adding the yield from glycolysis, the total is • Twelve reduced coenzymes and four ATP for each glucose molecule Coenzymes deliver electrons and hydrogen to the third stage of reactions Second Stage Reactions Acetyl–CoA Formation pyruvate NAD+ coenzyme A NADH CO2 acetyl–CoA coenzyme A Krebs Cycle oxaloacetate citrate CO2 NAD+ Krebs Cycle NADH NADH NAD+ NAD+ FADH2 CO2 NADH FAD ADP + Pi ATP Fig. 7.6a, p.113 Fig. 7.6b, p.113 7.5 Third Stage: Aerobic Respiration’s Big Energy Payoff Coenzymes deliver electrons and hydrogen ions to electron transfer chains in the inner mitochondrial membrane Energy released by electrons flowing through the transfer chains moves H+ from the inner to the outer compartment Hydrogen Ions and Phosphorylation H+ ions accumulate in the outer compartment, forming a gradient across the inner membrane H+ ions flow by concentration gradient back to the inner compartment through ATP synthases (transport proteins that drive ATP synthesis) The Aerobic Part of Aerobic Respiration Oxygen combines with electrons and H+ at the end of the transfer chains, forming water Overall, aerobic respiration yields up to 36 ATP for each glucose molecule Electron Transfer Phosphorylation Fig. 7.7a, p.114 INNER COMPARTMENT H+ NADH FADH2 H+ H+ H+ H+ H+ H+ H2O INNER MITOCHONDRIAL MEMBRANE OUTER COMPARTMENT H+ 1/2 O2 ATP ADP + Pi H+ H+ H+ H+ H+ Fig. 7.7b, p.114 Key Concepts: HOW AEROBIC RESPIRATION ENDS In the Krebs cycle (and a few steps before) • Pyruvate is broken down to carbon dioxide • Coenzymes pick up electrons and hydrogen atoms In electron transfer phosphorylation • Coenzymes deliver electrons to transfer chains that set up conditions for ATP formation Oxygen accepts electrons at end of chains glucose 2 ATP Glycolysis ATP (2 net) 2 NAD+ 2 NADH 2 pyruvate CYTOPLASM OUTER MITOCHONDRIAL COMPARTMENT 2 NADH 2 CO2 2 NADH 6 NADH 2 FADH2 INNER MITOCHONDRIAL COMPARTMENT 2 acetyl-CoA 4 CO2 Krebs Cycle 2 ATP ADP + Pi Electron Transfer Phosphorylation H+ water H+ H+ H+ 32 ATP H+ oxygen Fig. 7.8, p.115 7.6 Anaerobic Energy-Releasing Pathways Different fermentation pathways begin with glycolysis and end in the cytoplasm • Do not use oxygen or electron transfer chains • Final steps do not produce ATP; only regenerate oxidized NAD+ required for glycolysis to continue Anaerobic Pathways Lactate fermentation • End product: ATP & Lactate • Bacteria break down lactose in milk produce buttermilk, cheese, and yogurt • Yeast preserve pickles, cored beef, and sauerkraut Alcoholic fermentation • End product: ATP & Ethyl alcohol (or ethanol) • Yeast to make bread dough rises as CO2 forms bubbles Both pathways have a net yield of 2 ATP per glucose (from glycolysis) and NAD+ Fig. 7.9a, p.116 Fig. 7.9b, p.116 Glycolysis glucose 2 NAD+ 2 ATP 2 NADH 4 ATP pyruvate Lactate Fermentation 2 NADH 2 NAD+ lactate Fig. 7.9c, p.116 Alcoholic Fermentation Animal Skeletal Muscle Red fibers (legs of chicken) • A lot of mitochondria and Myoglobin (stores oxygen) • Produce ATP by aerobic respiration • Sustain prolonged activity (marathon runs) White fibers (wings of chicken) • Few mitochondria and no myoglobin can not carry out a lot of aerobic respiration • Most ATP produced by lactate fermentation • ATP produced quick by not for long • Short strenuous activity (Sprinting and weight lifting) Humans Mixed fibers Muscles and Lactate Fermentation Key Concepts: HOW ANAEROBIC PATHWAYS END Fermentation pathways start with glycolysis Substances other than oxygen are the final electron acceptor Compared with aerobic respiration, net yield of ATP is small 7.7 Alternative Energy Sources in the Body In humans and other mammals, foods enter aerobic respiration at various steps • Simple sugars from carbohydrates • Glycerol and fatty acids from fats • Carbon backbones of amino acids from proteins Disposition of Organic Compounds Alternative Energy Sources FOOD fats fatty acids COMPLEX CARBOHYDRATES glycerol glucose, other simple sugars PROTEINS amino acids acetyl-coA acetyl-coA PGAL Glycolysis NADH pyruvate oxaloacetate or another intermediate of the Krebs Krebs Cycle NADH, FADH2 Electron Transfer Phosphorylation Fig. 7.12a, p.119 Key Concepts: OTHER METABOLIC PATHWAYS Molecules other than glucose are common energy sources Different pathways convert lipids and proteins to substances that may enter glycolysis or the Krebs cycle Life’s Unity Photosynthesis and aerobic respiration are interconnected on a global scale In its organization, diversity, and continuity through generations, life shows unity at the bioenergetic and molecular levels Energy, Photosynthesis, and Aerobic Respiration Key Concepts: PERSPECTIVE AT UNIT’S END Life shows unity in its molecular and cellular organization and in its dependence on a oneway flow of energy Animation: Alternative energy sources Animation: Fermentation pathways Animation: Functional zones in mitochondria Animation: Glycolysis Animation: Overview of aerobic respiration Animation: The Krebs Cycle - details Animation: Third-stage reactions Animation: Where pathways start and finish