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How Cells Release Chemical Energy Chapter 7 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 Start with glycolysis in the cytoplasm • Convert glucose and other sugars to pyruvate Fermentation pathways • End in cytoplasm, do not use oxygen, yield 2 ATP per molecule of glucose Aerobic respiration • Ends in mitochondria, uses oxygen, yields up to 36 ATP per glucose molecule Pathways of Carbohydrate Breakdown a All carbohydrate breakdown pathways start in the cytoplasm, with glycolysis. b Fermentation pathways are completed in the semifluid matrix of the cytoplasm. c In eukaryotes, aerobic respiration is completed inside mitochondria. Fig. 7.2b, p.108 Animation: Where pathways start and finish CLICK HERE TO PLAY Overview of Aerobic Respiration Three main stages of aerobic respiration: 1. Glycolysis 2. Krebs cycle 3. Electron transfer phosphorylation Summary equation: C6H12O6 + 6O2 → 6CO2 + 6 H2O Overview of Aerobic Respiration Cytoplasm glucose 2 ATP ATP GLYCOLYSIS 4 ATP (2net) ATP 2 NADH 2 pyruvate a The first stage, glycolysis, occurs in the cell’s cytoplasm. Enzymes convert a glucose molecule to 2 pyruvate for a net yield of 2 ATP. During the reactions, 2 NAD+ pick up electrons and hydrogen atoms, so 2 NADH form. Mitochondrion Krebs Cycle 6 ATP CO 2 ATP 2 8 NADH, 2 FADH2 ATP oxygen Electron Transfer Phosphorylation 32 ATP b The second stage, the Krebs cycle and a few steps before it, occurs inside mitochondria. The 2 pyruvates are broken down to CO2, which leaves the cell. During the reactions, 8 NAD+ and 2 FAD pick up electrons and hydrogen atoms, so 8 NADH and 2 FADH2 form. 2 ATP also form. c The third and final stage, electron transfer phosphorylation, occurs inside mitochondria. 10 NADH and 2 FADH2 donate electrons and hydrogen ions at electron transfer chains. Electron flow through the chains sets up H+ gradients that drive ATP formation. Oxygen Fig. 7.3, p.109 Animation: Overview of aerobic respiration CLICK HERE TO PLAY 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 7.2 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 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 you are here Fig. 7.4a, p.110 glucose 2 ADP (net) Glycolysis 2 pyruvate to second stage of aerobic respiration or another pathway 2 NADH to third stage of aerobic respiration or another pathway Fig. 7.4b, p.110 Glycolysis Glycolysis glucose ATP ADP glucose–6–phosphate ATP ADP P P fructose–1,6–bisphosphate DHAP Fig. 7.4c1, p.111 2 PGAL 2 NAD+ + 2 Pi 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 Animation: Glycolysis CLICK HERE TO PLAY 7.3 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 Second Stage of Aerobic Respiration outer membrane (next to cytoplasm) glucose (glycolysis) 2 pyruvate OUTER COMPARTMENT inner membrane inner mitochondrial compartment outer mitochondrial compartment (in between the two membranes) a An inner membrane divides a mitochondrion’s interior into two compartments. The second and third stages of aerobic respiration take place at this membrane. 2 acetyl–CoA CO2 ATP NADH Krebs Cycle FADH2 INNER COMPARTMENT Breakdown of 2 pyruvate to 6CO2 yields 2 ATP. Also, 10 coenzymes are reduced (8 NADH, 2 FADH2). The coenzymes carry hydrogen ions and electrons to sites of the third stage of aerobic respiration. b The second stage starts after membrane proteins transport pyruvate from the cytoplasm, across both mitochondrial membranes, to the inner compartment. Six carbon atoms enter these reactions (in two pyruvate), and six leave (in six CO2). Many coenzymes form. Fig. 7.5, p.112 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 Fig. 7.6a, p.113 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 glucose Glycolysis you are here Krebs Cycle Electron Transfer Phosphorylation Fig. 7.6b, p.113 Animation: Functional zones in mitochondria CLICK HERE TO PLAY Animation: The Krebs Cycle - details CLICK HERE TO PLAY 7.4 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 glucose Glycolysis you are here Krebs Cycle Electron Transfer Phosphorylation Fig. 7.7a, p.114 Fig. 7.7b, 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 Summary: Aerobic Respiration 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 Animation: Third-stage reactions CLICK HERE TO PLAY 7.5 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: Lactate Alcoholic fermentation • End product: Ethyl alcohol (or ethanol) Both pathways have a net yield of 2 ATP per glucose (from glycolysis) Alcoholic and Lactate Fermentation Fig. 7.9a, p.116 glucose Glycolysis you are here Fermentation Pathway Fig. 7.9a, p.116 Fig. 7.9b, p.116 Glycolysis glucose 2 NAD+ 2 2 NADH ATP 4 ATP pyruvate Alcoholic Fermentation 2 CO2 acetaldehyde 2 NADH 2 NAD+ ethanol Fig. 7.9b, p.116 Fig. 7.9c, 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 Animation: Fermentation pathways CLICK HERE TO PLAY 7.6 The Twitchers Slow-twitch and fast-twitch skeletal muscle fibers can support different activity levels Aerobic respiration and lactate fermentation proceed in different fibers of muscles 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 Fig. 7.12b, p.119 Animation: Alternative energy sources CLICK HERE TO PLAY 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 7.8 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 sunlight energy in Photosynthesis Driven by energy input from the sun, electrons and hydrogen are used to form ATP. ATP energy drives the synthesis of glucose from hydrogen, electrons (delivered by coenzymes), and carbon dioxide’s atoms. glucose (stored chemical energy) carbon dioxide, water Aerobic Respiration Energy input from two ATP initiates three stages of reactions. Many ATP form during thecomplete breakdown of glucose to carbon dioxide and water. chemical energy in many ATP available to drive nearly all cellular tasks oxygen energy out (heat) energy out (heat) Fig. 7.13, p.120 sunlight energy in Photosynthesis Driven by energy input from the sun, electrons and hydrogen are used to form ATP. ATP energy drives the synthesis of glucose from hydrogen, electrons (delivered by coenzymes), and carbon dioxide’s atoms. energy out (heat) glucose (stored chemical energy) carbon dioxide, water oxygen Aerobic Respiration Energy input from two ATP initiates three stages of reactions. Many ATP form during thecomplete breakdown of glucose to carbon dioxide and water. chemical energy in many ATP available to drive nearly all cellular tasks energy out (heat) Stepped Art Fig. 7-13, p.120 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: Links with photosynthesis CLICK HERE TO PLAY Video: When Mitochondria Spin Their Wheels CLICK HERE TO PLAY