How Cells Release Chemical Energy. Chapter 7

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1 How Cells Release Chemical Energy Chapter 7

2 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

3 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

4 Pathways of Carbohydrate Breakdown

5 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

6 Overview of Aerobic Respiration Three main stages of aerobic respiration: 1. Glycolysis 2. Krebs cycle 3. Electron transfer phosphorylation Summary equation: C 6 H 12 O 6 + 6O 2 6CO H 2 O

7 Overview of Aerobic Respiration

8 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

9 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

10 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

11 Glycolysis

12 Glycolysis

13 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

14 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

15 Second Stage of Aerobic Respiration

16 Acetyl-CoA Formation Two pyruvates from glycolysis are converted to two acetyl-coa Two CO 2 leave the cell Acetyl-CoA enters the Krebs cycle

17 Krebs Cycle Each turn of the Krebs cycle, one acetyl-coa is converted to two molecules of CO 2 After two cycles Two pyruvates are dismantled Glucose molecule that entered glycolysis is fully broken down

18 Energy Products Reactions transfer electrons and hydrogen atoms to NAD + and FAD Reduced to NADH and FADH 2 ATP forms by substrate-level phosphorylation Direct transfer of a phosphate group from a reaction intermediate to ADP

19 Net Results Second stage of aerobic respiration results in Six CO 2, two ATP, eight NADH, and two FADH 2 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

20 Second Stage Reactions

21 Fig. 7.6b, p.113

22 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

23 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)

24 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

25 Electron Transfer Phosphorylation

26 Fig. 7.7a, p.114

27 INNER COMPARTMENT NADH FADH 2 H + H + H + H 2 O ADP + P i H + ATP INNER MITOCHONDRIAL MEMBRANE OUTER COMPARTMENT H + H + H + 1/2 O 2 H + H + H + H + H + H + Fig. 7.7b, p.114

28 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

29 Summary: Aerobic Respiration

30 glucose 2 ATP 2 NAD + Glycolysis ATP (2 net) 2 NADH 2 pyruvate CYTOPLASM OUTER MITOCHONDRIAL COMPARTMENT 2 NADH 2 NADH 6 NADH 2 FADH 2 2 acetyl-coa Krebs Cycle 2 CO 2 4 CO 2 2 ATP INNER MITOCHONDRIAL COMPARTMENT ADP + P i Electron Transfer Phosphorylation water 32 ATP H + H + H + H + H + oxygen Fig. 7.8, p.115

31 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

32 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)

33 Alcoholic and Lactate Fermentation

34 Fig. 7.9a, p.116

35 Fig. 7.9b, p.116

36 Glycolysis glucose 2 NAD + 2 ATP 2 NADH 4 ATP pyruvate Alcoholic Fermentation 2 CO 2 acetaldehyde 2 NADH ethanol 2 NAD + Fig. 7.9b, p.116

37 Fig. 7.9c, p.116

38 Glycolysis glucose 2 NAD + 2 ATP 2 NADH 4 ATP pyruvate Lactate Fermentation 2 NADH 2 NAD + lactate Fig. 7.9c, p.116

39 Alcoholic Fermentation

40 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

41 Muscles and Lactate Fermentation

42 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

43 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

44 Disposition of Organic Compounds

45 FOOD fats COMPLEX CARBOHYDRATES PROTEINS fatty acids glycerol glucose, other simple sugars amino acids acetyl-coa PGAL acetyl-coa Glycolysis NADH pyruvate Krebs Cycle oxaloacetate or another intermediate of the Krebs NADH, FADH 2 Electron Transfer Phosphorylation Fig. 7.12a, p.119

46 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

47 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

48 Energy, Photosynthesis, and Aerobic Respiration

49 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