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Cecie Starr Christine Evers Lisa Starr www.cengage.com/biology/starr Chapter 7 How Cells Release Chemical Energy (Sections 7.5 - 7.7) Albia Dugger • Miami Dade College 7.5 Aerobic Respiration’s Big Energy Payoff • The third stage of aerobic respiration, electron transfer phosphorylation, occurs at the inner mitochondrial membrane • Many ATP are formed in electron transfer phosphorylation Energy Transfer • The first two stages of cell respiration (glycolysis and Krebs cycle) have delivered electrons and hydrogen ions to electron transfer chains in the inner mitochondrial membrane • Electrons moving through the chains release energy which is used to move H+ from the matrix to the intermembrane space Hydrogen Ion Concentration • Hydrogen ions that accumulate in the intermembrane space form a gradient across the inner membrane • H+ ions follow the gradient back to the matrix through transport proteins (ATP synthases) that drive ATP synthesis • Oxygen combines with electrons and H+ at the end of the transfer chains, forming water 5 Steps in Electron Transfer Phosphorylation 1. NADH and FADH2 (high-energy molecules created by earlier steps) deliver electrons to electron transfer chains in the inner mitochondrial membrane 2. Electron flow through the chains causes hydrogen ions (H+) to be pumped from the matrix to the intermembrane space 3. The electron transfer chains cause an H+ gradient to form across the inner mitochondrial membrane 5 Steps in Electron Transfer Phosphorylation 4. Hydrogen ions flow back to the matrix through ATP synthases, driving the formation of ATP from ADP and phosphate (Pi) 5. Oxygen (O2) accepts electrons and hydrogen ions at the end of mitochondrial electron transfer chains, forming water Electron Transfer Phosphorylation 1 NADH and FADH2 deliver electrons to electron transfer chains in the inner mitochondrial membrane. Electron Transfer Phosphorylation 2 Electron flow through the Oxygen (O2) cytoplasm outer membrane intermembrane space 3 The activity of the electron 3 transfer chains causes a hydrogen ion gradient to form across the inner mitochondrial membrane. inner membrane 2 matrix 1 chains causes hydrogen ions (H+) to be pumped from the matrix to the intermembrane space. 5 4 4 Hydrogen ion flow back to the matrix through ATP synthases drives the formation of ATP from ADP and phosphate (Pi). 5 Oxygen (O2) accepts electrons and hydrogen ions at the end of mitochondrial electron transfer chains, so water forms. Stepped Art Fig, 7.8a, p. 114 Summing Up: The Energy Harvest • Overall, aerobic respiration typically yields 36 ATP for each glucose molecule • 32 ATP typically form in the third stage, but yield varies • Example: Typical yield of aerobic respiration in brain and skeletal muscle cells is 38 ATP, not 36 Summary: Aerobic Respiration Glycolysis A First stage: Glucose is converted to 2 pyruvate; 2 NADH and 4 ATP form. cytoplasm outer membrane B Second stage: 10 more coenzymes accept electrons and hydrogen ions during the second-stage reactions. C Third stage: Coenzymes that were reduced in the first two stages give up electrons and hydrogen ions to electron transfer chains. Energy lost by the electrons as they flow through the chains is used to move H+ across the membrane. intermembrane space inner membrane matrix Krebs Cycle oxygen Fig, 7.9, p. 115 Key Concepts • Aerobic Respiration • The final stages of aerobic respiration break down pyruvate to CO2 • High-energy molecules formed in the first two stages of cellular respiration deliver electrons and hydrogen ions to electron transfer chains, where ATP forms by electron transfer phosphorylation 7.6 Fermentation Pathways • Anaerobic fermentation pathways begin with glycolysis and finish in the cytoplasm • alcoholic fermentation • lactate fermentation • A molecule other than oxygen accepts electrons at the end of these reactions Key Terms • alcoholic fermentation • Anaerobic carbohydrate breakdown pathway that produces ATP and ethyl alcohol (ethanol) • Used in wine and breads • lactate fermentation • Anaerobic carbohydrate breakdown pathway that produces ATP and lactate • Used in cheeses and pickles Energy of Fermentation • Breakdown of one glucose molecule in either alcoholic or lactate fermentation yields only the 2 ATP that form in glycolysis reactions – enough energy to sustain single-celled anaerobic species Alcoholic Fermentation Products of Alcoholic Fermentation Lactate Fermentation Lactate Fermentation in Muscles • Skeletal muscle has two types of fibers: red and white • ATP is produced primarily by aerobic respiration in red muscle fibers, which sustain activities that require endurance • Lactate fermentation in white fibers supports activities that occur in short, intense bursts Lactate in Muscles Key Concepts • Fermentation • Fermentation pathways start with glycolysis • Substances other than oxygen accept electrons at the end of the pathways • Compared with aerobic respiration, the net yield of ATP from fermentation is small 7.7 Alternative Energy Sources in the Body • Simple sugars from carbohydrate breakdown, glycerol and fatty acids from fat breakdown, and carbon backbones of amino acids from protein breakdown may enter aerobic respiration at various reaction steps Energy From Fats • When blood glucose gets too high, excess dietary carbohydrates are converted to fatty acids • The body stores most fats as triglycerides, which have three fatty acid tails • When blood glucose falls, triglycerides provide energy Energy From Proteins • When you eat more protein than your body needs, the amino acids are broken down Food Fats 2 fatty acids 1 glycerol Complex Carbohydrates Proteins glucose, other simple sugars3 amino acids acetyl–CoA intermediate of glycolysis acetyl–CoA Glycolysis NADH pyruvate Krebs cycle Alternative Pathways of Aerobic Respiration intermediate of Krebs cycle NADH, FADH2 Electron Transfer Phosphorylation Fig, 7.12, p. 118 Key Concepts • Other Metabolic Pathways • Molecules other than carbohydrates are common energy sources in the animal body • Many different pathways can convert dietary lipids and proteins to molecules that may enter glycolysis or the Krebs cycle When Mitochondria Spin Their Wheels (revisited) • At least 83 proteins are directly involved in electron transfer phosphorylation in mitochondria • Defects in any of the thousands of other proteins used by mitochondria (such as frataxin) may be involved in many illnesses such as diabetes, hypertension, Alzheimer’s, Parkinson’s disease, and even aging