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Chapter 21: The Generation of Biochemical Energy 1.! What is the source of our energy, and what is its fate in the body? Be able to provide an overview of the sources of our energy and how we use it, identify the cellular location of energy generation, and explain the significance of exergonic and endergonic reactions in metabolism.! 2.! How are the reactions that break down food molecules organized? Be able to list the stages in catabolism and describe the role of each.! 3.! What are the major strategies of metabolism? Be able to explain and give examples of the roles of ATP, coupled reactions, and oxidized and reduced coenzymes in metabolic pathways. 4.! What is the citric acid cycle? Be able to describe what happens in the citric acid cycle and explain its role in energy production.! 5.! How is ATP generated in the final stage of catabolism? Be able to describe in general the electron-transport chain, oxidative phosphorylation , and how they are coupled.! 6. 1 What are the harmful by-products produced from oxygen, and what protects against them? Be able to identify the highly reactive oxygen-containing products formed during metabolism and the enzymes and vitamins that counteract them. Chapter 21: The Generation of Biochemical Energy 1.! What is the source of our energy, and what is its fate in the body? Energy and Life Energy must be released from food gradually. Energy must be stored in readily accessible forms. Release of energy from storage must be finely controlled so that it is available exactly when and where it is needed. Just enough energy must be released as heat to maintain constant body temperature. Energy and Biochemical Reactions Exergonic: 2 Chapter 21: The Generation of Biochemical Energy Endergonic: 3 Chapter 21: The Generation of Biochemical Energy Prokaryotic Cells: Eukaryotic Cells: Cell Components and Their Principal Function: Cilia: Golgi Apparatus: Rough Endoplasmic Reticulum: Nucleus: Ribosome: Microvilli: 4 Chapter 21: The Generation of Biochemical Energy Lysosome: Smooth Endoplasmic Reticulum: Cell Membrane: Cytoplasm: Cytosol: 5 Chapter 21: The Generation of Biochemical Energy Mitochondrion: Mitochondrial Matrix: Adenosine triphosphate (ATP): 6 Chapter 21: The Generation of Biochemical Energy 2.! How are the reactions that break down food molecules organized? Catabolism: Anabolism: 7 Chapter 21: The Generation of Biochemical Energy Stage 1: Digestion Enzymes in saliva, the stomach, and the small intestine convert the large molecules of lipids, carbohydrates, and proteins to smaller molecules. Carbohydrates are broken down to glucose and other sugars, proteins are broken down to amino acids, and triacylglycerols , the lipids commonly known as fats and oils, are broken down to glycerol plus long-chain carboxylic acids, the fatty acids. These smaller molecules are transferred into the blood for transport to cells throughout the body. Stage 2: Acetyl-S-coenzyme A production The small molecules from digestion follow separate pathways that move their carbon atoms into two-carbon acetyl groups. The acetyl groups are attached to coenzyme A by a bond between the sulfur atom of the thiol group at the end of the coenzyme A molecule and the carbonyl C atom of the acetyl group. Stage 3: Citric acid cycle Within mitochondria, the acetyl-group carbon atoms are oxidized to the carbon dioxide that we exhale. Most of the energy released in the oxidation leaves the citric acid cycle in the chemical bonds of reduced coenzymes. Stage 4: ATP production Electrons from the reduced coenzymes are passed from molecule to molecule down an electron-transport chain. Along the way, their energy is harnessed to produce more ATP. At the end of the process, these electrons— along with hydrogen ions from the reduced coenzymes—combine with oxygen we breathe to produce water. 8 Chapter 21: The Generation of Biochemical Energy 9 Chapter 21: The Generation of Biochemical Energy 3.! 10 What are the major strategies of metabolism? Chapter 21: The Generation of Biochemical Energy Oxidation: Reduction: Oxidation and reduction always occur together 11 Chapter 21: The Generation of Biochemical Energy A steady supply of oxidizing and reducing agents must be available, so a few coenzymes continuously cycle between their oxidized and reduced forms. 12 Chapter 21: The Generation of Biochemical Energy 13 Chapter 21: The Generation of Biochemical Energy 4.! 14 What is the citric acid cycle? Chapter 21: The Generation of Biochemical Energy 15 Chapter 21: The Generation of Biochemical Energy 16 Chapter 21: The Generation of Biochemical Energy The net result of the citric acid cycle is: Production of four reduced coenzyme molecules, 3 NADH and 1 FADH2 Conversion of an acetyl group to two CO2 molecules Production of one energy-rich molecule (GTP) ADP acts as an allosteric activator for the enzyme for Step 3. NADH acts as an inhibitor of the enzyme for Step 3. By such feedback mechanisms, the cycle is activated when energy is needed and inhibited when energy is in good supply. 5.! How is ATP generated in the final stage of catabolism? At the conclusion of the citric acid cycle, the reduced coenzymes formed in the cycle are ready to donate their energy to making additional ATP Hydrogen and electrons from NADH and FADH2 enter the electrontransport chain at enzyme complexes I and II, respectively. The enzyme for Step 6 of the citric acid cycle is part of complex II. FADH2 produced there does not leave complex II. Instead it is immediately oxidized there by reaction with coenzyme Q. Following formation of the mobile coenzyme Q, reductions occur when electrons are transferred. 17 Chapter 21: The Generation of Biochemical Energy 18 Chapter 21: The Generation of Biochemical Energy 19 Chapter 21: The Generation of Biochemical Energy ATP Synthase: Oxidative phosphorylation: 20 Chapter 21: The Generation of Biochemical Energy 6.! What are the harmful by-products produced from oxygen, and what protects against them? More than 90% of the oxygen we breathe is used in electron transport– ATP synthesis reactions. In these and other oxygen-consuming redox reactions, the product may not be water, but one or more of three highly reactive species. The superoxide ion, !O2- , and the hydroxyl free radical, !OH, can grab an electron from a bond in another molecule, which results in breaking that bond. The third oxygen by-product is hydrogen peroxide, H2 O2 , a relatively strong oxidizing agent. Conditions that can enhance production of these three reactive oxygen species are represented in the drawing below. Some causes are environmental, such as exposure to smog or radiation. Others are physiological, including aging and inflammation. 21 Chapter 21: The Generation of Biochemical Energy 22 Chapter 21: The Generation of Biochemical Energy 23