Darlene E. Berryman, PhD, RD and Matthew W. Hulver, PhD Total-body energy utilization is the sum of the collective energy-utilizing and energy-producing reactions occurring within individual cells throughout the body. Both energy in foods and energy expenditure by the body are typically measured in calories or joules. The various metabolic reactions that serve to sustain life are fueled by the energy released from the biological oxidation of energy-yielding nutrients in the food we consume. A fraction of the energy released from biological oxidation processes is captured in high-energy bonds in molecules, namely in the phosphate−phosphate bonds of adenosine 5′-triphosphate (ATP), which is the main energy currency of the cell. ATP is then used to fuel the various biochemical processes within cells that support basal metabolism, growth, and other essential functions. Animal cells obtain energy by oxidizing food molecules. The oxidation of fuel molecules in the body is referred to as catabolism. The metabolic reactions that result in breakdown of the fuel molecules are not completely efficient and result in loss of some of the inherent energy in the fuel molecule as heat. However, a substantial portion of the energy in the fuel molecules is conserved as either ATP or the reduced form of nicotinamide adenine dinucleotide phosphate (NADPH). Whereas ATP is the main source of energy for driving most cellular functions, NADPH is used mainly to drive reactions involved in the synthesis of fatty acids and sterols and a few other reactions such as reduction of oxidized glutathione (see Chapters 12, 16, and 17). Thus ATP and NADPH serve a unique role in coupling the energy-yielding processes of catabolism to the energy-consuming reactions of cellular work. In addition to serving as the major energy currency of the cell, ATP also serves as an important allosteric effector in the regulation of metabolism. Its concentration relative to those of ADP and AMP is an index of the energy status of the cell and determines the rates of regulatory enzymes situated at key points in metabolism. AMP-activated protein kinase (AMPK) is an important sensor of intracellular AMP/ATP ratios that exerts regulatory effects on multiple ATP-producing and ATP-consuming pathways (see Figure 19-1 in Chapter 19). Energy for the synthesis of ATP within cells comes mainly from the oxidation of energy-yielding molecules, which include predominantly carbohydrate and fat but also protein and alcohol (Flatt, 1985). The complete metabolic oxidation of fuel molecules to CO2 and H2O occurs with a similar stoichiometry to that for combustion of the fuels in a flame, but the processes involved are quite different. As summarized in Figure 21-3, catabolism of fuel molecules involves the oxidation of the fuel molecules coupled to the reduction of other molecules such as NAD(P) and FAD. Some ATP equivalents are formed by substrate-level phosphorylation (e.g., in two steps of glycolysis and in the citric acid cycle). CO2 is formed primarily from oxidation of acetyl-CoA by the citric acid cycle with H2O as the source of the additional oxygen needed to convert the acetyl moiety to CO2. However, most ATP production and almost all molecular O2 consumption occur at the level of oxidative phosphorylation in the mitochondria. The electrons carried by the reduced NADH and FADH2 generated in the citric acid cycle as well as by glycolysis and β-oxidation are transferred to the mitochondrial electron transport chain, which fuels phosphorylation of ADP to yield ATP. Molecular O2 is the terminal electron (and proton) acceptor in electron transport; O2 is reduced to produce H2O. In macronutrient oxidation, the release of CO2 generally occurs at earlier stages in nutrient catabolism and thus prior to O2 consumption and H2O production. The coupling of oxidation of fuel molecules to ATP synthesis (with the exception of some substrate-level ATP formation) is localized within the mitochondria of mammalian cells, as illustrated in Figure 21-3. The specialized proteins and enzymes required for oxidative phosphorylation are specifically localized on the inner mitochondrial membrane. Oxidative phosphorylation is the process by which a molecule of inorganic phosphate is condensed with ADP to form ATP, a process driven by the step-by-step transfer of electrons along a chain of electron carriers termed the electron transport chain (Figure 21-4).
Cellular and Whole-Animal Energetics
Cellular Energetics
Adenosine Triphosphate and Nicotinamide Adenine Dinucleotide Phosphate
Metabolic Sources of Heat Production
Oxidation of Fuel Molecules
Oxidative Phosphorylation
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Cellular and Whole-Animal Energetics
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