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Release of Energy from Foods, and the Concept of “Free Energy”

  Release of Energy from Foods, and the Concept of “Free Energy” A great proportion of the chemical reactions in the cells is concerned with making the energy in foods available to the various physiologic systems of the cell. For instance, energy is required for muscle activity, secretion by the glands, maintenance of membrane potentials by the nerve and muscle fibers, synthesis of substances in the cells, absorption of foods from the gastrointestinal tract, and many other functions. Coupled Reactions.  All the energy foods—carbohydrates, fats, and proteins—can beoxidized in the cells, and during this process, large amounts of energy are released. These same foods can also be burned with pure oxygen outside the body in an actual fire, also releasing large amounts of energy; in this case, however, the energy is released suddenly, all in the form of heat. The energy needed by the physiologic processes of the cells is not heat but energy to cause mechanical movement in the case of muscle
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Role of Adenosine Triphosphate in Metabolism

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  Role of Adenosine Triphosphate in Metabolism Adenosine triphosphate (ATP) is an essential link between energy-utilizing and energy-producing functions of the body (Figure 67–1). For this reason, ATP has been called the energy currency of the body, and it can be gained and spent repeatedly.         Energy derived from the oxidation of carbohydrates, proteins, and fats is used to convert adenosine diphosphate (ADP) to ATP, which is then consumed by the various reactions of the body that are necessary for (1) active transport of mole-cules across cell membranes; (2) contraction of muscles and performance of mechanical work; (3) various synthetic reactions that create hormones, cell mem-branes, and many other essential molecules of the body; (4) conduction of nerve impulses; (5) cell division and growth; and (6) many other physiologic functions that are necessary to maintain and propagate life.       ATP is a labile chemical compound that is present in all cells. It has the chemical stru
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Central Role of Glucose in Carbohydrate Metabolism

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  Central Role of Glucose in Carbohydrate Metabolism The final products of carbohydrate digestion in the alimentary tract are almost entirely glucose, fructose, and galactose—with glucose representing, on average, about 80 per cent of these. After absorption from the intestinal tract, much of the fructose and almost all the galactose are rapidly con- verted into glucose in the liver. Therefore, little fructose and galactose are present in the circulating blood Glucose thus becomes the final common pathway for the transport of almost all carbohydrates to the tissue cells.          In liver cells, appropriate enzymes are available to promote  interconversions  among  the  monosaccha-  rides—glucose, fructose, and galactose—as shown in  Figure 67–3. Furthermore, the dynamics of the reactions are such that when the liver releases the monosaccha- rides back into the blood, the final product is almost entirely glucose. The reason for this is that the liver cells contain large amounts of  glu
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Transport of Glucose Through the Cell Membrane

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  Transport of Glucose Through the Cell Membrane Before glucose can be used by the body’s tissue cells, it must be transported through the tissue cell membrane into the cellular cytoplasm. However, glucose  cannoteasily diffuse through the pores  of the cell membranebecause the maximum molecular weight of particles that can diffuse readily is about 100, and glucose has a molecular weight of 180. Yet glucose does pass to the interior of the cells with a reasonable degree of freedom by the mechanism of  facilitated diffusion.  Basi-cally, they are the following. Penetrating through the lipid matrix of the cell membrane are large numbers of protein  carrier  molecules that can bind with glucose. In this bound form, the glucose can be transported by the carrier from one side of the membrane to the other side and then released. Therefore, if the concentration of glucose is greater on one side of the membrane than on the other side, more glucose will be transported from the high-concentratio
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Glycogen Is Stored in Liver and Muscle

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  Glycogen Is Stored in Liver and Muscle After absorption into a cell, glucose can be used immediately for release of energy to the cell, or it can be stored in the form of  glycogen,  which is a large polymer of glucose. All cells of the body are capable of storing at least some glycogen, but certain cells can store large amounts, especially  liver cells,  which can store up to 5 to 8 per cent of their weight as glycogen, and  muscle cells,  which can store up to 1 to 3 per cent glycogen. The glycogen molecules can be polymerized to almost any molecular weight, with the average molecular weight being 5 million or greater; most of the glycogen precipitates in the form of solid granules. This conversion of the monosaccharides into a high-molecular-weight precipitated compound (glycogen) makes it possible to store large quantities of carbohy-drates without significantly altering the osmotic pres-sure of the intracellular fluids. High concentrations of low-molecular-weight soluble monosac
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Release of Energy from the Glucose Molecule by the Glycolytic Pathway

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  Release of Energy from the Glucose Molecule by the Glycolytic Pathway Because complete oxidation of 1 gram-molecule of glucose releases 686,000 calories of energy and only 12,000 calories of energy are required to form 1 gram-molecule of ATP, energy would be wasted if glucose were decomposed all at once into water and carbon dioxide while forming only a single ATP mole-cule. Fortunately, all cells of the body contain special protein enzymes that cause the glucose molecule to split a little at a time in many successive steps, so that its energy is released in small packets to form one mole-cule of ATP at a time, forming a total of 38 moles of ATP for each mole of glucose metabolized by the cells. The next sections describe the basic principles of the processes by which the glucose molecule is pro-gressively dissected and its energy released to form ATP. Glycolysis and the Formation of Pyruvic Acid By far the most important means of releasing energy from the glucose molecule is initiat
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Summary of ATP Formation During the Breakdown of Glucose

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  Summary of ATP Formation During the Breakdown of Glucose We can now determine the total number of ATP mole-cules that, under optimal conditions, can be formed by the energy from one molecule of glucose. 1. During glycolysis, four molecules of ATP are formed, and two are expended to cause the initial phosphorylation of glucose to get the process going. This gives a net gain of  two molecules ofATP.   2. During each revolution of the citric acid cycle, one molecule of ATP is formed. However, because each glucose molecule splits into two pyruvic acid molecules, there are two revolutions of the cycle for each molecule of glucose metabolized, giving a net production of  two more molecules of ATP.   3. During the entire schema of glucose breakdown, a total of 24 hydrogen atoms are released during glycolysis and during the citric acid cycle. Twenty of these atoms are oxidized in conjunction with the chemiosmotic mechanism shown in Figure 67–7, with the release of three ATP molecules per two
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