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Energy metabolism, or bioenergetics, is the study of energy changes that accompany biochemical reactions. Energy sustains the work of biosynthesis of cellular and extracellular components, the transport of ions and organic chemicals against concentration gradients (osmotic work), the conduction of electrical impulses in the nervous system, and the movement of cells and the whole organism. Sunlight is the ultimate source of energy for life. Photosynthetic cells use light energy to produce chemical energy and reducing compounds, used to convert carbon dioxide into organic chemicals such as glucose. The energy from the oxidation of carbohydrates, fats, and proteins sustains the biochemical reactions required for life.
The main sources of chemical energy for most organisms are carbohydrates, fats, and protein. Energy content is expressed in calories or joules. The nutritional calorie, or kilocalorie (kcal), in foodstuffs is equivalent to 1000 calories. The energy content per gram of carbohydrate is 4 kcal (16 J); protein, 4 kcal (16 J); and fat, 9 kcal (36 J). The metabolism of foodstuffs yields chemical energy and heat.
Energy is defined as the ability to do work, and metabolism represents the biochemical reactions that a cell can perform to produce energy. The most important thermodynamic parameter in bioenergetics is the free energy change, ΔG, occurring at constant temperature and pressure (the usual conditions for chemical reactions inside the cell). The Gibbs free energy change is defined as the free energy content of the final state minus the free energy content of the initial state.
All feasible reactions occur with a negative free energy change; the final state has less free energy than the initial state; that is, ΔG < 0 (process is exergonic). If the free energy of the final state is more than that of the initial state, ΔG is positive and the reaction is not feasible without the input of energy; ΔG > 0 (process is endergonic). When the free energy change is zero, the reaction or process is at equilibrium; ΔG = 0 (process is isoergonic).
The complete oxidation of one mole of glucose to carbon dioxide and water is associated with the liberation of free energy. Energy is released in a stepwise fashion and is coupled to the biosynthesis of adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and inorganic phosphate (Pi). The reaction of ATP with water to produce ADP and Pi results in the liberation of a large amount of energy (30 kJ, or 7 kcal per mole). Such compounds are said to be energy-rich and to possess a high-energy bond. Lipmann's law is the cornerstone of energy metabolism: ATP serves as the common currency of energy exchange in living systems (animals, plants, and bacteria). The ATP-ADP couple receives and distributes chemical energy in all living systems. Creatine phosphate is an energy-rich compound found in vertebrate muscle and brain; it is a storage form of chemical energy and can energize the regeneration of ATP from ADP. Such a reaction occurs in vigorously exercising skeletal muscle when ATP is expended to produce contraction. See Adenosine diphosphate (ADP), Adenosine triphosphate (ATP)