metabolism (redirected from Metabolic Reaction)

metabolism, sum of all biochemical processes involved in life. Two subcategories of metabolism are anabolism, the building up of complex organic molecules from simpler precursors, and catabolism, the breakdown of complex substances into simpler molecules, often accompanied by the release of energy. Organic molecules involved in these processes are called metabolites, and their interconversions are catalyzed by enzymes. The transformation of one molecule into another, and then into another and another in sequence, is termed a metabolic pathway; the intermediates in these pathways are often identified with the aid of a chemical tracer. Exercise, food, and environmental temperature influence metabolism. Basal metabolism is the caloric expenditure of an organism at rest; it represents the minimum amount of energy required to maintain life at normal body temperature. The basal metabolism rate is usually measured indirectly by calculation from measurements of the amounts of oxygen and carbon dioxide exchanged during breathing under certain standard conditions, i.e., complete rest in a room temperature of 68°F; (20°C;), 12 to 14 hours after ingestion of food. A less cumbersome method of estimating basal metabolic rate involves the quantitative assay of the hormone thyroxine, known to regulate the body's rate of metabolism. Often the word metabolism is associated with a particular organic compound or class of compounds, as in phenylalanine metabolism or amino acid metabolism. In this usage the word refers to the sum of all interconversions, both anabolic and catabolic, in which the particular compound or class of compounds is involved.

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metabolism

Sum of all the chemical reactions that take place in every cell of a living organism, providing energy for the processes of life and synthesizing new cellular material. The term intermediary metabolism refers to the vast web of interconnected chemical reactions by which all the cell's constituents, many rarely found outside it, are created and destroyed. Anabolic reactions use energy to build complex molecules from simpler organic compounds (e.g., proteins from amino acids, carbohydrates from sugars, fats from fatty acids and glycerol); catabolic reactions break complex molecules down into simpler ones, releasing chemical energy. For most organisms, the energy comes ultimately from the Sun, whether they obtain it by photosynthesis and store it in organic compounds or by consuming those organisms that do so. In some bacteria in special environments such as deep-sea vents, the energy comes from chemical reactions instead. Energy is transferred within the cell and the organism by ATP; anabolic reactions consume it, and catabolic reactions generate it. Every cellular chemical reaction is mediated by a specific enzyme. The process that breaks down a substance is usually not the reverse of the process that makes it, using a different enzyme. See also digestion; fermentation; glycolysis; tricarboxylic acid cycle.

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metabolism

1. the sum total of the chemical processes that occur in living organisms, resulting in growth, production of energy, elimination of waste material, etc.

2. the sum total of the chemical processes affecting a particular substance in the body

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Metabolism

All the physical and chemical processes by which living, organized substance is produced and maintained and the transformations by which energy is made available for use by an organism.

In defining metabolism, it is customary to distinguish between energy metabolism and intermediary metabolism, although the two are, in fact, inseparable. Energy metabolism is primarily concerned with overall heat production in an organism, while intermediary metabolism deals with chemical reactions within cells and tissues. In general, the term metabolism is interpreted to mean intermediary metabolism. See Energy metabolism

Metabolism thus includes all biochemical processes within cells and tissues which are concerned with their building up, breaking down, and functioning. The synthesis and maintenance of tissue structure generally involves the union of smaller into larger molecules. This part of metabolism, the building of tissues, is termed anabolism. The process of breaking down tissue, of splitting larger protoplasmic molecules into smaller ones, is termed catabolism. Growth or weight gain occurs when anabolism exceeds catabolism. On the other hand, weight loss results if catabolism proceeds more rapidly than anabolism, as in periods of starvation, serious injury, or disease. When the two processes are balanced, tissue mass remains the same.

The metabolism of the three major foodstuffs, carbohydrates, fats, and proteins, is intimately interrelated, so any clearcut division of the three is arbitrary and inaccurate. Thus the metabolism of protoplasm is concerned with all three of these foodstuffs. The metabolic pathways of carbohydrates, fats, and proteins cross at many points; thus certain pathways of metabolism are shared in common by fragments of these different classes of foodstuffs.

Some of the metabolic processes of the protoplasm of both plant and animal cells occur along common pathways; carbohydrate metabolism in plants is similar in many details to carbohydrate metabolism in animals. Therefore the study of metabolism in any organism is, in a sense, the study of metabolism in all protoplasm. See Carbohydrate metabolism, Lipid metabolism, Protein metabolism