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Definition

metabolism

(mĕ-tab'ŏ-lizm )

[Gr. metabolē, a change + -ism]

All of the energy and material transformations that occur within living cells. It includes material changes undergone by substances during all periods of life (growth, maturity, and senescence) and energy changes (transformations of chemical energy from food to mechanical energy or heat). Metabolism involves the two fundamental processes of anabolism and catabolism.

SEE: anabolism; catabolism.

metabolic,

(metă-bol'ik )

adj.

basal m.The lowest level of energy expenditure, determined when the body is at complete rest. For an average person, basal metabolism is measured in various ways. In terms of large calories (Cal), measurement is about 1500 to 1800 per day; in terms of body weight, measurement is 1 Cal/kg per hour; in terms of body surface, measurement is 40 Cal/m2/hr.

carbohydrate m.The sum of the physical and chemical changes involved in the breakdown and synthesis of carbohydrates in the body. Carbohydrates are digested to hexose monosaccharides that are absorbed by the small intestine; the liver converts fructose and galactose to glucose. In the liver and muscles, glucose may be converted to glycogen. In all cells, glucose is oxidized to carbon dioxide and water, with energy released in the forms of adenosine triphosphate and heat. Lactic acid, a product of the anaerobic breakdown of glucose, causes muscle fatigue.

constructive m.Anabolism.

destructive m.Catabolism.

fat m.The sum of the physical and chemical changes involved in the breakdown and synthesis of fats in the body. Dietary fats are digested to fatty acids and glycerol in the small intestine, absorbed, and reformed into triglycerides that are transported in the form of chylomicrons. Fats may be stored in adipose tissue as potential energy or may be broken down to provide immediate energy. The liver has enzymes for the beta-oxidation of fatty acids and their use in the Krebs cycle. Fats may be formed from excess dietary carbohydrate or amino acids. Synthetic reactions produce phospholipids and steroids.

first-pass m.The metabolism of a substance that occurs immediately as it enters the body, and before it can exert any effect, or before it can be measured at its target organ.

intermediary m.The series of intermediate compounds formed during digestion before the final excretion or oxidation products are formed or eliminated from the body.

muscle m.The consumption of energy by all cells, including those of muscle tissue, to perform work. The source of chemical energy, adenosine triphosphate (ATP), is metabolized to adenosine diphosphate (ADP). If the energy requirement is short-term, the ADP is converted back to ATP. This process is too slow to keep up with energy demand s during long-term exercise; thus, consumption of other fuels is required. The main sources of fuel for muscles are carbohydrates and lipids. Before being available for intracellular metabolism, glycogen is obtained when glucose is converted to glycogen. The main lipid fuel is free fatty acids present in plasma. Carbohydrates can be metabolized either in the presence of oxygen (aerobically) or in its absence (anaerobically), but lipids can be metabolized only aerobically. During light exercise or when the body is at rest, muscle metabolism is usually entirely aerobic, and the source of fuel is the free fatty acids in plasma. During intense exercise, metabolism of the fatty acids cannot keep up with the demand , and glycogen is used for energy. However, as intense exercise continues, glycogen stores are exhausted, and free fatty acids become the principal source of energy.

protein m.The sum of the physical and chemical changes involved in the breakdown and synthesis of proteins in the body. Dietary proteins are digested to amino acids, which are absorbed by the small intestine and used to synthesize enzymes and the structural proteins essential for growth and repair of tissue. Amino acids in excess of protein synthesis requirements are deaminated; the amino group (NH2) is removed and converted to urea, which is excreted by the kidneys. The remaining carbon chain may be converted to a simple carbohydrate and oxidized to produce energy.

purine m.Metabolism involving nucleic acids, present in nuclei of cells, in which they are combined with proteins to form nucleoproteins. In the breakdown of nucleic acid, uric acid, a nitrogenous waste product, is formed.