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a succession or recurring series of events.
cardiac cycle a complete cardiac movement, or heart beat, including systole, diastole, and the intervening pause.
cell cycle the cycle of biochemical and morphological events occurring in a reproducing cell population; it consists of the S phase, occurring toward the end of interphase, in which DNA is synthesized; the G2 phase, a relatively quiescent period; the M phase, consisting of the four phases of mitosis; and the G1 phase of interphase, which lasts until the S phase of the next cycle.
citric acid cycle tricarboxylic acid cycle.
estrous cycle the recurring periods of estrus in adult females of most mammalian species and the correlated changes in the reproductive tract from one period to another.
hair cycle the successive phases of the production and then loss of hair, consisting of anagen, catagen, and telogen.
Krebs cycle tricarboxylic acid cycle.
menstrual cycle see menstrual cycle.
ovarian cycle the sequence of physiologic changes in the ovary involved in ovulation; see also ovulation and reproduction.
reproductive cycle the cycle of physiologic changes in the reproductive organs, from the time of fertilization of the ovum through gestation and childbirth; see also reproduction.
sex cycle (sexual cycle)
1. the physiologic changes that recur regularly in the reproductive organs of nonpregnant female mammals.
2. the period of sexual reproduction in an organism that also reproduces asexually.
TCA cycle tricarboxylic acid cycle.
tricarboxylic acid cycle the cyclic metabolic mechanism by which the complete oxidation of the acetyl portion of acetyl-coenzyme A is effected; the process is the chief source of mammalian energy, during which carbon chains of sugars, fatty acids, and amino acids are metabolized to yield carbon dioxide, water, and high-energy phosphate bonds. Called also citric acid cycle, Krebs cycle, and TCA cycle.
urea cycle a cyclic series of reactions that produce urea; it is a major route for removal of the ammonia produced in the metabolism of amino acids in the liver and kidney.
a biomolecule that carries an activated form of the 2-carbon acetyl unit found in the course of several important metabolic processes. The formation of acetylcoenzyme A is the critical intermediate step between anaerobic glycolysis and the citric acid cycle. Also called acetyl-CoA.
Acetyl-CoAA coenzyme derivative in the metabolism of glucose and fatty acids that contributes substrates to the Krebs cycle. Acetyl CoA provides the acetyl for multiple biochemical reactions and plays a key role in intermediary metabolism—synthesis, catabolism, or use of nutrients for energy production and growth.
acetylcoenzyme ASee Acetyl-CoA.
acetylcoenzyme A (acetyl-CoA)an organic compound important in the process of OXIDATION of energy-rich compounds to CO2 and water. In the presence of oxygen the three-carbon pyruvic acid produced in GLYCOLYSIS is broken down in the MITOCHONDRIA to produce CO2, two hydrogen atoms (reducing NAD to NADH2 ), and an active form of two-carbon acetic acid bonded to coenzyme A, called acetylcoenzyme A. Metabolism of fats and proteins also produces acetyl-CoA, either via pyruvic acid or directly. Thus acetyl-CoA is common to the metabolism of all major types of food, forming a step leading to the KREBS CYCLE. See Fig. 6 .
acetyl-CoA, the carrier of acetyl groups into the tricarboxylic acid (Krebs) cycle and the chief precursor of lipids; it is formed by the attachment to coenzyme A of an acetyl group during the oxidation of pyruvate, fatty acids or amino acids.