cycle (sI´kl)

1. A recurrent series of events. 2. A recurring period of time. 3. One successive compression and rarefaction of a wave, as of a sound wave. [G. kyklos, circle]
anovulatory c. a sexual c. in which no ovum is discharged.
brain wave c. the complete upward and downward excursion of a single wave, complex, or impulse as seen on an electroencephalogram.
carbon dioxide c. , carbon c. the circulation of carbon as CO2 from the expired air of animals and decaying organic matter to plant life where it is synthesized (through photosynthesis) to carbohydrate material, from which, as a result of catabolic processes in all life, it is again ultimately released to the atmosphere as CO2.
cardiac c. the complete round of cardiac systole and diastole with the intervals between, or commencing with, any event in the heart's action to the moment when that same event is repeated.
cell c. the periodic biochemical and structural events occurring during proliferation of cells such as in tissue culture; the c. is divided into periods called: G0, Gap1 (G1), synthesis (S1), Gap2 (G2), and mitosis (M). The period runs from one division to the next.mitotic c;
chewing c. a complete course of movement of the mandible during a single masticatory stroke.
citric acid c. tricarboxylic acid c
Cori c. the phases in the metabolism of carbohydrate: 1) glycogenolysis in the liver; 2) passage of glucose into the circulation; 3) deposition of glucose in the muscles as glycogen; 4) glycogenolysis during muscular activity and conversion to lactate, which is converted to glycogen in the liver.
dicarboxylic acid c. 1. that portion of the tricarboxylic acid c. involving the dicarboxylic acids (succinic, fumaric, malic, and oxaloacetic acids); 2. a cyclic scheme in which certain steps of the tricarboxylic acid c. are used with the glyoxylate c.; important in the utilization of glyoxylic acid in microorganisms.
endogenous c. the portion of a parasitic life cycle occurring within the host.
estrous c. the series of physiologic uterine, ovarian, and other changes that occur in higher animals, consisting of proestrus, estrus, postestrus, and anestrus or diestrus.
exoerythrocytic c. that nonpathogenic portion of the vertebrate phase of the life cycle of malarial organisms that takes place in liver cells, outside of the blood cells.
exogenous c. the portion of a parasitic life cycle occurring outside the host.
fatty acid oxidation c. a series of reactions involving acyl-coenzyme A compounds, whereby these undergo beta oxidation and thioclastic cleavage, with the formation of acetyl-coenzyme A; the major pathway of fatty acid catabolism in living tissue.
forced c. a cardiac c. (atrial or ventricular) that is cut short by a forced beat.
futile c. a c. of phosphorylation and dephosphorylation catalyzed by two enzymes which normally function in two different metabolic pathways; the net effect is the hydrolysis of ATP and the generation of heat; e.g., the futile c. from the unregulated action of 6-phosphofructokinase and fructose-1,6-bisphosphatase in muscle; such c.'s may have important roles in heat production, in the fine tuning of the regulation of certain pathways and may be a factor in malignant hyperthermia.substrate c;
genesial c. the reproductive period of a woman's life.
gamma-glutamyl c. a proposed pathway for the glutathione-dependent transport of certain amino acids (most notably l-cystine, l-methionine, and l-glutamine) and dipeptides into certain cells; this c. requires the formation of gamma-glutamyl amino acids and gamma-glutamyl dipeptides as well as a protein for the translocation of these di- and triisopeptides into the cells.
glycine-succinate c. a series of metabolic steps in which glycine is condensed with succinyl-CoA and is then oxidized to CO2 and H2O with regeneration of the succinyl-CoA; important in the synthesis of delta-aminolevulinic acid and in the metabolism of red blood cells.Shemin c;
glyoxylic acid c. a catabolic c. in plants and microorganisms like that of the tricarboxylic acid c. in animals; its key reaction is the condensation of acetyl-CoA with glyoxylic acid to malic acid (analogous to the condensation of acetyl-CoA and oxaloacetic acid to form citric acid in the tricarboxylic acid c.).Krebs-Kornberg c;
gonadotrophic c. (go´nad-o-trof´ik) one complete round of ovarian development in the insect vector from the time when the blood meal is taken to the time when the fully developed eggs are laid.
hair c. the cyclical phases of growth (anagen), regression (catagen), and quiescence (telogen) in the life of a hair.
Krebs c. tricarboxylic acid c
Krebs-Henseleit c. , Krebs ornithine c. , Krebs urea c. urea c
Krebs-Kornberg c. glyoxylic acid c
life c. the entire life history of a living organism.
masticating c.'s the patterns of mandibular movements formed during the chewing of food.
menstrual c. the period in which an ovum matures, is ovulated, and enters the uterine lumen via the fallopian tubes; ovarian hormonal secretions effect endometrial changes such that, if fertilization occurs, nidation will be possible; in the absence of fertilization, ovarian secretions wane, the endometrium sloughs, and menstruation begins; this c. lasts an average of 28 days, with day 1 of the c. designated as that day on which menstrual flow begins.
mitotic c. cell c
nitrogen c. the series of events in which the nitrogen of the atmosphere is fixed, thus made available for plant and animal life, and is then returned to the atmosphere: nitrifying bacteria convert N2 and O2 to NO2- and NO3-, the latter being absorbed by plants and converted to protein; if plants decay, the nitrogen is in part given up to the atmosphere and the remainder is converted by microorganisms to ammonia, nitrites, and nitrates; if the plants are eaten, the animals' excreta or bacterial decay return the nitrogen to the soil and air.
ornithine c. urea c
ovarian c. the normal sex c. which includes development of an ovarian (graafian) follicle, rupture of the follicle with discharge of the ovum, and formation and regression of a corpus luteum.
pentose phosphate c. pentose phosphate pathway
reproductive c. the c. which begins with conception and extends through gestation and parturition.
restored c. an atrial or ventricular cardiac c. that follows the returning c. and resumes the normal rhythm.
returning c. an atrial or ventricular cardiac c. that begins with an extrasystole or a forced beat.
Ross c. the life c. of the malaria parasite.
Shemin c. glycine-succinate c
substrate c. futile c
succinic acid c. a series of oxidation reduction reactions in which succinic acid and other 4-carbon atoms acids (fumaric, malic, oxaloacetic) take part in the oxidation of pyruvic acid as part of the tricarboxylic acid c. See also dicarboxylic acid c.
tricarboxylic acid c. together with oxidative phosphorylation, the main source of energy in the mammalian body and the end toward which carbohydrate, fat, and protein metabolism are directed; a series of reactions, beginning and ending with oxaloacetic acid, during the course of which a two-carbon fragment is completely oxidized to carbon dioxide and water with the production of 12 high-energy phosphate bonds. So called because the first four substances involved (citric acid, cis-aconitic acid, isocitric acid, and oxalosuccinic acid) are all tricarboxylic acids; from oxalosuccinate, the others are, in order, a-ketoglutarate, succinate, fumarate, l-malate, and oxaloacetate, which condenses with acetyl-CoA (from fatty acid degradation) to form citrate (citric acid) again.citric acid c., Krebs c;
urea c. the sequence of chemical reactions, occurring primarily in the liver, that results in the production of urea; the key reaction is the hydrolysis of l-arginine by arginase to l-ornithine and urea; l-ornithine is then converted to l-citrulline by a carbamoylation and then to l-arginine again by an amination reaction involving l-aspartic acid.Krebs-Henseleit c., Krebs ornithine c., Krebs urea c., ornithine c;
visual c. the transformation of carotenoids involved in the bleaching and regeneration of the visual pigment.


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