cycle /cy·cle/ (si´k'l) a succession or recurring series of events.
nitrogen cycle
carbon cycle the steps by which carbon (in the form of carbon dioxide) is extracted from the atmosphere by living organisms and ultimately returned to the atmosphere. It comprises a series of interconversions of carbon compounds beginning with the production of carbohydrates by plants during photosynthesis, proceeding through animal consumption, and ending and beginning again in the decomposition of the animal or plant or in the exhalation of carbon dioxide by animals.
cardiac cycle a complete cardiac movement, or heart beat, including systole, diastole, and intervening pause. 
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Events of the cardiac cycle.
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 c.
Cori cycle the mechanism by which lactate produced by muscles is carried to the liver, converted back to glucose via gluconeogenesis, and returned to the muscles.
γ-glutamyl cycle a metabolic cycle for transporting amino acids into cells.
Krebs cycle tricarboxylic acid c.
Krebs-Henseleit cycle urea c.
menstrual cycle the period of the regularly recurring physiologic changes in the endometrium, occurring during the reproductive period of female humans, culminating in partial shedding of the endometrium and some bleeding per vagina (menstruating). 
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Changes in the menstrual cycle in the human female. Solid lines indicate the course of events when the oocyte is not fertilized; dotted lines indicate the course of events when fertilization occurs. Arrows indicate the actions of hormones of the pituitary and the ovary in regulating the cycle.
mosquito cycle that period in the life of a malarial parasite that is spent in the body of the mosquito host.
nitrogen cycle the steps by which nitrogen is extracted from the nitrates of soil and water, incorporated as amino acids and proteins in living organisms, and ultimately reconverted to nitrates: (1) conversion of nitrogen to nitrates by bacteria; (2) the extraction of the nitrates by plants and the building of amino acids and proteins by adding an amino group to the carbon compounds produced in photosynthesis; (3) the ingestion of plants by animals, and (4) the return of nitrogen to the soil in animal excretions or on the death and decomposition of plants and animals.
ornithine cycle urea c.
ovarian cycle the sequence of physiologic changes in the ovary involved in ovulation.
reproductive cycle the cycle of physiologic changes in the female reproductive organs, from the time of fertilization of the oocyte through gestation and parturition.
sex cycle , sexual cycle
1. the physiologic changes recurring regularly in the genital organs of nonpregnant female mammals; in humans, the menstrual cycle.
2. the period of sexual reproduction in an organism that also reproduces asexually.
tricarboxylic acid cycle the final common pathway for the oxidation to CO2 of fuel molecules, most of which enter as acetyl coenzyme A; it also provides intermediates for biosynthetic reactions and generates ATP by providing electrons to the electron transport chain. 
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Tricarboxylic acid cycle. Diagrammatic representation of reactions by which carbon chains of sugars, fatty acids, and amino acids are metabolized to yield carbon dioxide. Water produced by the cycle and components of the high-energy phosphate pool generated by the associated electron chain are not shown.
urea cycle a series of metabolic reactions in the liver, by which ammonia is converted to urea using cyclically regenerated ornithine as a carrier. 
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Urea cycle. Diagrammatic representation of reactions by which excess nitrogen in the form of ammonia is converted to soluble urea, using l-ornithine as a recyclable carrier.
uterine cycle the phenomena occurring in the endometrium during the menstrual cycle, preparing it for implantation of the blastocyst.
visual cycle the cyclic interconversion of 11-cis- retinal and all-trans- retinal and association with opsins, creating an electric potential and initiating the cascade generating a sensory nerve impulse in vision. 
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Visual cycle of retinal rod cells; an analogous cycle occurs with iodopsins in the cones.
| nitrogen cycle n. 1. The circulation of nitrogen in nature, consisting of a cycle of chemical reactions in which atmospheric nitrogen is compounded, dissolved in rain, and deposited in the soil, where it is assimilated and metabolized by bacteria and plants, eventually returning to the atmosphere by bacterial decomposition of organic matter. 2. See carbon-nitrogen cycle. |
nitrogen cycle
Etymology: Gk, nitron, soda, genein, to produce, kyklos, circle
the circulation of nitrogen through natural processes in either of two ways: from the soil to organisms that excrete nitrogen products back into the soil or by bacterial fixation of atmospheric nitrogen through other organisms that decay and release the element back into the atmosphere.
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