citric acid cycle


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cycle

 [si´k'l]
a succession or recurring series of events.
cardiac cycle a complete cardiac movement, or heart beat, including systole, diastole, and the intervening pause.
Cardiac cycle. From Applegate, 2000.
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.
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.
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.
 Central pathways of metabolism: How the body produces energy from the energy-containing nutrients using the tricarboxylic acid cycle. From Davis and Sherer, 1994.
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.

tri·car·box·yl·ic ac·id cy·cle

together with oxidative phosphorylation, the main source of energy in the mammalian body and the end toward which carbohydrate, fat, and protein metabolism is 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, α-ketoglutarate, succinate, fumarate, l-malate, and oxaloacetate, which condenses with acetyl-CoA (from fatty acid degradation) to form citrate (citric acid) again.

citric acid cycle

citric acid cycle

Etymology: Gk, kitron, citron; L, acidus, sour; Gk, kyklos, circle
a sequence of enzymatic reactions involving the metabolism of carbon chains of sugars, fatty acids, and amino acids to yield carbon dioxide, water, and high-energy phosphate bonds. The cycle is initiated when pyruvate combines with coenzyme A (CoA) to form a two-carbon unit, acetyl-CoA, which enters the cycle by combining with four-carbon oxaloacetic acid to form six-carbon citric acid. In subsequent steps, isocitric acid, produced from citric acid, is oxidized to oxalosuccinic acid, which loses carbon dioxide to form alpha-ketoglutaric acid. Succinic acid, resulting from the oxidative decarboxylation of alpha-ketoglutaric acid, is oxidized to fumaric acid, and its oxidation regenerates oxaloacetic acid, which condenses with acetyl-CoA, closing the cycle. The citric acid cycle provides a major source of adenosine triphosphate energy and also produces intermediate molecules that are starting points for a number of vital metabolic pathways including amino acid synthesis. Also called Krebs cycle, tricarboxylic acid cycle. See also acetylcoenzyme A.

cycle

(si'kel) [Gr. kyklos, circle]
A regular, complete series of movements or events.

anovular cycle

Menstrual cycle in which ovulation is absent.
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CARDIAC CYCLE (ONE HEARTBEAT, PULSE 75): The outer circle represents the ventricles, the middle circle the atria, and the inner circle the movement of blood and its effect on the heart valves.

cardiac cycle

The period from the beginning of one heartbeat to the beginning of the succeeding beat, including systole and diastole. Normally, the atria contract immediately before the ventricles. The ordinary cycle lasts 0.8 sec with the heart beating approx. 60 to 85 times a minute in the adult at rest. Atrial systole lasts 0.1 sec, ventricular systole 0.3 sec, and diastole 0.4 sec. Although the heart seems to be working continuously, it actually rests for a good portion of each cardiac cycle.
A wart, typically found on the genitals, the perineum, the anus, or the mucosal surfaces of the vagina or mouth, usually spread by sexual contact. It is caused by various types of human papilloma virus and may be spread by physical contact with an area containing a wart. The spread of a wart from one labium to the other by autoinoculation is possible. The virus that causes the wart is usually transmitted sexually. Synonym: genital wart

Treatment

Topically applied liquid nitrogen, imiquimod cream, fluorouracil, or podophyllin may prove effective; multiple treatments are usually needed, including occasionally surgery, electrosurgery, or laser ablation. Extremely large lesions (Buschke-Lowenstein tumor) may need radical excision.

See: Cardiac Cycle illustration

cell cycle

The cycle of the growth and development of a cell. The cell cycle consists of mitosis, during which chromosomes actively divide to form two sister cells, and the interphase, during which the cell grows, begins to synthesize DNA, and prepares for chromosomal division. The interphase consists of several gap or G phases and the S (DNA Synthesis) phase.
See: interphase; meiosis and mitosis for illus.

cell growth cycle

The order of physical and biochemical events that occur during the growth of cells. In tissue culture studies, the cyclic changes are divided into specific periods or phases: the DNA synthesis or S period, the G2 period or gap, the M or mitotic period, and the G1 period.

citric acid cycle

Krebs cycle.

Cori cycle

See: Cori cycle

duty cycle

During chest compressions of a victim of cardiac arrest, the relative amount of time that the chest is compressed compared to the time that the chest is allowed to recoil to its fully inflated position. A cycle of 50% occurs when chest compression equals chest recoil.

estrus cycle

The sequence from the beginning of one estrus period to the beginning of the next. It includes proestrus, estrus, and metestrus, followed by a short period of quiescence called diestrus.

gastric cycle

The progression of peristalsis through the stomach.

genesial cycle

1. The period from puberty to menopause.
2. The period of sexual maturity.

glycolytic cycle

The cycle by which glucose is broken down in living tissue.

initiated cycle

In assisted reproduction, any month when a woman is treated with drugs that stimulate the ovary to produce follicles.

Krebs cycle

See: Krebs cycle

life cycle

All of the developmental history of an organism, whether in a free-living condition or in a host (e.g., as a parasite that experiences part of its cycle inside another organism).
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PHASES OF THE MENSTRUAL CYCLE

menstrual cycle

The periodically recurrent series of changes occurring in the uterus and associated sex organs (ovaries, cervix, and vagina) associated with menstruation and the intermenstrual period. The human cycle averages 28 days in length, measured from the beginning of menstruation. The menstrual cycle is, however, quite variable in length, even in the same person from month to month. Variations in the length of the cycle are due principally to variation in the length of the proliferative phase. See: illustration

The menstrual cycle is divided into four phases characterized by histological changes that take place in the uterine endometrium. They are:

Proliferative Phase: Following blood loss from the endometrium, the uterine epithelium is restored to normal; the endometrium becomes thicker and more vascular; the glands elongate. During this period, the ovarian follicle is maturing and secreting estrogens; with the estrogen stimulation, the endometrium hypertrophies, thickening and becoming more vascular, and the glands elongate. The phase is terminated by the rupture of the follicle and the liberation of the ovum at about 14 days before the next menstrual period begins. Fertilization of the ovum is most likely to occur in the days immediately following ovulation.

Luteal or Secretory Phase: After releasing the ovum, the corpus luteum secretes progesterone. With the progesterone stimulation, the endometrium becomes even thicker; the glands become more tortuous and produce an abundant secretion containing glycogen. The coiled arteries make their appearance; the endometrium becomes edematous; the stroma becomes compact. During this period, the corpus luteum in an ovary is developing and secreting progesterone. This phase lasts 10 to 14 days.

Premenstrual or Ischemic Phase: If pregnancy has not occurred, the coiled arteries constrict and the endometrium becomes anemic and shrinks a day or two before menstruation. The corpus luteum of the ovary begins involution. This phase lasts about 2 days and is terminated by the opening up of constricted arteries, the breaking off of small patches of endometrium, and the beginning of menstruation with the flow of menstrual fluid.

Menstruation: The functional layer of the endometrium is shed.

The menstrual cycle is altered by pregnancy, the use of contraception, intercurrent illnesses, diet, and exercise.

nitrogen cycle

A series of natural processes in which nitrogen is discharged from animal life into the soil; the nitrogen is taken up from the soil by nitrogen-fixing bacteria and converted to nitrates usable by plants for their nourishment; and in turn nitrogen is taken up by plant-eating animals.

sleep-wake cycle

The amount of time spent asleep and awake and the cycle of that schedule from day to day.

stimulated cycle

A cycle in assisted reproduction in which a woman receives drugs to stimulate her ovaries for production of additional follicles.
See: unstimulated cycle

stretch-shortening cycle

An eccentric muscle contraction followed immediately by a concentric contraction of the same muscle group. The elastic potentiation that occurs during the eccentric phase increases the force of output of the concentric contraction. These exercises replicate functional movement patterns and are typically used in the advance phase of rehabilitation, particularly in sports rehab. Exercises incorporating this phenomenon are called plyometrics. See: plyometrics

tricarboxylic acid cycle

Krebs cycle.

unstimulated cycle

A cycle in assisted reproduction in which a woman does not receive drugs to stimulate her ovaries for production of additional follicles.
See: stimulated cycle

urea cycle

The complex cyclic chemical reactions in some (ureotelic) animals, including humans, that produce urea from the metabolism of nitrogen-containing foods. This cycle, first described by Sir Hans Krebs, provides a method of excreting the nitrogen produced by the metabolism of amino acids as urea.

Wald cycle

See: Wald cycle

citric acid cycle

See KREBS CYCLE.

cycle, Krebs 

A series of reactions in which the intermediate products of carbohydrate, fat and protein metabolism are converted to carbon dioxide and hydrogen atoms (electrons and hydrogen ions). This cycle can only operate in the presence of oxygen. Further oxidation yields carbon dioxide, water and ATP. This cycle occurs in the mitochondria that are found in the cytoplasm of cells of living organisms. It forms one of the processes in the metabolism of glucose providing energy (stored in ATP) to maintain the vital functions of the cells (e.g. mitosis). This cycle represents the principal energy pathway of the corneal endothelium. Syn. citric acid cycle; tricarboxylic acid cycle. See mitosis.
Table C9 Relationship between the minimum angle of resolution, the Snellen fraction and the equivalent spatial frequency of a sine wave
Snellen fraction
resolution (min of
arc)
(m)(ft)Spatial frequency (cpd)
0.56/320/1060
0.66/3.620/1250
0.756/4.520/1540
1.06/620/2030
1.256/7.520/2524
1.56/920/3020
2.06/1220/4015
2.56/1520/5012
4.06/2420/807.5
5.06/3020/1006
8.06/4820/1603.8
10.06/6020/2003
20.06/12020/4001.5

citric acid

a tricarboxylic acid occurring in citrus fruits and acting as an antiscorbutic and diuretic. It functions as an anticoagulant in the blood preservatives, acid citrate dextrose and citrate phosphate dextrose. See also citrate.

citric acid cycle

cycle

a succession or recurring series of events.

cardiac cycle
a complete cardiac movement, or heartbeat, including systole, diastole, and the intervening pause.
The cycle includes eight separate phases: (1) isovolumetric contraction; (2) maximum ejection; (3) reduced ejection; (4) protodiastole (onset of ventricular relaxation); (5) isovolumetric relaxation; (6) rapid flow; (7) diastasis (onset of atrial contraction); (8) atrial systole.
cell cycle
the cycle of biochemical and morphological events occurring in a dividing cell population; it consists of the S phase, occurring toward the end of interphase, in which DNA is synthesized; the G2 phase, for gap 2, the interval between S and M; the M phase, for mitosis, consisting of the four phases of mitosis; and the G1 phase, which lasts from the end of M until the start of S phase of the next cycle. Fully differentiated cells are nondividing and are said to be in G0.
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Cell cycle. By permission from Booth DM, Small Animal Clinical Pharmacology and Therapeutics, Saunders, 2000
citric acid cycle
estrus cycle
see estrous cycle.
Krebs cycle
ovarian cycle
the sequence of physiological changes in the ovary involved in ovulation. See also ovulation and reproduction.
reproductive cycle
the cycle of physiological changes in the reproductive organs, from the time of fertilization of the ovum through gestation and parturition. See also reproduction.
sex cycle, sexual cycle
1. the physiological changes recurring regularly in the reproductive organs of female mammals when pregnancy does not supervene.
2. the period of sexual reproduction in an organism that also reproduces asexually.
tricarboxylic acid cycle
urea cycle
a cyclic series of reactions that produce urea, a major route for removal of the ammonia produced in the metabolism of amino acids in the liver and kidney. See also urea.
References in periodicals archive ?
Pyruvate is converted (through the actions of the thiamine-dependent enzyme pyruvate dehydrogenase) into a molecule called acetyl-CoA, which can then be fed into the second set of reactions, the citric acid cycle (also known as the tricarboxylic acid [TCA] cycle).
He also was drawn to the primitive power of the citric acid cycle.
This book includes considerable detail of metabolic pathways related to proteins: the citric acid cycle, biosynthesis of proteins, degradation of proteins via proteasomes, and folding inside GroEC-GroES barrels, for example; it thus goes well beyond protein chemistry.