citric acid cycle

citric acid cycle:

see Krebs cycleKrebs cycle,
series of chemical reactions carried out in the living cell; in most higher animals, including humans, it is essential for the oxidative metabolism of glucose and other simple sugars.
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Citric acid cycle

In aerobic cells of animals and certain other species, the major pathway for the complete oxidation of acetyl coenzyme A (the thioester of acetic acid with coenzyme A); also known as the Krebs cycle or tricarboxylic acid cycle. Reduced electron carriers generated in the cycle are reoxidized by oxygen via the electron transport system; water is formed, and the energy liberated is conserved by the phosphorylation of adenosine diphosphate (ADP) to adenosine triphosphate (ATP). Reactions of the cycle also function in metabolic processes other than energy generation. The role of the cycle in mammalian tissues will be emphasized in this article. See Adenosine diphosphate (ADP), Coenzyme, Enzyme

The first step in the cycle involves the condensation of the acetyl portion of acetyl coenzyme A (CoA) with the four-carbon compound oxaloacetate to form citrate, a tricarboxylate containing six carbons (see illustration). A shift of the hydroxyl group of citrate to an adjacent carbon results in the formation of d -threo-isocitrate, which in turn is oxidized to the five-carbon compound α-ketoglutarate and carbon dioxide (CO₂). In a second oxidative decarboxylation reaction, α-ketoglutarate, in the presence of CoA, is converted to succinyl CoA and another molecule of CO₂. In the subsequent formation of the four-carbon compound succinate and CoA, the energy in the thioester bond of succinyl CoA is conserved by the formation of guanosine triphosphate (GTP) from guanosine diphosphate (GDP) and inorganic phosphate. Fumarate is formed from succinate by the removal of two atoms of hydrogen, and the unsaturated compound is then hydrated to l -malate. The dehydrogenation of malate forms oxaloacetate, the starting four-carbon compound of the metabolic cycle. Thus, beginning with the two-carbon acetyl group, one completion of the cycle results in the formation of two molecules of carbon dioxide.

Citric acid cycle

The oxidation of acetyl CoA to CO₂ in the cycle occurs without direct reaction with molecular oxygen. The oxidations occur at dehydrogenation reactions in which hydrogen atoms and electrons are transferred from intermediates of the cycle to the electron carriers nicotinamide adenine dinucleotide (NAD⁺) and flavin adenine dinucleotide (FAD). The electrons from NADH and FADH₂ are transferred to molecular oxygen via a series of electron transport carriers, with regeneration of NAD⁺ and FAD. The energy liberated in the electron transport chain is partially conserved by the formation of ATP from ADP and inorganic phosphate, by a process called oxidative phosphorylation. The energy generated as oxygen accepts electrons from the reduced coenzymes generated in one turn of the cycle results in the maximal formation of 11 molecules of ATP. Because GTP obtained by phosphorylation of GDP at the succinyl CoA to succinate step of the cycle is readily converted to ATP by nucleotide diphosphokinase, the yield is 12 molecules of ATP per molecule of acetyl CoA metabolized. See Nicotinamide adenine dinucleotide (NAD)

The electron transport and oxidative phosphorylation systems and the enzymes required for the citric acid cycle are located in the mitochondria of cells, which are the major source of ATP for energy-consuming reactions in most tissues. The citric acid cycle does not occur in all cells. For example, mature human red blood cells do not contain mitochondria and the cycle is absent. In these cells, ATP is formed by the anaerobic conversion of glucose to lactate (anaerobic glycolysis). See Mitochondria

Acetyl CoA is formed from carbohydrates, fats, and the carbon skeleton of amino acids. The origin of a precursor and the extent of its utilization depend on the metabolic capability of a specific tissue and on the physiological state of the organism. For example, most mammalian tissues have the capacity to convert glucose to pyruvate in a reaction called glycolysis. Pyruvate is then taken up from cellular cytosol by mitochondria and oxidatively decarboxylated to acetyl CoA and carbon dioxide by pyruvate dehydrogenase. Acetyl CoA is also the end product of fatty acid oxidation in mitochondria. However, the fatty acid oxidation pathway occurs in fewer tissues than does glycolysis or the citric acid cycle. The amino acids follow varied pathways for forming compounds that can enter the citric acid cycle. See Amino acids

In addition to the cycle's role in yielding catabolic energy, portions of it can supply intermediates for synthetic processes, such as the synthesis of the fatty acid moiety of triglycerides from glucose (lipogenesis), and formation of glucose from the carbon skeletons of certain amino acids, lactate, or glycerol (gluconeogenesis). See Carbohydrate metabolism, Cell (biology), Glucose, Glycogen, Lipid metabolism, Metabolism

citric acid cycle

[′si‚trik ′as·əd ′sī·kəl] (biochemistry) Krebs cycle

citric acid cycle

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.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.

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" >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. Synonym(s): citric acid cycle, Krebs cycle

citric acid cycle

n. See Krebs cycle.

cycle

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

anovular cycle

Menstrual cycle in which ovulation is absent.

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).

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)
0.5	6/3	20/10	60
0.6	6/3.6	20/12	50
0.75	6/4.5	20/15	40
1.0	6/6	20/20	30
1.25	6/7.5	20/25	24
1.5	6/9	20/30	20
2.0	6/12	20/40	15
2.5	6/15	20/50	12
4.0	6/24	20/80	7.5
5.0	6/30	20/100	6
8.0	6/48	20/160	3.8
10.0	6/60	20/200	3
20.0	6/120	20/400	1.5

citric acid cycle

citric acid cycle

citric acid cycle

Krebs′ cy`cle

citric acid cycle

citric acid cycle

citric acid cycle:

Citric acid cycle

citric acid cycle

citric acid cycle

cycle

tri·car·box·yl·ic ac·id cy·cle

citric acid cycle

cycle

anovular cycle

cardiac cycle

Treatment

cell cycle

cell growth cycle

citric acid cycle

Cori cycle

duty cycle

estrus cycle

gastric cycle

genesial cycle

glycolytic cycle

initiated cycle

Krebs cycle

life cycle

menstrual cycle

nitrogen cycle

sleep-wake cycle

stimulated cycle

stretch-shortening cycle

tricarboxylic acid cycle

unstimulated cycle

urea cycle

Wald cycle

citric acid cycle

cycle, Krebs

citric acid cycle

Synonyms for citric acid cycle

noun in all plants and animals: a series of enzymatic reactions in mitochondria involving oxidative metabolism of acetyl compounds to produce high-energy phosphate compounds that are the source of cellular energy

Synonyms

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