Krebs cycle

Krebs cycle,

series of chemical reactions carried out in the living cell; in most higher animals, including humans, it is essential for the oxidative metabolismmetabolism,
sum of all biochemical processes involved in life. Two subcategories of metabolism are anabolism, the building up of complex organic molecules from simpler precursors, and catabolism, the breakdown of complex substances into simpler molecules, often accompanied by
..... Click the link for more information. of glucoseglucose,
dextrose,
or grape sugar,
monosaccharide sugar with the empirical formula C₆H₁₂O₆ . This carbohydrate occurs in the sap of most plants and in the juice of grapes and other fruits.
..... Click the link for more information. and other simple sugars. The breakdown of glucose to carbon dioxide and water is a complex set of chemical interconversions called carbohydrate catabolismcatabolism
, subdivision of metabolism involving all degradative chemical reactions in the living cell. Large polymeric molecules such as polysaccharides, nucleic acids, and proteins are first split into their constituent monomeric units, such as amino acids, after which the
..... Click the link for more information. , and the Krebs cycle is the second of three major stages in the process, occurring between glycolysisglycolysis
, term given to the metabolic pathway utilized by most microorganisms (yeast and bacteria) and by all "higher" animals (including humans) for the degradation of glucose. Glycolysis means, literally, the dissolution of sugar.
..... Click the link for more information. and oxidative phosphorylationphosphorylation,
chemical process in which a phosphate group is added to an organic molecule. In living cells phosphorylation is associated with respiration, which takes place in the cell's mitochondria, and photosynthesis, which takes place in the chloroplasts.
..... Click the link for more information. . This cycle, also known as the citric acid cycle, was named in recognition of the German chemist Hans Krebs, whose research into the cellular utilization of glucose contributed greatly to the modern understanding of this aspect of metabolism. The name citric acid cycle is derived from the first product generated by the sequence of conversions, i.e., citric acidcitric acid
or 2-hydroxy-1,2,3-propanetricarboxylic acid,
HO₂CCH₂C(OH)(CO₂H)CH₂CO₂H, an organic carboxylic acid containing three carboxyl groups; it is a solid at room temperature, melts at 153&degC;, and
..... Click the link for more information. . The reactions are seen to comprise a cycle inasmuch as citric acid is both the first product and the final reactant, being regenerated at the conclusion of one complete set of chemical rearrangements. Citric acid is a so-called tricarboxylic acid, containing three carboxyl groups (COOH). Hence the Krebs cycle is sometimes referred to as the tricarboxylic acid (TCA) cycle. The Krebs cycle begins with the condensation of one molecule of a compound called oxaloacetic acid and one molecule of acetyl CoA (a derivative of coenzyme A; see coenzymecoenzyme
, any one of a group of relatively small organic molecules required for the catalytic function of certain enzymes. A coenzyme may either be attached by covalent bonds to a particular enzyme or exist freely in solution, but in either case it participates intimately in
..... Click the link for more information. ). The acetyl portion of acetyl CoA is derived from pyruvic acid, which is produced by the degradation of glucose in glycolysis. After condensation, the oxaloacetic acid and acetyl CoA react to produce citric acid, which serves as a substrate for seven distinct enzyme-catalyzed reactions that occur in sequence and proceed with the formation of seven intermediate compounds, including succinic acid, fumaric acid, and malic acid. Malic acid is converted to oxaloacetic acid, which, in turn, reacts with yet another molecule of acetyl CoA, thus producing citric acid, and the cycle begins again. Each turn of the citric acid cycle produces, simultaneously, two molecules of carbon dioxide and eight atoms of hydrogen as byproducts. The carbon dioxide generated is an ultimate end product of glucose breakdown and is removed from the cell by the blood. The hydrogen atoms are donated as hydride ions to the system of electron transport molecules, which allow for oxidative phosphorylation. In most higher plants, in certain microorganisms, such as the bacterium Escherichia coli, and in the algae, the citric acid cycle is modified to a form called the glyoxylate cycle, so named because of the prominent intermediate, glyoxylic acid.

Krebs cycle

[′krebz ‚sī·kəl] (biochemistry) A sequence of enzymatic reactions involving oxidation of a two-carbon acetyl unit to carbon dioxide and water to provide energy for storage in the form of high-energy phosphate bonds. Also known as citric acid cycle; tricarboxylic acid cycle.

Krebs 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

Krebs cycle

(krĕbz)n. A series of enzymatic reactions in aerobic organisms involving oxidative metabolism of acetyl units and producing high-energy phosphate compounds such as ATP, which serve as the main source of cellular energy. Also called citric acid cycle, tricarboxylic acid cycle.

tri·car·box·yl·ic ac·id cy·cle

(TCA cycle) (trī-kahr-bok-sil'ik as'id sī'kĕl) 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): Krebs cycle.

Krebs cycle

A cyclical sequence of 10 biochemical reactions, brought about by mitochondrial enzymes, that involves the oxidation of a molecule of acetyl-CoA, to two molecules of carbon dioxide and water. Each turn of the cycle can result in the formation of 12 molecules of ATP per molecule of acetyl-CoA. ATP is the direct source of energy for all work performed in any cell. The Krebs cycle is one of the most important in all body biochemistry and occurs in all organisms that oxidise food totally to carbon dioxide and water. Also known as the citric acid cycle or the tricarboxylic acid cycle. (Hans Adolf Krebs, 1900–89, German-born English biochemist)

Fig. 202 Krebs cycle . The major steps of the Krebs cycle.

Krebs cycle or tricarboxylic acid cycle (TCA cycle) or (formerly) citric-acid cycle

a circular series of reactions that forms part of CELL RESPIRATION in the presence of oxygen, occurring in many microorganisms as well as higher organisms. In eukaryotes the cycle occurs in the matrix of MITOCHONDRIA. The steps of the cycle were deduced by Sir Hans Krebs (1900–81), for which he received a Nobel Prize. The reactions leading up to the cycle by which ACETYLCOENZYME A is produced, together with the overall role of the cycle in the breakdown of complex molecules is described fully under AEROBIC RESPIRATION.

Each turn of the cycle releases 2 molecules of carbon dioxide, 8 hydrogen atoms that produce 11 molecules of ATP via the ELECTRON TRANSPORT SYSTEM, and 1 molecule of ATP produced by SUBSTRATE-LEVEL PHOSPHORYLATION. Two turns of the cycle are needed to complete the breakdown of one glucose molecule.

Krebs,

Sir Hans Adolph, German biochemist in England and Nobel laureate, 1900-1981. Krebs cycle - 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. Synonym(s): tricarboxylic acid cycleKrebs-Henseleit cycle - the sequence of chemical reactions, occurring primarily in the liver, that results in the production of urea. Synonym(s): urea cycleKrebs-Ringer solution - a modification of Ringer solution.

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

Krebs cycle

Krebs cycle

Krebs cycle

Krebs′ cy`cle

Krebs cycle

Krebs cycle

Krebs cycle

Krebs cycle,

Krebs cycle

Krebs cycle

cycle

tri·car·box·yl·ic ac·id cy·cle

Krebs cycle

tri·car·box·yl·ic ac·id cy·cle

Krebs cycle

Krebs cycle or tricarboxylic acid cycle (TCA cycle) or (formerly) citric-acid cycle

Krebs,

cycle, Krebs

Krebs cycle

Synonyms for Krebs 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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