سبتمبر 16th, 2018

 

absorption

Either the taking up of matter in bulk by other matter, as in the dissolving of a gas by a liquid; or the taking up of energy from radiation by the medium through which the radiation is passing. In the first case, an absorption coefficient is defined as the amount of gas dissolved at standard conditions by a unit volume of the solvent. Absorption in this sense is…

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absorbance

absorbance symbol: A; a measure of the ability of a substance or a solution to absorb electromagnetic radiation incident upon it. It equals the logarithm of the ratio of the radiant power of the incident radiation, Φ0, to the radiant power of the transmitted radiation, Φ. For a solution, absorbance is expressed as the logarithm of the ratio of the radiant power of light transmitted through the reference…

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absolute reaction rate theory

absolute reaction rate theory a theory that sets out to predict the absolute reaction rate of a chemical reaction from the quantum mechanical description of the potential energy changes during the interaction between chemical species. It is most widely drawn upon in applying thermodynamic reasoning to equilibria between reactants in the ground state and chemical species in the activated state or transition state.

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absolute alcohol

absolute alcohol the common name for pure ethanol, i.e. ethanol that has been freed of water. It may contain small amounts of benzene that have been added to aid in removing water. Substances may be added to absolute alcohol to render it unfit for human consumption and hence free of excise duty: industrial spirit contains 5% v/v methanol, while methylated spirit also contains pyridine, petroleum oil, and methyl…

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absolute

absolute 1 pure, unmixed; e.g. absolute alcohol. 2 not relative; e.g. absolute configuration. 3 describing a measurement defined in fundamental units of mass, length, and time that does not depend on the characteristics of the measuring apparatus; e.g. absolute temperature.

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abscisic acid

abscisic acid or (formerly) abscisin II or dormin or ABA; 5-(1- hydroxy-2,6,6-trimethyl-4-oxocyclohex-2-en-1-yl)-3-methylpenta- 2,4-dienoic acid; a chiral sesquiterpene. The naturally occurring form, the 2Z,4E,S isomer, also designated (S)-abscisic acid, is a phytohormone formed by the degradation of carotenoids. It controls abscission in flowers and fruit but probably not in leaves, and is also implicated in geotropism, stomatal closure, bud dormancy, dormancy of seeds requiring stratification (i.e. those that will…

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abrin

abrin a highly toxic ∼65 kDa glycoprotein obtained from the seeds of jequirity, or Indian liquorice (Abrus precatorius L.), a tropical Asian vine that also occurs in Florida. It consists of an ∼30 kDa acidic A chain, and an ∼35 kDa neutral B chain, held together by disulfide bonds. The A chain is a powerful inhibitor of protein synthesis, while the B chain functions as a carrier to…

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ablation

ablation 1 (in surgery) the removal or destruction of tissue by a surgical procedure. 2 (in genetics) a technique for the removal of a tissue or a particular cell type during development. It depends on the tissue-specific expression of a toxin gene such as diphtheria A (dipA) in a transgenic organism.

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abl

abl an oncogene from murine Abelson leukemia virus. The human equivalent is ABL (locus at 9q34), which encodes a tyrosine protein kinase. In humans, inappropriate activation of ABL occurs via a reciprocal translocation between chromosomes 9 and 22 in which ABL is joined at the breakpoint cluster region (bcr) of the ph1 gene on chromosome 22(q11), resulting in an altered chromosome 22, referred to as the Philadelphia chromosome…

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ABH antigens

ABH antigens one of the systems of blood group antigens having determinants associated with oligosaccharide structures. It is the basis of the ABO system, which was the first human blood group antigen system to be detected, by Austrian-born US pathologist Karl Landsteiner (1868–1943) in 1901, and it remains the most important in blood transfusion. Individuals having neither A nor B antigen express the H antigen, the product of…

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