مصادر الكيمياء موقع لتعليم الكيمياء بصورة مبسطة

الامتصاص Absorption تعريف الامتصاص وأمثلة يحدث الامتصاص عندما تنتقل الأيونات أو الذرات أو الجزيئات إلى المادة الماصة. هذه الجسيمات (الممتصة) تنتشر أو تذوب في المادة الماصة. و المثال المألوف على ذلك هو امتصاص منديل ورقي للماء، و يحدث الامتصاص بشكل سلبي (الانتشار) أو بشكل نشط (الانتشار الميسر أو النقل النشط) وهو عملية ماصة للحرارة. و يعتمد معدل الامتصاص على عدة …

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 …

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 …

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.

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 …

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 …

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.

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 …

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 …

abetalipoproteinemia or (Brit.) abetalipoproteinaemia an autosomal recessive disorder in which plasma lipoproteins lack apolipoprotein B. There is defective assembly and secretion both of chylomicrons in intestinal mucosa and of very-low-density lipoproteins in the liver. The cause is a deficiency of the 88 kDa subunit of microsomal triglyceride transfer protein.