الرئيسية » Simplified Chemistry (صفحه 20)

Simplified Chemistry

bacteriorhodopsin

A membrane­ bound protein of the halophilic (salt­resistant) bacterium Halobacterium halobium. When activated by light, it pumps protons out of the cell; this creates a concentration gradient, which enables ATP to be synthesized. Bacteriorhodopsin is composed of seven a-helix segments, which span the membrane and are joined to­gether by short …

أكمل القراءة »

back titration

A technique in vol­umetric analysis in which a known excess amount of a reagent is added to the solution to be estimated. The unreacted amount of the added reagent is then determined by titra­tion, allowing the amount of sub­stance in the original test solution to be calculated.

أكمل القراءة »

background radiation

Low inten­sity ionizing radiation present on the surface of the earth and in the atmosphere as a result of cosmic radiation and the presence of radio­isotopes in the earth’s rocks, soil, and atmosphere. The radioisotopes are either natural or the result of nu­clear fallout or waste gas from power stations. …

أكمل القراءة »

backbiting

A rearrangement that can occur in some polymerization reactions involving free radicals. A radical that has an unpaired electron at the end of the chain changes into a radical with the unpaired electron elsewhere along the chain, the new radical being more stable than the one from which it originates. …

أكمل القراءة »

Babbit metal

Any of a group of re­lated alloys used for making bear­ings. They consist of tin containing antimony (about 10%) and copper (1-2%), and often lead. The original alloy was invented in 1839 by the US inventor Isaac Babbit (1799-1862).

أكمل القراءة »

allosteric enzyme

An enzyme that has two structurally distinct forms, one of which is active and the other inactive. In the active form, the quaternary structure of the enzyme is such that a substrate can interact with the enzyme at the active site. The conformation of the substrate-binding site becomes al­tered in …

أكمل القراءة »

allenes

Compounds that contain the group >C=C=C<, in which three carbon atoms are linked by two adja­cent double bonds. The outer carbon atoms are each linked to two other atoms or groups by single bonds. The simplest example is 1,2-propadiene, CH2CCH2 . Allenes are dienes with typical reactions of alkenes. Under …

أكمل القراءة »

alkyd resin

A type of polyester resin used in paints and other sur­face coating. The original alkyd resins were made by copolymerizing phthalic anhydride with glycerol, to give a brittle cross-linked polymer. The properties of such resins can be modified by adding monobasic acids or alcohols during the polymeriza­tion.

أكمل القراءة »

(algin (alginic acid

A complex poly­saccharide occurring in the cell walls of the brown algae (Phaeophyta). Algin strongly absorbs water to form a viscous gel. It is produced commer­cially from a variety of species of Laminaria and from Macrocystis pyrifera in the form of alginates, which are used mainly as a stabilizer and …

أكمل القراءة »

aldoximes

Compounds formed by reaction between hydroxylamine and an aldehyde RCOH + H2NOH → RCH:NOH + H2O  If R is an aliphatic group, the al­doxime is generally a liquid or low ­melting solid. If R is aromatic, the aldoxime is a crystalline solid. Al­doximes have a planar structure and can exist …

أكمل القراءة »