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abomasum
In Ruminantia, the fourth and final region of the specialized stomach, corresponding to the stomach in other mammals and the zone in which digestion proceeds with the usual mammalian digestive ...
abscission
The separation of a leaf, fruit, or other part from the body of a plant. It involves the formation of an abscission zone, at the base of the part, within which a layer of cells ( abscission layer) ...
abzyme
Catalytic antibodies. A class of monoclonal antibodies that bind to and stabilize molecules in the transition state through which they must pass to form products. See enzyme.
acarbose
A group of complex carbohydrates (oligosaccharides) which inhibit the enzymes of starch and disaccharide digestion; used experimentally to reduce the digestion of starch. It has been marketed for use ...
ACAT
Acyl CoA cholesterol acyl transferase, the enzyme that catalyses esterification of cholesterol by transfer of fatty acids from fatty acyl CoA. See also LCAT.
accelase
A mixture of enzymes that hydrolyse proteins, used to shorten the maturation time of cheeses and intensify the flavour of processed cheese.
acetylcholine
(ass-i-tyl-koh-leen)the acetic acid ester of the organic base choline: the neurotransmitter released at the synapses of parasympathetic nerves and at neuromuscular junctions. See also cholinesterase.
acid–base homeostasis
All living things depend on water. Life consists of a highly complex series of chemical reactions occurring in aqueous media. Among the most important factors in the composition of these ...
acrosome
A thin-walled vesicle that forms a cap on the head of a spermatozoon. On contact with an ovum it bursts, releasing powerful hydrolytic enzymes which cause a localized softening of the vitelline ...
activation energy
The energy that must be delivered to a system in order to increase the incidence within it of reactive molecules, thus initiating a reaction.
activator
A molecule that converts a repressor into a stimulator of operon transcription; e.g., the repressor of a bacterial arabinose operon becomes an activator when combined with the substrate.
active site
The part of an enzyme molecule that binds it to the substrate or substrates to form an enzyme–substrate complex. The conformation is not absolute and may alter according to reaction conditions.
active transport
The movement of an ion or molecule across a cell membrane against a concentration or electrochemical gradient. The process requires specific enzymes and energy supplied by ATP.
alactasia
n. absence or deficiency of the enzyme lactase, which is essential for the digestion of milk sugar (lactose). All babies have lactase in their intestines, but the enzyme disappears during childhood ...
albinism
Hereditary lack of pigmentation (see melanin) in an organism. Albino animals and human beings have no colour in their skin, hair, or eyes (the irises appear pink from underlying blood vessels). The ...
aleurone layer
The outer layer of living cells of the endosperm of wheat and other grain species. It is a single layer of cells that synthesizes the enzyme α-amylase, which is secreted during germination into the ...
alimentary canal
(ali-ment-er-i)the long passage, extending from the mouth to the anus, through which food passes to be digested and absorbed.The alimentary canal
alkaline phosphatase
An enzyme that catalyses the hydrolysis of phosphoric acid esters under conditions of alkaline pH. In humans the blood levels of alkaline phosphatase are measured as part of the assessment of liver ...
allosteric
Applied to a protein molecule that has two or more sites at which it can bind (combine) with other molecules, such that binding with one influences its binding with another. Some (allosteric) ...
