Abiotic
Abiotic | Any nonliving factor making up the environment of living organisms. e.g. Water, Rainfall, pH, Temperature or light intensity
Abscisic Acid (ABA)
Abscisic Acid (ABA) | Plant growth substance.
Plant growth substance that stimulates closing of stomatal pores minimising water loss.
Also inhibits premature seed germination.
Absorption
Absorption | Process by which fluids or dissolved substances are moved across cell membranes.
Absorption may involve diffusion, facilitated diffusion, active transport, osmosis or pinocytosis.
e.g. movement of solutes across cells lining the ileum or nephrons. In plants substances are absorbed from the soil through root hairs.
Absorption Spectrum
Absorption Spectrum | Graph showing the relative amounts of light at particular wavelengths and the absorption of specific pigments.
e.g. chlorophyll absorbs blue light and red light, but reflects green light thus, leaves appear green. Not to be confused with action spectrum.
Accommodation (1.)
Accommodation | 1. (in animal physiology)
A reflex adjustment to the shape of the lens of the eye - the process by which the eye focuses upon an object.
Accommodation is the process that allows clear images to be formed on the retina over various ranges of distance.
Accommodation (2.)
Accommodation | 2. (in plant physiology)
The ability for a plant to adapt to gradually changing environmental conditions.
Accommodation (3.)
Accommodation | 3. (in animal behaviour)
The Adjustments made to the nervous system of animals or their sensory systems in response to continuously changing environmental conditions.
Acellular
Acellular | Organisms (or tissues) which are not composed of separate cells.
Often having more than one nucleus.
e.g. the structures of muscle fibres are acellular.
Acetic Acid
Acetic Acid (Ethanoic Acid) | CH3COOH
A carboxylic acid that is used as a Carbon source by some green algae.
Combined with Acetyl coenzyme A acetic acid plays a critical role in energy metabolism for all organisms.
Acetyl
Acetyl | Chemical group derived from acetic acid.
Acetyl groups are important in metabolism and are added covalently to some proteins as a post-translational modification.
Acetylation
Acetylation | The combination of an acetyl group into a compound. e.g. the acetylation of coenzyme A to acetyl coenzyme A during the Krebs cycle.
Acetylcholine (ACh)
Acetylcholine (ACh) | Neurotransmitter of the vertebrate nervous system.
Acetylcholine initiates muscular contraction at neuromuscular junctions and allows for the transmission of an impulse from one neurone to another.
Once ACh has been released its effect is transitory since it is rapidly broken down by the enzyme acetylcholinesterase.