Biology

acclimatization

depth are chemoautotrophic prokaryotes and not photosynthetic producers.

See also OCEANIC ZONE.

acclimatization Acclimatization is the progressive

physiological adjustment or adaptation by an organism to a change in an environmental factor, such as

temperature, or in conditions that would reduce the

amount of oxygen to its cells. This adjustment can

take place immediately or over a period of days or

weeks. For example, the human body produces more

erythrocytes (red blood cells) in response to low partial pressures of oxygen at high altitudes; short-term

responses include shivering or sweating in warmblooded animals.

accommodation The automatic reflex adjustment

that allows the focal length of the lens of an eye to

change to focus on an object. The lens shape, more

convex for near objects and less convex for distant

objects, is caused by ciliary muscles acting on the elastic property of the lens.

acetylcholine (ACh) One of the most common

neurotransmitters of the vertebrate nervous system,

ACh is a chemical (CH3COOCH2CH2N+(CH3)3) that

transmits impulses between the ends of two adjacent

nerves or neuromuscular junctions. Released by nerve

stimulation (exciting or inhibiting), it is confined largely to the parasympathetic nervous system, where it diffuses across the gap of the synapse and stimulates the

adjacent nerve or muscle fiber. It rapidly becomes inactive by the enzyme cholinesterase, allowing further

impulses to occur.

acetyl CoA A compound formed in the mitochondria when the thiol group (–SH) of coenzyme A combines with an acetyl group (CH3CO–). It is important

in the Krebs cycle in cellular respiration and plays a

role in the synthesis and oxidation of fatty acids.

Fritz Albert Lipmann (1899–1986), a biochemist,

is responsible for discovering coenzyme A and cofactor

A, or CoA (A stands for acetylation), in 1947. He

shared the 1953 Nobel Prize in physiology or medicine

with HANS KREBS.

See also KREBS CYCLE.

achiral See CHIRALITY.

acid A chemical capable of donating a HYDRON (proton, H+) or capable of forming a covalent bond with

an electron pair. An acid increases the hydrogen ion

concentration in a solution, and it can react with certain metals, such as zinc, to form hydrogen gas. A

strong acid is a relatively good conductor of electricity.

Examples of strong acids are hydrochloric (muriatic),

nitric, sulfuric, while examples of mild acids are sulfurous and acetic (vinegar). The strength of an acidic

solution is usually measured in terms of its pH (a logarithmic function of the H+ ion concentration). Strong

acid solutions have low pHs (typically around 0–3),

while weak acid solutions have pHs in the range 3–6.

See also BASE; PH SCALE.

acidity constant The equilibrium constant for splitting off a HYDRON from a BRØNSTED ACID.

acid-labile sulfide Refers to sulfide LIGANDs, e.g.,

the BRIDGING LIGANDs in IRON–SULFUR PROTEINS,

which are released as H2S at acid pH.

See also FERREDOXIN.

acid precipitation Because pure precipitation (e.g.,

rain) is slightly acidic (due to the reaction between

water droplets and carbon dioxide, creating carbonic

acid) with a potential pH of 5.6, acid precipitation

refers to precipitation with a pH less than 5.6. Acid

precipitation includes rain, fog, snow, and dry deposition. Anthropogenic (man-made) pollutants (carbon

dioxide, carbon monoxide, ozone, nitrogen and sulfur

oxides, and hydrocarbons) react with water vapor to

produce acid precipitation. These pollutants come primarily from burning coal and other fossil fuels. Sulfur

dioxide, which reacts readily with water vapor and

droplets (i.e., has a short residence time in the atmosphere as a gas), has been linked to the weathering

2 acclimatization

(eating away) of marble structures and the acidification of freshwater lakes (consequently killing fish).

Natural interactions within the biosphere can also lead

to acid precipitation.