Atomic number
The number of protons in the nucleus of an atom, unique for each element and designated
by a subscript to the left of the elemental symbol.
Atomic Theory
The physical theory of the structure, properties, and behavior of the atom.
Atomic weight
The total atomic mass, which is the mass in grams of one mole of the atom.
ATP
Abbreviation of adenosine triphosphate, the principal energy-carrying compound of the cell.
ATP synthase
A cluster of several membrane proteins found in the mitochondrial cristae (and bacterial
plasma membrane) that function in chemiosmosis with adjacent electron transport chains,
using the energy of a hydrogen-ion concentration gradient to make ATP. ATP synthases
provide a port through which hydrogen ions diffuse into the matrix of a mitrochondrion.
Atrioventricular node
A group of slow-conducting fibers in the atrium of the vertebrate heart that are stimulated
by impulses originating in the sinoatrial node (the pacemaker) and that conduct impulses to
the bundle of His, a group of fibers that stimulate contraction of the ventricles.
Atrioventricular valve
A valve in the heart between each atrium and ventricle that prevents a backflow of blood
when the ventricles contract.
Atrium
A chamber that receives blood returning to the vertebrate heart.
Auricles
The flap of the ear in the form of a funnel-like organ which collects the sound waves. Called
also pinna.
Autocatalysis
A single chemical reaction is said to have undergone autocatalysis, or be autocatalytic, if
the reaction product is itself the catalyst for that reaction.
Autoecious
Having all stages of a life cycle occurring on the same host. Eg.; fungi
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