Absorption
The movement of digested food molecules from the gut into the blood stream
Abstinence
Not doing, such as avoiding sexual intercourse
Abundance
The number of organisms of a particular species in an area, related to the population size
Accommodation
The adjustment of the shape of the lens in the eye to focus images at different distances properly to the retina
Acid Rain
Rain that contains higher than normal concentrations of dissolved acidic gases, such as sulfur dioxide and nitrogen oxides, which causes the rain to have lower pH than normal
Active site
a 3D-shaped space in the molecule of an enzyme that matches the shape of another molecule that reacts