A kind of transport by which ions or molecules move along a concentration gradient, which means movement from an area of higher concentration to an area of lower concentration.
Since the movement of substances is by passive transport, this process does not require chemical energy (in contrast to active transport).
In moving substances across a biological membrane, a passive transport may or may not need the assistance of a membrane protein.
There are four major types of passive transport:
Compare: active transport
See also: concentration gradient