Although there are many variations of these basic
mechanisms, diffusion means random molecular movement
of substances molecule by molecule, either through
intermolecular spaces in the membrane or in combination
with a carrier protein. The energy that causes diffusion
is the energy of the normal kinetic motion of matter.
By contrast, active transport means movement of ions
or other substances across the membrane in combination
with a carrier protein in such a way that the carrier
protein causes the substance to move against an
energy gradient, such as from a low-concentration state
to a high-concentration state. This movement requires
an additional source of energy besides kinetic energy.
Following is a more detailed explanation of the basic
physics and physical chemistry of these two processes.