Active transport refers to the process in which
the cell uses its energy to transport materials in and out of the cell. This
process occurs across the cell membrane. Since the liquid inside and outside of
the cell are composed of different substances, the cell has to work extra and
use some of its energy to maintain a proper balance of ions and molecules.
Proteins that are attached to the lipid bilayer are responsible for doing much
of the work in the process of active transport.
