Active transport is the pumping of individual ions or molecules across a cell
membrane, through the use of a protein,
from a region of lower concentration to one of higher concentration. Because
the ion or molecule is moved in a direction other than the one in which
simple diffusion would take it, this transport
requires energy, which is typically supplied by the expenditure of ATP (adenosine triphosphate).
Active transport is used to:
There are two main types of active transport: primary and
secondary. Primary active transport uses energy directly
to convey molecules across a membrane. Most of the enzymes that perform this type of transport are transmembrane ATPases. A primary
ATPase universal to all cellular life is the sodium-potassium
pump, which helps maintain the cell's resting
potential. In primary transport, energy is directly coupled to the movement
of a desired substance across a membrane independent of any other species.
In secondary active transport, there is no direct coupling
of ATP; instead, the electrochemical potential difference created by pumping
ions out of cells is used.
- Generate charge gradients. For example, in the mitochondrion,
hydrogen ion pumps pump hydrogen ions into the intermembrane space of
the organelle as part of making ATP.
- Concentrate ions, minerals, and nutrients inside the
cell that are in low concentration outside.
- Keep unwanted ions or other molecules out of the cell
that are able to diffuse through the cell membrane.
Active transport is inhibited by substances that interfere with cellular
metabolism (e.g., high doses of digitalis).