active transport
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:
- 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.
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.
Active transport is inhibited by substances that interfere with cellular
metabolism (e.g., high doses of digitalis).
Related categories
• ANATOMY
AND PHYSIOLOGY
• BIOCHEMISTRY
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