From Wikipedia,
the free encyclopedia.
Active transport is the
mediated transport of
biochemicals, and other
atomic/molecular
substances, across
membranes. Unlike
passive transport, this
process requires
chemical
energy. In this form of
transport,
molecules move against either
an electrical or concentration
gradient (collectively termed an
electrochemical gradient).
This is achieved by either
altering the affinity of the
binding site or altering the rate
at which the protein changes
conformations.
Types
There are two main types,
primary and secondary.
In primary transport energy is
directly coupled to movement of
desired substance across a
membrane, independent of any other
species. Secondary transport
concerns the diffusion of one
species across a membrane to drive
the transport of another.
Primary
Primary active transport
directly uses energy to transport
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 potential.
Secondary
In secondary active transport,
there is however no direct
coupling of
ATP; instead, the
electrochemical potential
difference created by pumping
ions out of cells is used. The two
main forms of this are
counter-transport (antiport)
and co-transport (symport).
Counter-transport
In counter-transport two
species of ion or other solute are
pumped in opposite directions
across a membrane. One of these
species is allowed to flow from
high to low concentration, which
yields the
entropic energy to drive the
transport of the other solute from
a low concentration region to a
high one. An example is the
sodium-calcium exchanger or
antiporter, which allows three
sodium ions into the cell to
transport one calcium out.
Many cells also possess a
calcium ATPase, which can operate
at lower intracellular
concentrations of calcium and sets
the normal or resting
concentration of this important
second messenger. But the
ATPase exports calcium ions more
slowly: only 30 per second versus
2000 per second by the exchanger.
The exchanger comes into service
when the calcium concentration
rises steeply or "spikes" and
enables rapid recovery. This shows
that a single type of ion can be
transported by several enzymes,
which need not be active all the
time (constitutively), but may
exist to meet specific,
intermittent needs.
Co-transport
Co-transport also uses the flow
of one solute species from high to
low concentration to move another
molecule against its preferred
direction of flow; but here, both
solutes move in the same direction
across the membrane. An example is
the glucose symporter, which
cotransports two sodium ions for
every molecule of glucose it
imports into the cell.
