In certain instances, molecules
are transported from an area of low concentration
to an area of high concentration.
Examples of this type of transport
include the epithelial linig of the small
intestines that transport glucose from the lumen
into the cells and the transport of calcium from
the intracellular compartment to the
extracellular compartment so that the
extracellular concentration of calcium is from
1000 to 10,000 times the intracellular
This transport requires carrier
molecules and energy.
Energy is obtained from the
enzymatic hydrolysis of one phosphate bond from
an adenosine triphosphate molecule (ATP).
In a similar fashion to
facilitated diffusion, active transport shows the
characteristics of Specificity,
Competition and Saturation.
The difference is the fact
that active transport is completely inhibited if
the enzyme system is poisoned e.g. cyanide while
facilitated diffusion is not affected by