Active transport

The exchange of substances across cellular membranes may be accomplished by diffusion, facilitated diffusion, or active transport. The result of unaided diffusion is an equilibration of concentrations on both sides of the membrane. In facilitated diffusion the establishment of equilibrium is sped up by the function of a protein. Active transport is neccessary to accumulate a substance against a concentration gradient. Thermodynamics require some kind of energy to perform this, so there has to be another downhill gradient that may be dissipated or some other form of chemical energy.

Depending on the source of energy primary transport is differentiated from secondary transport. Primary transport uses energy directly: light or chemical energy is converted to electrochemical energy as electrochemical potential of the substances to be transported. This category comprises photosynthetic electron transport, light driven ion pumps, redoxenergy dependent respiratory chains, transport ATPases and sodium pumps utilizing decarboxylation energy.

In secondary transport the electrochemical energy originates from the electrochemical potential of another substance that is used up in symport or antiport.

Normally a molecule passes the membrane unchanged. However, in group translocation there is a chemical modification. This method is used by bacteria in the phosphoenolpyruvate:sugar phosphotransferase system (PTS).

Transport systems for nutrients are directed into the cell; all prokaryotic as well as eukaryotic cells contain additionally systems with a reverse transport direction to get rid of toxic substances. These systems may severely impair therapeutic efforts (multidrug efflux systems). Special transport systems protect producers of antibiotics or toxins against their own poisons.