Active transport is the movement of a molecule across a membrane or another barrier that is driven by energy other than stored in the concentration gradient or the electrochemical gradient of the transported molecule. This type of transport requires usually the expenditure of ATP and the help of specific transport proteins. In this way can even large molecules can be channelled through the membrane. For the understanding of the details of the mechanism are both the structure of the involved transport molecules and the question how the energy for the transport is supplied and how it is used important. We will meet the electrical membrane gradient again in this context.
Active transport can only occur at intact, closed membranes.
Such membranes can envelop very different compartments , like the whole cell, vesicles, the vacuole, the mitochondrial matrix , the inner thylacoid space of the chloroplasts, etc. As a result of active transport can ions and metabolites be concentrated within the respective compartment or the cell and the steady state of the metabolism can be kept constant despite of large fluctuations in the external medium's composition. Ions, especially potassium, calcium, magnesium and phosphate have an important part in the regulation of the metabolism. The transport direction is thermodynamically determined by coupling the transport with a gradient, usually an electron gradient. The direction can be reversed if the appropriate substrate concentrations are chosen.