Active Transport is a term grouping all means of actively transferring molecules across a biological membrane, from small ions to entire proteins.

This transport is calledactivebecause it requires energy to trigger the transfer, as opposed topassivetransport (such as diffusion) which is the result of simple gradients of concentrations and osmotic pressure.

Active transport is often required when the transfer is against a gradient, whether a concentration gradient or an electrochemical gradient. For example, with a higher concentration of sodium ions inside the cell compared the external environment, the natural diffusion is for sodium to exit the cell. If in reverse, the cell requires more sodium to enter, it has to be done actively to fight the opposing gradient.

Such a process usually requires energy for the transfer, either through the consumption of ATP or using a different electrochemical gradient. The former is calledprimary active transportwith a direct source of energy, the latter is calledsecondary active transportusing another gradient (also called co-transport). In more details, if this secondary gradient is in the same direction as the transport, it is called asymport. If the secondary gradient goes in the opposite direction, it is called anantiport

The most obvious examples of active transport are the transmembraneions channels, although there are countless variations.

The energetical needs for active transport to be functional often requires the presence of cofactors with molecules such as ATP, or ions and molecules such as oxygen or hydrogen. If the transport can proceed in the absence of oxygen or hydrogen, it is most likely a passive process instead.

This is an example of ions transport in the thick ascending limb in the renal nephron.

The paracellular Na⁺route (bottom) is passive diffusion through the membrane. The Na⁺/K⁺泵(右上角)是一个主要的主动运输,再保险quiring the consumption of molecules of ATP to transfer ions through the membrane. The Na⁺/K⁺/Cl^-co-transport (top left) is a secondary active transport, using gradients of concentration as an energy source (in the same direction, so it is a symport).

Of course, pumps require the presence of all the co-factors (whether it is a symport or an antiport) to function correctly. If one of the co-factors, for example K⁺or Cl^-is missing, then the Na⁺/K⁺/Cl^-co-transport is inactivated. The paracellular route for Na⁺would be however unaffected because it is not dependent from K⁺or Cl^-.