Active Transport

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 called active because it requires energy to trigger the transfer, as opposed to passive transport (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 called primary active transport with a direct source of energy, the latter is called secondary active transport using another gradient (also called co-transport). In more details, if this secondary gradient is in the same direction as the transport, it is called a symport. If the secondary gradient goes in the opposite direction, it is called an antiport

The most obvious examples of active transport are the transmembrane ions 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.

Figure 1. Diagram showing the ions transportation in the renal nephron

The paracellular Na⁺ route (bottom) is passive diffusion through the membrane. The Na⁺/K⁺ pump (top right) is a primary active transport, requiring the consumption of molecules of ATP to transfer ions through the membrane. The Na⁺/K⁺/Cl^- sodium route, bottom, is passive diffusion through the membrane. The sodium potassium pump (top right) is a primary active transport, requiring the consumption of molecules of ATP to transfer ions through the membrane. The sodium potassium chlorine 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^-. potassium or chlorine is missing, then the sodum potassium chlorine co-transport is inactivated. The paracellular route for sodium would be however unaffected because it is not dependent from potassium or chlorine.