Active Transport Definition

Another example involves active transport wherein there is protons movement across the inner mitochondrial membrane against a concentration gradient, and it is driven by the redox energy of NADH. During the primary active transport of protons across the thylakoid membrane, photon energy drives the process. This leads to the generation of a proton gradient.

There is no direct ATP coupling involved in secondary active transport. In this transport, the transport is driven by the energy from electrochemical potential difference because the ions are pumped into and out of the cell. In secondary active transport, one ion moves down its electrochemical gradient which results in increased entropy and can be used as a source of energy.

E.g., When Na+ ions move down the electrochemical gradient across the plasma membrane, it drives the transport of a second ion against its gradient, e.g. H+ ions. Such type of transport is also termed coupled transport or cotransport as it involves simultaneous transport of two substances across a biological membrane.

Depending on the direction of movement of the two substances, it may be a symport or antiport. In symport type of coupled transport, both substances move in the same direction. Conversely, if the substances move in opposite directions it is called antiport.