Biology – 3.3 Active transport | e-Consult

Active transport is the movement of molecules across a cell membrane against their concentration gradient – meaning from an area of lower concentration to an area of higher concentration. This process requires energy, which is supplied by respiration, typically in the form of ATP (adenosine triphosphate).

The movement is facilitated by specific carrier proteins embedded in the cell membrane. These proteins bind to the molecule being transported. The binding causes the protein to change shape, and this conformational change allows the molecule to be moved across the membrane. Some carrier proteins are always in the 'on' state, while others can switch between 'on' and 'off' states depending on the availability of ATP.

A key feature of active transport is saturation. There are a finite number of carrier proteins in the cell membrane. As the concentration of the transported molecule increases, more and more carrier proteins become occupied. Eventually, all the carrier proteins are bound to the molecule, and the rate of transport reaches a maximum. At this point, the transport is said to be saturated. Increasing the concentration of the molecule beyond this point will not increase the rate of transport.

In summary, active transport uses energy to move molecules against their concentration gradient, relying on carrier proteins that bind to the molecule and undergo a conformational change to facilitate movement. The process is limited by the number of available carrier proteins, leading to saturation.