Active Transport:

All the mechanisms discussed so far are for nonliving membranes, where permeation of chemical substances takes place in the direction from high concentration to low concentration. For living (biological) membranes, such as cell membranes the actual mechanism of permeation is generally quite complicated. In a few cases, a substance might be transported by a cell membrane against its concentration gradient. This does not mean which the second law of thermodynamics is violated. Further to diffusion procedure, there are other reactions occurring within the membrane which supplies energy to the system. Therefore, the cell is doing work to move the substance by the cell membrane against a concentration gradient. Because the cell membrane is 'actively' transporting the matter, the word 'active transport' is used for such a process. A schematic of active transport mechanism is given in Figure, and is quite similar to facilitated diffusion. The schematic give details the active transport of a chemical substance A through cell membrane from low concentration to high concentration with the help of enzymatic reactions at the two surfaces of the membranes.