It is a vital procedure. It is the movement of molecules or ions alongside the concentration gradient that is, the molecules or ions move from the area of lower concentration towards the area of higher concentration. Movement of molecules can be compared with the mounting movement of water.
Figure: Active transport-a scheme
Energy is needed to counteract the force of diffusion and the energy comes from ATP generated by oxidative phosphorylation or by concentration gradient of the ions. Therefore active transport is stated as the energy dependent transport of molecules or ions across a semi permeable membrane alongside the concentration gradient.The active transport occurs with the help of carrier proteins which are present in the plasma membrane. In plasma membrane there are a number of carriers molecules termed as permeases or translocases. For each kind of solute molecule there is a particular carrier molecule. It has got two binding positions; one for the transportant and other for ATP molecule. The carrier proteins combine the transportant molecule on the external side of the plasma membrane. This outcome in the formation of carrier–transportant–complex. Since the ATP molecule binds it to the other binding position of the carrier protein it is hydrolyzed to form ADP and energy is discharged. This energy brings conformational modification in the carrier-transportant-complex and the transportant is carried however the channel on the other side of the membrane. The carrier molecule recovers its original form and repeats the procedure.
There are two forces that govern the movement of ions across selectively permeable membranes that are the membrane electric potential and the ion concentration gradient. ATP driven ion pumps produce and sustain ionic gradients across the plasma membrane.
Figure: Role of carrier protein in active membrane transport
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