Active transport is the movement of molecules or ions against the concentration gradient, i.e. the molecules or ions from the region of low Concentration towards the region of high concentration. The movement of molecules can be compared with the uphill movement of water.
Energy is required to counteract the force of diffusion. For Example in order to maintain a low intracellular concentration of Na+ the cell extrudes sodium against higher concentration of sodium outside the cell. Simultaneously, K+ ions are transported inside. This is known as sodium pump.
1. Demonstration of active transport:
The phenomenon of active transport can be illustrated by conducting the following experiment. When isolated kidney tubules are immersed in a solution of phenol red, it has been observed that after some time dye passes through cells and becomes deposited in the lumen of tubules. In due course of time the concentration of in the lumen becomes much greater than in its surrounding medium. This indicates that the dye is moving through the cell from low to high concentration.
(i) Carrier molecule mechanism:
It is presumed that a carrier molecule which is a component of the plasma membrane picks up the molecule of transportation (i.e. the molecule which is to be transferred to the cell) and forms a carrier-transportant complex. The carrier may be a protein, lipid or an enzyme. For example, in the active transport of Na+ ions Mg++ activated ATPase acts as a carrier. Its ATP provides energy for transport.
The sodium ion is picked up from the outside forming a temporary complex which is carried to the opposite side of plasma membrane, i.e. inside the cell to be released there. The transportant undergoes metabolic changes along with the chemical component of plasma membrane, the carrier. The carrier is known as translocase or permease. Certain enzymes are found to assist in this active transport.
Na+ C → Na+C
Na+C+ATP → ATP-C+Na+
ATP-C (ATPase) → ADP+ C
Where, C is a Carrier.
(ii) Revolving door models:
Monod, Cohen and Rickenberg have described that during the transport of lactose across the plasma membrane in E.coli. The carrier protein has a slot facing outside. The molecule of the substance to be transported fits into this slot. The carrier protein changes its shape as the substance enters the slot and rotates so that the slot comes to lie on the inner side. The substance. is released in the cell and protein rotates back to its original form. The energy is spent during the process of rotation.
(iii) Fixed pore mechanism:
Fixed pore mechanism a fixed pore or channel lies between the integral protein sub-units. The anions (e g. chlorides, bicarbonates) pass through these channels into the cell cytoplasm.
3. Enzyme for active transport:
The enzyme for active trasport is the enzyme ATPase catalyses the hydrolysis of ATP on the intracellular side by utilizing HO− from the inside and H+ from the outside. As a result, the outer side of the plasma membrane becomes more alkaline and inside becomes more acidic.
Recently, it has been presumed that ATPase enzyme substrate complex constitutes a carrier complex mechanism bining the internal Na+ and releasing outside the membrane. A similar but reverse mechanism is postulated for K+ ions. Importance of Active Transport.
It maintains ionic and water balance between cells and extracellular fluids.
It enables the rapid and selective intake of many nutrients by the cells.
It aids in maintaining membrane potential by keeping the inner side of the membrane relatively electronegative to its outer side.