The sodium-potassium pump is an example of:

Ans. is 'a' i.e. Active transport Transport of substances through the cell membraneCell membrane is a 'lipid bilayer1 that allows the diffusion of only water and lipid soluble substances (O2, N2, & CO2) through it.Lipid bilayer is impermeable to most essential molecules and ions. These are transported inside the cell with the help of transport proteins.These transport proteins are of two typesIon channels- some transport proteins provide watery spaces all the way through the molecule. This allows free movement of water as well as selected ions or molecules through them.Carrier proteins - These carter proteins bind with molecules or ions that are to be transported and then changes its configuration. This conformational change in the carrier protein transports the substance across the membrane.Transport by carrier proteins are of 2 typesfacilitated diffusion - when carrier protein moves substances in the direction of their chemical or electrical gradient. Here no energy input is required, eg-glucose transport by glucose transporter, which moves glucose down its cone, gradient from the ECF to cytoplasm of cell.active transport - when carrier protein transports substances against their electrical and chemical gradients. This form of transport requires energy.In man this energy is provided by hydrolysis of ATP therefore active transport proteins are ATPase's, the most important of which is Na+, K+ATPase also k/a Na+ - K+ pump.