 Dear students, in this topic we shall discuss the active transport and the sodium potassium pump. Dear students, all living cells spend energy to maintain a transmembrane concentration gradient or difference of various solutes which is far away from equilibrium. This maintenance of concentration difference requires an uphill movement of solutes against the concentration gradient by utilizing the energy of ATP as it is an energy requiring process so it is called active transport. Dear students, the active transport of solutes is carried out by specialized carrier proteins. These proteins can move the solutes against their concentration gradients that is from their lower concentration to higher concentration. The mechanisms through which these carrier proteins actively transport substances are known as membrane pumps. We shall discuss here the sodium potassium pump as the model pump that explains most of the features of active transport. This sodium potassium pump maintains a steep concentration of sodium and potassium ions across the plasma membrane. This pump is virtually found in all living animal cells. The sodium potassium pump maintains the concentration of potassium ions 10 to 30 times higher inside the cell than outside. It also maintains the sodium ion concentration 10 to 20 times lower inside the cell than outside. Dear students, now we shall discuss the nature and function of sodium potassium pump. This pump is actually a protein which is called sodium potassium ATPase. This protein is an antiport. Antiport proteins couple the transport of two ions at a time. Sodium potassium ATPase is the transfer of sodium and potassium ions to these opposite directions. In this way, three sodium ions are sent out of the cell, while two potassium ions are brought out of the cell. This unequal stoichiometry of sodium potassium pump makes it an electrogenic pump because it involves a net transport of charge across the membrane. This is why membrane potential is generated. Dear students, now we shall discuss the important features of active transport. The first feature of the active transport is that it takes place against substantial concentration gradient. Second feature of active transport is that it exhibits a high degree of selectivity for the substance or ion that transport. The active transport utilizes ATP as energy source. The importance of ATP comes in light when we see that those substances are those metabolites which inhibit ATP also inhibit the process of active transport in the cells. Dear students, the energy of ATP is released with the help of certain enzymes which are called ATPases. Such ATPases are involved with the membrane pumps and are involved with sodium potassium pump as sodium potassium ATPase. These certain ATPases are also specific so sodium potassium ion dependent ATPases are associated with sodium potassium pump. Similarly, the calcium activated ATPases are associated with those membranes which are calcium pumping membranes. Dear students, some membrane pumps, for example sodium potassium pumps, are antiports. Antiports exchange these two types of molecules. For example, sodium potassium pump is an antiport that extracts sodium ions from the cells and takes potassium ions into the cells instead of them. Such pumps exchange ions and do electrical work and also do net flux of charge. For example, sodium potassium pump produces a net outward movement of one positive charge in each cycle in the form of three sodium ions outside and two potassium ions inside. Similarly, ion pumps which produce net charge movement are known as rheogenic. If the current produces a measurable effect on the voltage across the membrane, the pump is known as electro-genic. In this way, sodium potassium pump is the major electro-genic pump of the cell. Dear students, another important feature of active transport is that it is controlled by enzyme proteins because the active transport follows Mekili's Menten kinetics and also exhibits competitive inhibition by analogue molecules. These properties show that active transport is controlled by enzymes.