Transport across the cell membrane is essential for maintaining the internal environment of cells and for carrying out physiological functions. The movement of substances across the membrane occurs by two major mechanisms: active transport and passive transport. These mechanisms differ in their energy requirement, direction of movement, transport proteins involved, and physiological significance. Passive transport is the movement of molecules or ions across the cell membrane without the expenditure of cellular energy (ATP). In passive transport, substances move along their concentration or electrochemical gradient, that is, from a region of higher concentration to a region of lower concentration until equilibrium is reached. The process depends on the natural kinetic energy of molecules. Passive transport is important for the exchange of oxygen, carbon dioxide, water, and various small solutes between cells and their surroundings. Active transport is the movement of substances across the cell membrane against their concentration or electrochemical gradient, from a region of lower concentration to a region of higher concentration. Since this movement is against the natural direction of diffusion, it requires energy in the form of ATP. Active transport is carried out by specific carrier proteins or pumps present in the plasma membrane. It is essential