Membrane Transport

Every living cell has to exchange molecules across the membrane for cellular functions. The hydrophobic or lipophilic molecules do not require energy for crossing the membrane. They can diffuse freely from higher to lower concentration till equilibrium is established. This process is called passive transport or diffusion. This is not true for polar or lipophobic molecules because of the nonpolar nature of membrane lipid bilayer. For polar molecules, the cell recruits mediator or transporter proteins in the membrane to facilitate their movement. If the direction of transport is from higher to lower concentration region, the transporter proteins do not use external energy and the process is called facilitated transport, because the energy needed for transport is dispensed by molecule’s own concentration gradient (unequal distribution across the membrane) in the favorable direction. The free energy is minimal in unequal distribution of polar molecules. If polar molecule is transported ...