Transport of Molecules Across Cell Membrane ❖ It is one of the most important functions of plasma membrane. ❖ The membrane is selectively permeable to some molecules present on either side of it. ❖ Many molecules can move briefly across the membrane without any requirement of energy and this is called the passive transport. ❖ Neutral solutes may move across the membrane by the process of simple diffusion along the concentration gradient, i.e., from higher concentration to the lower concentration. ❖ Water may also move across this membrane from higher to lower concentration. Movement of water by diffusion is called osmosis. ❖ As the polar molecules cannot pass through the nonpolar lipid bilayer, they require a carrier protein of the membrane to facilitate their transport across the membrane. ❖ A few ions or molecules are transported across the membrane against their concentration gradient, i.e., from lower to the higher concentration. Such a transport is an energy dependent process, in which ATP is util