The movement across the plasma membrane is of two types i.e., active transport and passive transport. Active transport uses specific transporter proteins called pumps which use metabolic energy (ATP) to move ions or molecules against the concentration gradient. In passive transport an ion or molecule crossing a membrane moves down its electrochemical or concentration gradient. It is of two type. [a] Osmosis: The process by which the water molecules pass through a membrane from a region of higher water concentration to a region of lower water concentration is known as osmosis. [b] Diffusion: The movement of particles of different substances from the region of their higher concentration to region of their lower concentration is called diffusion.