1. Diffusion is the movement of substances from an area of high concentration to an area of low concentration . Diffusion occurs in liquids and gases where the particles collide randomly and spread out . Passive transport : Passive transport is an example of diffusion and it do not requires any input of energy and the molecules moves from a region of high concentration to a region of lower concentration. Active transport : It is the movement of molecules or ions across a cell membrane from a region of low concentration to a region of higher concentration . Actiev transport requires energy to transport molecules. Osmosis : It is the movement of a solvent such as water from a region of high water concentration to a region of low water concentration through a semi-permeable membrane . Concentration gradient refers to the gradual change in the concentration of solutes in a solution . The change in the concentration gradient helps in the movement of solutes from one region to another . 2. Biological membranes are selectively permeable membrane which allows only certain substances to pass through it . The larger molecules cannot pass through the cellular membrane and it allows only small molecules to pass through it.