The cell membrane serves following functions: 1. The cell membrane provides mechanical support to the cell. 2. It maintains the shape of the cell. 3. It protects the cell from injury. 4. The cell membrane is selectively permeable and able to regulate what enters and exits the cell, thus facilitating the transport of material needed for survival. The movement of substances across the membrane can be either ?passive?, occurring without the expenditure of cellular energy or ?active?, requiring the cell to expend energy. Passive Transport: It is the movement of ions and other atomic or molecular substances across cell membrane without expenditure of energy. The rate of passive transport depends on the permeability of the cell membrane, which is turn depend on the organization and characteristics of the membrane lipids and proteins. The four main kinds of passive transports: (a) Diffusion (Simple): Movement of material from an area of its high concentration to the area of its low concentration along the concentration gradient. (b) Facilitated Diffusion: Movement of material from an area of its lower concentration to the area of its higher concentration via specific transmembrane integral proteins. (c) Filtration: Movement of molecules through semi - permeable membranes with pressure developed by external factors. (d) Osmosis: Movement of solvent molecules (diffusion of water) from the region of its higher concentration to the region of its lower concentration through the semi permeable membrane.