Cell membrane separates cells from their external environment and divides the interior of the cell into compartments. Fluid mosaic model of cell membrane is the famous and the accepted model. Simple diffusion, facilitated diffusion, active transport, sodium pump, endocytosis, osmosis and filtration are elucidated. Information on acids, bases, dissociation of water, pH and buffer is given in the sub-chapter. Henderson-Hasselbalch equation and various body buffers are explained. Role of lungs and kidney in maintenance of acid-base balance and also disturbances in acid base balance such as acidosis and alkalosis, compensatory mechanisms and anion gap are discussed extensively to understand the concept.