Hydrophilic and some lipophilic hormones bind to cell surface receptors. These are necessary membrane proteins located in the plasma membrane which bind the signaling molecule (ligand) with high affinity. Ligand binds to a particular site on the receptor in much the same way as a substrate binds to an enzyme. The Binding of the ligand to the receptor should cause a conformational modification in the receptor or endorse dimerization of two receptors which initiates a series of reactions in the goal cell (often referred to as signal transduction) leading to a modification in cellular function. The sharing of receptors varies on variety cells and there is frequently more than one kind of receptor for a specific ligand permitting variety goal cells to respond differently to the same signaling molecule. The Cell surface receptors can be classified into three major classes depending on how they transmitted the information from the ligand to the interior of the cell: ion channel-linked receptors, enzyme-linked receptors and receptors of G protein-coupled .