A change in shape is often caused by the Ligand binding

Although epinephrine encounters many types of cells in the blood, only the target cells detect and react to it. The nature of a signal molecule is like that of a Ligand, which attaches to the site receptor. A change in shape is often caused by the Ligand binding (Recce, 2005). Most of the receptors are activated by this change and are permitted to interact with other cellular molecules. Messenger cells have to pass through the plasma membrane of the target cell in order to interact with intercellular molecules. A number of signaling molecules are able to cross the phospholipid interior of the membrane because of their hydrophobic nature or small size. Steroid hormones and thyroid hormones of animals are two examples of a hydrophobic chemical messenger. Many of the intracellular receptor proteins are structurally similar, suggesting an evolutionary affiliation.