**5. Mechanisms of drug absorption across membranes**

In order for drugs to elicit their pharmacological effects, they have to cross the biological membranes into systemic circulation and reach the site of action. Therefore an insight into the structure and function of the membrane leads to a better understanding of drug absorption. Membranes are phospholipid bi-layers with interspersed integral and peripheral proteins which behave either as molecular 'gates' or 'pumps'. Molecular gates are non-specific. The intake of molecules into the cell depends on the charged groups in the pore and the size of molecule to be transported across the membrane. Molecular pumps, however, are highly specific and require energy for molecular transport. There are several mechanisms by which drugs traverse membranes to reach their intended target site and they include the following:
