The function of biological membranes:
Biomembranes assist in the compartmentalization of specialized cellular function and provide a semi-permeable “barrier” for the selective passage of solutes including both nutrients and waste by-products. This function of a biomembrane is subserved by its unique structure and composition. In general, biomembranes are comprised of both lipid (fat) and protein constituents. Of these molecules, the lipids found within biomembranes include primarily cholesterol and phospholipids. The polar head groups of phospholipids contain negatively charged phosphate groups that readily mix with the aqueous environment of the cell (both intracellular and extracellular fluid), while the non-polar tails consist of uncharged lipid chains that aggregate with other lipids.
The organization and composition of membranes determines the function:
The organization of the phospholipid components into a bilayer (or “sandwich-like” structure) allows the hydrophilic (water-loving) heads to mix with the aqueous solutions that bathe the surfaces of the biomembrane and the hydrophobic (water-fearing) tails to face one another in the middle (figure). This bilayer structure provides a flexible scaffold for the protein components of the membrane. However, the membrane is not a static organelle/compartment.
The composition and function of proteins within the membrane is dynamic and contributes significantly to the selectivity and specialization of biomembranes within an organism. For example, some membrane proteins form pores or channels that allow the selective transport of ions, while others are carriers or transporters of nutrients and macromolecules. The protein composition of any membrane determines which nutrients and molecules will be selectively transported into and out of the cell. This is an example of how structure is intimately connected to function in biology.
The physiochemical properties and unique structure of biomembranes allow the preferential passage of lipophilic, non-ionized (uncharged) molecules through the semi-permeable phospholipid bilayer. The physiochemical properties of the solute or “drug” molecule are also important in determining its absorption rate. This is due, in part, to the ability of the charged (ionized) or polar phospholipids head groups to interact with a polar charged “drug”. Interactions between the polar functional groups of the “drug” and the membrane will generally slow its rate of absorption through the membrane.