<P> Also, coatings on a tablet or a pellet may act as a barrier to reduce the rate of dissolution . Coating may also be used to modify where dissolution takes place . For example, enteric coatings may be applied to a drug, so that the coating only dissolves in the basic environment of the intestines . This will prevent release of the drug before reaching the intestines . </P> <P> Since solutions are already dissolved, they do not need to undergo dissolution before being absorbed . Lipid - soluble drugs are less absorbed than water - soluble drugs, especially when they are enteral . </P> <P> The gastrointestinal tract is lined with epithelial cells . Drugs must pass or permeate through these cells in order to be absorbed into the circulatory system . One particular cellular barrier that may prevent absorption of a given drug is the cell membrane . Cell membranes are essentially lipid bilayers which form a semipermeable membrane . Pure lipid bilayers are generally permeable only to small, uncharged solutes . Hence, whether or not a molecule is ionized will affect its absorption, since ionic molecules are charged . Solubility favors charged species, and permeability favors neutral species . Some molecules have special exchange proteins and channels to facilitate movement from the lumen into the circulation . </P> <P> Ions cannot passively diffuse through the gastrointestinal tract because the epithelial cell membrane is made up of a phospholipid bilayer . The bilayer is made up of two layers of phospholipids in which the charged hydrophilic heads face outwards and the non-charged hydrophobic fatty acid chains are in the middle of the layer . The uncharged fatty acid chains repel ionized, charged molecules . This means that the ionized molecules cannot pass through the intestinal membrane and be absorbed . </P>

Where does the majority of drug absorption occur