The majority of drugs have molecular weights higher than 200 and have to pass from the gastrointestinal tract into the blood supply by traveling through the cells lining the gut wall. In the cell membrane, the phospholipid molecules are arranged in two layers measuring about 80 Å in thickness. However, the important thing is that the center of the cell membrane is hydrophobic and fatty. Carrier proteins are essential to a cell’s survival since they smuggle the highly polar building blocks required for various biosynthetic pathways. The vesicle then fuses with the membrane to release the drug on the other side of the cell. Most orally active drugs have a molecular weight less than 500. This is because high molecular weight compounds are more likely to have a large number of polar functional groups, and it is the presence of these that limits absorption.