Illustration depicting the semi-permeability of cell membranes. Cell membranes are formed primarily from a phospholipid bilayer. They serve as barriers and gatekeepers to cells. They are semi-permeable, which means that some molecules can diffuse across the lipid bilayer but others cannot. Small (low molecular weight) molecules such as oxygen, water and carbon dioxide are soluble in the lipid bilayer and therefore can readily cross cell membranes. Larger (higher molecular weight) molecules such as glucose and charge-carrying ions cannot.