Cell surfaces guide embryonic cells to form tissues and organs. The human leukocyte antigen cell surface markers establish self and are associated with increased risk of developing certain disorders. A biological membrane consists of a phospholipid bilayer embedded with movable proteins, glycoproteins, and glycolipids, like a fluid mosaic. The percentage and distribution of membrane proteins varies in different cell types. Membrane proteins include receptors, carriers, adhesion molecules, and cell surface proteins establishing self.
Substances cross cell membranes in several ways.
In diffusion, a molecule passes through openings in a membrane following its concentration gradient. Osmosis is the simple diffusion of water across the cell membrane or other semipermeable membrane. Several adaptations maintain cell shape in the face of solute concentration changes. In facilitated diffusion, molecules cross a membrane following the concentration gradient with the aid of a carrier protein. In exocytosis, vesicles inside the cell carry substances to the cell membrane, where they fuse with the membrane and release the cargo outside.