6.1 Membrane Structure, Fluidity and Adhesion

Every cellular membrane has a phospholipid bilayer roughly 7–10 nm thick. A phospholipid is amphipathic: its phosphate-containing head is polar and hydrophilic, while its fatty-acid tails are non-polar and hydrophobic. In water, heads interact with the aqueous cytosol or extracellular fluid and tails cluster away from water. A bilayer closes at exposed edges because closure removes energetically unfavourable tail–water contacts.

The bilayer is called fluid because phospholipids and many proteins diffuse laterally rather than occupying fixed coordinates. It is a mosaic because chemically different lipids, integral proteins, peripheral proteins, glycoproteins and glycolipids share the surface. The model predicts flexibility, self-sealing and local reorganization—properties required when vesicles bud, cells change shape or two membranes fuse.

Integral proteins contain hydrophobic regions that associate with lipid tails; transmembrane proteins also expose hydrophilic regions on both surfaces. They act as selective channels, carriers, ATP-driven pumps, receptors, adhesion molecules and immobilized enzymes. Peripheral proteins associate with a membrane surface or an integral protein and can anchor the cytoskeleton, organize signalling complexes or catalyse reactions.

Carbohydrate chains occur only on the non-cytosolic face when attached to proteins or lipids. Their varied shapes create a recognition code used in cell–cell identification, immune recognition and attachment. A receptor binds a ligand because of complementary chemistry, but binding is only the first step: a conformational change must transmit information to intracellular proteins.