A phospholipid bilayer with proteins floating in it
Expansion
The fluid mosaic model: a phospholipid bilayer with hydrophilic heads outward and hydrophobic tails inward, in which proteins float.
Components
- Phospholipids, arranged asymmetrically. Phosphatidylserine is normally confined to the inner leaflet, and its exposure on the outer leaflet signals apoptosis, marking the cell for phagocytosis
- Cholesterol, which buffers fluidity in both directions: it restricts movement at high temperature and prevents tight packing at low
- Integral (transmembrane) proteins: channels, carriers, receptors, pumps
- Peripheral proteins, attached to one face
- Glycoproteins and glycolipids, forming the glycocalyx, which carries blood group antigens and mediates cell recognition and adhesion
Lipid rafts are cholesterol and sphingolipid rich microdomains that concentrate receptors and signalling molecules.
“Only small, uncharged, lipid soluble molecules cross freely”: oxygen, carbon dioxide, steroid hormones, urea and ethanol. Ions, glucose and water need channels or carriers, which is why aquaporins exist despite water being small.
Clinical relevance: the membrane is the target of amphotericin (binds ergosterol), alcohol (increases fluidity), and complement’s membrane attack complex, and its failure defines necrosis as distinct from apoptosis.