II. The Fluid Mosaic Model: Composition and Dynamics

The fluid mosaic model describes the plasma membrane as a fluid structure of lipids with a “mosaic” of embedded proteins.

1. The Lipid Bilayer

The bilayer is freely permeable to small, lipid-soluble, nonpolar molecules but impermeable to charged ions.

• Molecular Composition:
  • Phospholipids: The most abundant lipid component. They are amphipathic, with a polar (hydrophilic) head and two nonpolar (hydrophobic) fatty acyl tails. The tails are typically 16–18 carbon chains and are linked by weak noncovalent bonds. The distribution of phospholipids is asymmetrical between the two leaflets.
  • Glycolipids: These are restricted to the outer leaflet. Their polar carbohydrate residues extend into the extracellular space, contributing to the glycocalyx.
  • Cholesterol: Constitutes about 2% of membrane lipids and is found in both leaflets, where it helps maintain structural integrity.
• Lipid Rafts: Cholesterol and phospholipids can form microdomains known as lipid rafts, which can influence the movement of integral proteins.
• Fluidity: The fluidity of the bilayer is crucial for processes like endocytosis, exocytosis, and membrane trafficking.
  • Fluidity increases with higher temperatures and with decreased saturation of fatty acyl tails.
  • Fluidity decreases with higher cholesterol content.

1. Membrane Proteins

Proteins constitute approximately 50% of the plasma membrane’s composition in most cells.

• Integral Proteins: These proteins are dissolved within the lipid bilayer.
  • Transmembrane Proteins: Span the entire thickness of the membrane. Most are glycoproteins and are often folded into multipass proteins that cross the membrane multiple times. They function as receptors, enzymes, transport proteins, and cell adhesion molecules.
  • Anchored Proteins: Some integral proteins are anchored to a leaflet via fatty acyl or prenyl groups.
  • Freeze-Fracture: In freeze-fracture preparations, integral proteins preferentially remain with the P-face (the protoplasmic, or outer, surface of the inner leaflet).
• Peripheral Proteins: These proteins do not extend into the lipid bilayer.
  • Location: They are typically found on the cytoplasmic side of the inner leaflet, attached to integral proteins or phospholipid polar groups via noncovalent interactions.
  • Functions: They often act as electron carriers (e.g., cytochrome c), components of the cytoskeleton, or elements of intracellular second messenger systems.
  • Examples: Include annexins (ion channel formation), synapsin I (binds synaptic vesicles to cytoskeleton), and spectrin (stabilizes red blood cell membranes).

1. The Glycocalyx (Cell Coat)

• Location and Composition: The glycocalyx is a “fuzzy” layer up to 50 nm thick on the outer surface of the plasmalemma. It is composed of the polar oligosaccharide side chains of glycolipids and glycoproteins, as well as proteoglycans.
• Functions:
  • Aids in cell attachment to the extracellular matrix (e.g., fibroblasts).
  • Binds antigens and enzymes to the cell surface.
  • Facilitates cell-cell recognition and interaction.
  • Protects the cell from physical and enzymatic injury.
  • In blood vessels, it reduces friction and fluid loss.