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6.4.2. The structure of the membrane. The fluid mosaic model

The structure of the plasma membrane. Singer and Nicholson's fluid mosaic model

Singer and Nicholson (1972), proposed the fluid mosaic model. This model considers that:

The plasma membrane is like a fluid mosaic, where the lipids are arranged forming a phospholipid bilayer, located with their hydrophilic heads towards the external environment or towards the cytosol, and their hydrophobic tails arranged in a palisade. Proteins that intercalate into bilayer lipid depending on interactions with regions of the lipid area. There are three types of proteins  according to their arrangement in the bilayer:

  • Integral or intrinsic proteins.
  • Glycoproteins.
  • Peripheral or extrinsic proteins.

Membranes are asymmetric structures in terms of the distribution of all their chemical components: lipidsproteins and carbohydrates.

The membrane is not a rigid structure, but a fluid one, and allows the movement of proteins within the lipid bilayer.

Like lipids, integral proteins are also amphipathic, since they present hydrophilic and hydrophobic areas, so they can be partially embedded in the bilayer.

Whether the membrane has more or less fluidity depends on several factors:

  • Degree of saturation of fatty acids in membrane lipids. The higher the degree of saturation of the fatty acids, the fluidity is lower.

Efectos de la insaturación de los ácidos grasos de los fosfolípidos

Por MDougM (Trabajo propio) [Public domain], undefined

  • Length of fatty acid chains in membrane lipids. Fluidity decreases as the length of the chains increases.
  • Temperature. The fluidity decreases as the temperature decreases.
  • Cholesterol ratio. The cholesterol is a molecule located between the phospholipids that stabilizes membranes, ie, making them less flexible and fluid. Cholesterol is present in almost all plasma membranes of eukaryotic cells and in some prokaryotes devoid of cell walls.

Animation: Membrane fluidity and temperature.