Membrane Asymmetry and Fluidity

Membrane Asymmetry

• Definition: Differences in lipid composition between the two monolayers of a lipid bilayer.

  • Lipid bilayer has two layers, arbitrarily called layer one and layer two.

• Variations:

  • Specific lipids present.
  • Degree of saturation of fatty acids.

• Examples:

  • Glycolipids: Predominantly in the outer monolayer (layer one, facing the outside of the cell).
  • Phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol: Mainly in the inner monolayer (layer two).

Membrane Synthesis

• Location: Endoplasmic Reticulum (ER)
• Process: Fatty acids are added to glycerol phosphatidic acid within the ER membrane.
• Initial Placement: Lipids are initially added to the monolayer facing the cytosol.
• Flipases: Determines if a lipid ends up in the other monolayer.

Membrane Fluidity

• Definition: The movement of lipids within a monolayer.
• Movement Types:
  • Rotation: A lipid spins on its axis.
  • Diffusion: A lipid moves laterally within the monolayer.
• Transverse Diffusion (Flip-Flop):
  • Movement of a lipid from one monolayer to the other.
  • Requires a specific enzyme called a flipase.
  • Occurs primarily during membrane synthesis.
  • Different flipases exist for different lipids (e.g., phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol).

Establishing Membrane Asymmetry

• Membrane asymmetry is caused because some lipids can be flipped to the other monolayer while others cannot (due to the presence or absence of specific flipases).