Membranes and Receptors S3 Lecture 1

Learning Outcomes

• Recognize the main types of membrane lipids.
• Understand properties of amphipathic molecules and lipid bilayer formation.
• Describe movements in lipid bilayers and effects of unsaturated fatty acids and cholesterol on membrane fluidity.

Functions of Biological Membranes

• Serve as a selective permeability barrier, regulating chemical environments.
• Facilitate communication and information flow between cells and their environments.
• Enable recognition through signaling molecules, adhesion proteins, and immune mechanisms.
• Generate signals in response to stimuli (electrical and chemical).

Specialized Functions of Biological Membranes

• Plasma membrane: encompasses all mentioned functions.
• Mitochondrial membrane: crucial for energy conservation via oxidative phosphorylation.

Membrane Lipids

• Amphipathic molecules: possess both hydrophilic and hydrophobic regions.

Types of Membrane Lipids

• Phospholipids:
  • Derived from glycerol-3-phosphate.
  • Common types include phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, and phosphatidylinositol.
  • Fatty acids typically range from 16-18 carbons; can be saturated or unsaturated (cis conformation).
• Sphingomyelin:
  • Unique as it is not based on glycerol, using sphingosine as a backbone.
• Glycolipids:
  • Sugars attached to lipids, with glycerol or sphingosine backbone.
  • Subtypes include cerebrosides (sugar monomers) and gangliosides (sugar multimers).
• Cholesterol:
  • Integral to membrane fluidity and stability.

Lipid Distribution in Tissues

• Phosphatidylcholine: 45-55%
• Phosphatidylethanolamine: 15-25%
• Phosphatidylinositol: 10-15%
• Phosphatidylserine: 5-10%
• Cholesterol: 10-20%

Lipid Bilayer Formation

• Amphipathic molecules form micelles or bilayers in water, with bilayers being preferred for phospholipids and glycolipids.
• Bilayer formation is spontaneous due to van der Waals forces among hydrophobic tails.
• Stabilization occurs through non-covalent interactions (electrostatic and hydrogen bonds) between hydrophilic moieties and water.
• Pure lipid bilayers exhibit low permeability to ions and most polar molecules.

Dynamics of Lipid Bilayers

• Membranes exhibit fluidity with lipid molecules able to move in four ways:
  • Lateral diffusion: Rapid movement within the bilayer plane.
  • Flip-flop: Transfer of lipid molecules from one bilayer layer to the opposite in a one-for-one exchange.
  • Intra-chain motion: Kink formation within fatty acyl chains.
  • Axial rotation: Fast rotation around the lipid axes.

Importance of Unsaturated Fatty Acids and Cholesterol

• Unsaturated fatty acids increase membrane fluidity by disrupting hexagonal packing of phospholipids, thereby enhancing flexibility and function.

This quiz covers the foundational concepts of membrane bilayers, including the types of membrane lipids, properties of amphipathic molecules, and the mechanisms behind lipid bilayer formation. You'll also explore the dynamics of movements within lipid bilayers and the role of unsaturated fatty acids and cholesterol in membrane fluidity.