U02 p1

Unit 2: Overview of Cell Membranes

Plasma Membrane

  • Defines the cell and separates inner cell content from the environment.

  • Creates intracellular compartments characteristic of eukaryotic cells.

  • Composed of a thin layer of lipid molecules with embedded proteins, held by noncovalent interactions.

  • Provides membrane fluidity, liquidity, and dynamic properties.

Lipid Composition

  • Membranes are typically impermeable to most water-soluble molecules.

  • Image shows phosphatidylcholine molecules forming a lipid bilayer with:

    • Red lipid head groups and yellow fatty acid tails.

    • Water molecules on either side of the bilayer.

  • Gradients form across membranes affecting ion and solute distribution.

Ion Gradients and Functions

  • Ion gradients are crucial for:

    • ATP production.

    • Electrical signaling in nerve and muscle cells.

    • Transmembrane movement of solutes.

  • Transmembrane proteins:

    • Facilitate movement of ions/solutes.

    • Serve structural roles and act as cell surface receptors.

    • Approximately 30% of proteins in the eukaryotic genome are membrane proteins.

Lipid Bilayer Structure

  • Lipid bilayer is about 5 nanometers thick, visible via Transmission Electron Microscopy (TEM).

  • Lipids assemble spontaneously into bilayers due to their amphiphilic nature:

    • Hydrophobic tails (nonpolar) face inward.

    • Hydrophilic heads (polar) face outward.

  • Animal cell membranes are generally 50% lipid and 50% protein.

Phospholipids

  • Phospholipids consist of:

    • Polar head groups and two hydrocarbon tails (12-24 carbons in length).

    • One tail usually saturated and the other unsaturated.

    • Main types include:

      • Phosphatidylethanolamine (PE).

      • Phosphatidylserine (PS).

      • Phosphatidylcholine (PC).

    • PS has a charge at neutral pH; PE and PC are neutral.

    • Sphingolipids or sphingomyelin also present in cell membranes.

Lipid Shapes and Membrane Formation

  • Lipids can form:

    • Micelles (conical shape) or lipid bilayers (rectangular shape).

  • Lipid bilayers can be synthesized in labs, forming liposomes or black membranes depending on lipid concentration.

  • Bilayer integrity is crucial; tears lead to segregation to minimize exposure of hydrophobic regions.

Phospholipid Movement

  • Phospholipid movements include:

    • Lateral diffusion: rapid movement across the membrane (equivalent to a bacterium per second).

    • Flip-flopping: rare movement from one leaflet to the other.

    • Rapid rotation and flexion of tails.

  • Fluidity depends on composition and temperature:

    • Shorter hydrocarbon chains and more double bonds increase fluidity.

    • Phase transition occurs when shifting from liquid to gel state as temperatures drop.

Role of Cholesterol

  • Cholesterol (a steroid molecule) is common in eukaryotic membranes (ratio can be 1:1 with phospholipids).

  • Enhances membrane permeability barrier and impacts fluidity; concentration-dependent effects.

  • Cholesterol orientation within the bilayer:

    • Polar head groups embedded in phospholipid heads.

    • Hydrocarbon region associated with fatty acid tails.

  • Maintains membrane fluidity and prevents phase transition.

Lipid Rafts

  • Lipid rafts are regions with high concentrations of specific lipids and proteins:

    • Composed of single lipids, glycolipids, and cholesterol.

    • Play roles in vesicular transport, signal transduction, and protein aggregation.

  • Distinct domains can be observed within membranes, signaling different functions.

Lipid Storage and Asymmetry

  • Lipid droplets allow cells to store lipids and can be retrieved for membrane biogenesis (example: adipocytes).

  • Noted for:

    • Surrounded by a monolayer, unlike organelles.

    • Rapid formation upon high fatty acid concentrations.

  • Asymmetry in membranes:

    • Outer leaflet (red blood cells) predominantly has phosphatidylcholine and sphingomyelin; inner leaflet has phosphatidylserine and phosphatidylethanolamine.

    • Asymmetry is maintained by membrane-bound enzymes (phospholipid translocators).

Glycolipids in Cell Membranes

  • Glycolipids are sugar-containing lipids found on the outer leaflet.

  • Important in cell recognition, protection, and adhesion, particularly in epithelial cells.

  • Gangliosides, a type of glycolipid, are found in nerve cells and have implications for membrane properties and ion concentrations.

Conclusion

  • The intricate structure and fluidity of cell membranes are vital for cell integrity and function in signaling, transport, and cellular organization.