1/28 Lecture

Study Notes on Cell Membranes and Their Composition

Composition of Cell Membranes

  • Main Components: Cell membranes in eukaryotic cells consist of phospholipids and proteins.

  • Phospholipids: Form bilayers and have a polar head and two hydrocarbon tails, creating a hydrophobic (nonpolar) and hydrophilic (polar) region.

    • Polar Head Structure:

      • Centrally linked glycerol molecule

      • Phosphate group attached to the third carbon

      • Example: Choline can vary among different phospholipids to form distinct types.

    • Hydrocarbon Tails: Composed of long nonpolar structures that are hydrophobic.

  • Phospholipid Types in Eukaryotic Cells:

    • Phosphatidylethanolamine

    • Phosphatidylserine

    • Phosphatidylcholine

    • Sphingomyelin: Similar to phospholipids but differs by its attachment of a second fatty acid tail.

    • Sphingosine: Has a single fatty acid tail, found in smaller quantities in cell membranes.

  • Cholesterol:

    • Enhances membrane fluidity and stability, has a small polar region and a large nonpolar region, fitting into phospholipid bilayers.

    • Amphipathic Nature: Both hydrophobic and hydrophilic properties allow cholesterol to integrate into membranes.

Membrane Structure Dynamics

  • Bilayer Formation:

    • Micelles form from cone-shaped phospholipids when placed in water.

      • Hydrophobic regions avoid water, while polar regions face outward.

      • Phospholipids form bilayers rather than micelles as they maximize hydrophobic interactions among tails.

  • Compartment Formation:

    • Bilayers form closed compartments naturally in aqueous environments, creating cellular boundaries.

  • Lipid Bilayer Characteristics:

    • 2D Fluidity: Membranes are not macromolecules and consist of phospholipid subunits linked by weak Van der Waals interactions, allowing for lateral movement without flip-flopping except under rare circumstances.

  • Fluidity Factors:

    • Variations in fatty acid saturation affect membrane fluidity.

      • Unsaturated (kinked) fatty acids allow for more fluidity, while saturated (straight) fatty acids contribute to stiffness.

  • Cholesterol Effects: Reduces membrane fluidity by filling space between kinked fatty acids.

Membrane Composition Variations

  • Different Cell Membrane Compositions:

    • Examples of different compositions between cell types (e.g., liver and red blood cells).

    • Organelles like mitochondria and ER possess distinct phospholipid profiles differing from the plasma membrane.

    • Bacterial membranes contain unique phospholipid structures compared to eukaryotic cells.

  • Leaflet Asymmetry:

    • Membranes may be asymmetric with different phospholipid distributions across leaflets (e.g., cytosolic leaflet rich in phosphatidylserine; extracellular leaflet rich in phosphatidylcholine and sphingomyelin).

  • Importance of Asymmetry:

    • Necessary for signaling processes, for example, phosphatidylserine on cell surfaces signals immune cells for removal in unhealthy cells.

Membrane Proteins and Their Functions

  • Protein Integration:

    • Membrane proteins, particularly transmembrane proteins, span the membrane and can be categorized in different ways based on their integration method:

      1. Transmembrane Proteins: Span the membrane (alpha helices or beta barrels).

      2. Amphipathic Alpha Helices: Partially embedded, one side hydrophilic, other side hydrophobic.

      3. Lipid Anchors: Covalently linked to proteins, similar to hydrophobic side chains of fatty acids.

      4. Peripheral Proteins: Loosely associated with the membrane, interact with transmembrane proteins.

  • Permanent vs. Temporary Integration:

    • Transmembrane proteins are permanently integrated, while others can be transiently associated or dissociated from the membrane.

Membrane Protein Characteristics

  • Lipophilicity vs. Hydrophilicity: Hydrophobic amino acids are required for protein interaction with the hydrophobic membrane core, whereas hydrophilic regions function in aqueous environments.

  • Transmembrane Protein Structure: Usually contains hydrophobic regions that signal membrane spanning capabilities as identified in hydropathy plots.

  • Function of Lateral Diffusion: Proteins can move laterally within the membrane despite immobilization by interactions with lipids or cytoskeletal components.

Membrane Segregation and Diversity

  • Lipid Rafts: Membrane regions enriched in saturated fatty acids and cholesterol which can localize specific proteins, affecting membrane properties.

  • Epithelial Cell Asymmetries: Different proteins localized at different regions of epithelial sheets can be regulated by interactions between neighboring cells.

Conclusion of Membrane Structure and Functions

  • Membrane Diversity: Each cellular membrane has a unique composition that affects its function and organization. Examples illustrate the importance of understanding membrane variations for cellular processes.