8.- Feb 13 Membrane Structure

Membrane Structure and Functions

• Membrane Formation

  • Membranes create specialized compartments through selective permeability.

  • Different internal environments are established (concentration of molecules, pH, charge differences).

  • Asymmetrical distribution of proteins plays a role in cellular function.

• Key Functions of Membranes

  • Cell-cell recognition.

  • Receptor molecules facilitate signaling; ligand binding induces intracellular reactions.

  • Control of reaction sequences facilitates metabolic pathways (product of one enzyme can be a substrate for the next).

  • Sequences of redox reactions are crucial for cellular metabolism.

Membranes as Compartments

• The plasma membrane encloses cell molecules from the external environment.

• Internal compartments are created by internal membranes, allowing for diverse cellular functions.

Membrane Structure Models

• Davson-Danielli Model

  • Visualized through electron microscopy (2 dark lines with light interior).

  • Features a lipid bilayer with hydrophobic tails (inside) and hydrophilic heads (outside).

  • Proteins coat the exterior surface creating a protein-lipid sandwich.

• Fluid Mosaic Model by Singer and Nicolson

  • Membranes comprise a fluid mosaic of phospholipids and proteins.

  • Integral proteins span the lipid bilayer.

  • Peripheral proteins associate with the membrane surface.

  • Lateral movement of proteins within the lipid layer is significant.

Membrane Fluidity

• Membrane fluidity allows for the lateral movement of proteins and lipids, which is essential for various cellular functions including signaling.

• Factors Affecting Fluidity

  • Lipid composition: unsaturated fatty acids allow greater flexibility compared to saturated fatty acids.

  • Cholesterol maintains fluidity, preventing membranes from becoming too fluid or solid.

Cholesterol in Membranes

• Cholesterol molecules fill spaces between fatty acid tails, influencing fluidity, and stability of membranes.

• Cholesterol can create regions of varying fluidity within the membrane.

Lipid Bilayer Formation

• Lipids are amphipathic, causing them to arrange into bilayers spontaneously in aqueous environments.

• The hydrophilic heads face outward, while the hydrophobic tails face inward, forming a sealed barrier.

Asymmetrical Distribution of Lipids

• The lipid bilayer shows different lipid compositions on each side.

  • Phospholipids are synthesized on the cytoplasmic side and later transferred across the bilayer by enzymes called flippases.

  • Glycolipids are predominantly found on the extracellular side of the plasma membrane.

Membrane Proteins

• Types of Membrane Proteins

  • Integral Proteins: Span the membrane, involved in transport and signaling.

  • Peripheral Proteins: Attach to the membrane surface and are involved in maintaining structure and signaling.

  • Lipid-Linked Proteins: Associated with membranes through lipid anchors.

• Experimentation

  • Detergents can extract membrane proteins for study. Amphipathic properties allow detergents to solubilize proteins without denaturing them.

Membrane Carbohydrates

• Membrane carbohydrates play crucial roles in cell signaling, cell recognition, and interactions.

  • Function as glycoproteins or glycolipids, providing specificity for interactions like blood type recognition.

  • Lectins bind specific oligosaccharides for cellular recognition and response to infections.

Summary of Membrane Dynamics

• Membranes are not static structures; they constantly adapt their composition and fluidity in response to environmental changes.

• Maintaining membrane fluidity is critical for cellular health and function, enabling the movement and interaction of membrane proteins and lipids.