Chapter 7 Membrane Structure & Function

• Phospholipids:

  • Key components of the membrane.

  • Arranged in a bilayer, forming the fundamental structure.

  • Fluid Mosaic Model: Proteins float in or on the fluid lipid bilayer.

• Components of Membrane:

  • Phospholipid Bilayer: Flexible matrix, acts as a permeability barrier.

  • Transmembrane Proteins: Integral membrane proteins extending across the lipid bilayer.

  • Interior Protein Network: Peripheral membrane proteins supporting the structure.

  • Cell Surface Markers: Glycoproteins and glycolipids that identify cells.

Phospholipid Structure

• Components:

  • Glycerol: A three-carbon molecule.

  • Fatty Acids: Two nonpolar, hydrophobic fatty acids.

  • Phosphate Group: Polar and hydrophilic, forming the head.

• Bilayer Formation:

  • Spontaneously forms with fatty acids facing inward and phosphate heads facing outward.

Membrane Fluidity

• Fluidity Characteristics:

  • Lateral Movement: Phospholipids move laterally ~107 times per second.

  • Flip-Flop Movement: ~Once per month, where phospholipids switch layers.

• Factors Influencing Fluidity:

  • Saturated vs Unsaturated Fatty Acids:

    • Unsaturated fatty acids introduce kinks, preventing tight packing, leading to increased fluidity.

  • Cholesterol: Acts as a buffer for fluidity depending on temperature.

Membrane Proteins

• Functions:

  1. Transporters: Move substances across the membrane.

  2. Enzymes: Catalysts for reactions.

  3. Cell-Surface Receptors: Communicate signals.

  4. Identity Markers: Help cells recognize each other.

  5. Adhesion Proteins: Attach cells to each other or extracellular matrix.

  6. Cytoskeleton Attachment: Contributes to the shape and structure of the cell.

• Integral Membrane Proteins:

  • Span the lipid bilayer with nonpolar regions inside and polar regions protruding outward.

  • Transmembrane Domain: Contains hydrophobic amino acids typically arranged in alpha helices.

Types of Membrane Transport

• Passive Transport:

  • No Energy Required: Movement down a concentration gradient.

  • Diffusion: Molecules move from high to low concentrations until equilibrium is reached.

• Facilitated Diffusion:

  • Molecules that cannot easily cross membranes move through protein channels or carriers.

  • Channel Proteins: Form hydrophilic channels for ions.

  • Carrier Proteins: Specific to the solute, change shape to transport it.

• Active Transport:

  • Energy Required: Moves substances against their concentration gradients using ATP.

  • Example: Sodium-Potassium Pump: Moves Na+ out of the cell and K+ into the cell.

Osmosis

• Definition: Net diffusion of water across a selectively permeable membrane towards higher solute concentration.

• Types of Solutions:

  • Hypertonic: Higher solute concentration leading to water loss in cells.

  • Hypotonic: Lower solute concentration leading to water intake and possible cell lysis.

  • Isotonic: Equal solute concentrations; no net movement.

Bulk Transport

• Mechanism: Large quantities or molecules use vesicles to cross membranes.

  • Exocytosis: Expulsion of materials from the cell.

  • Endocytosis: Intake of materials into the cell through invagination of the plasma membrane.

    • Types of endocytosis:

      • Phagocytosis: Engulfing of particles.

      • Pinocytosis: Intake of fluids.

      • Receptor-Mediated Endocytosis: Specific molecules bind to receptors to trigger a vesicle formation.