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.

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