Membrane Structure & Function - Quick Notes

Membrane Structure

  • Phospholipids and globular proteins arranged in a bilayer.
  • Fluid mosaic model: proteins float in/on fluid lipid bilayer.

Membrane Components

  • Phospholipid bilayer: flexible, permeable barrier.
  • Transmembrane proteins: integral membrane proteins.
  • Interior protein network: peripheral membrane proteins.
  • Cell surface markers: glycoproteins and glycolipids.

Phospholipids

  • Glycerol + 2 fatty acids (hydrophobic) + phosphate group (hydrophilic).
  • Forms bilayer spontaneously with fatty acids inside and phosphate groups on both surfaces.

Membrane Fluidity

  • Individual phospholipids and unanchored proteins can move.
  • Saturated fatty acids: less fluid.
  • Unsaturated fatty acids: more fluid due to kinks.
  • Cholesterol: increases or decreases fluidity depending on temperature.
  • Warm temperatures: more fluid.
  • Cold temperatures: less fluid.

Membrane Proteins - Functions

  • Transporters, enzymes, cell-surface receptors, cell-surface identity markers, cell-to-cell adhesion, attachments to cytoskeleton.
  • Integral membrane proteins span the bilayer with nonpolar regions inside and polar regions protruding.
  • Transmembrane domain: hydrophobic amino acids in α helices.

Membrane Transport

  • Passive transport: no energy required, movement down concentration gradient (high to low).

Diffusion

  • Movement of molecules from high to low concentration until equilibrium.
  • Nonpolar molecules cross easily.
  • Limited permeability to polar molecules and ions.

Facilitated Diffusion

  • Molecules move through proteins (channel or carrier) from high to low concentration.
  • Channel proteins: hydrophilic channel, e.g., ion channels (gated).
  • Carrier proteins: bind specifically to molecules, transport rate limited by number of transporters (saturation).

Osmosis

  • Net diffusion of water across a membrane toward higher solute concentration.
  • Hypertonic: higher solute concentration.
  • Hypotonic: lower solute concentration.
  • Isotonic: same solute concentration.
  • Aquaporins facilitate osmosis.
  • Hypertonic solution: cell loses water and shrivels.
  • Hypotonic solution: cell gains water and swells.

Active Transport

  • Requires energy (ATP) to move substances from low to high concentration.
  • Uses selective carrier proteins.
  • Sodium-potassium pump: moves 3 Na+ out and 2 K+ in against concentration gradient.

Coupled Transport

  • Uses ATP indirectly; energy from diffusion of one molecule drives active transport of another (e.g., glucose-Na+ symporter).

Bulk Transport

  • Large molecules cross membrane via vesicles.
  • Exocytosis: movement out of cell.
  • Endocytosis: movement into cell.
    • Phagocytosis: takes in particulate matter.
    • Pinocytosis: takes in fluid.
    • Receptor-mediated endocytosis: specific molecules taken in after binding to receptors.