Membrane Structure Function

Membrane Structure and Function

Overview of Plasma Membrane

  • Acts as the boundary separating living cells from their surroundings.

  • Exhibits selective permeability, allowing certain substances to cross more easily than others.

Membrane Composition

  • Composed of fluid mosaics of lipids and proteins, primarily phospholipids.

  • Phospholipid characteristics:

    • Most abundant lipid in the plasma membrane.

    • Contains amphipathic structure with both hydrophobic and hydrophilic regions.

  • Fluid mosaic model:

    • Membrane is a fluid structure with various proteins embedded within.

Historical Models of Membrane Structure

  • 1935 Model (Davson-Danielli):

    • Proposed sandwich model with a bilayer of lipids between globular proteins.

    • Initial issues with protein placement due to amphipathic properties.

  • 1972 Proposal (Singer-Nicolson):

    • Proposed the mosaic model with proteins embedded in the bilayer, exposing hydrophilic regions.

Membrane Fluidity

  • Motion of phospholipids:

    • Lateral movement occurs frequently (~10^7 times/sec); transverse flip-flop is rare (once/month).

  • Factors affecting fluidity:

    • Unsaturated fatty acids promote fluidity (due to kinks in tails).

    • Saturated fatty acids contribute to viscosity.

    • Cholesterol modulates fluidity; restrains movement at high temperatures and prevents tight packing at low temperatures.

  • Optimal fluidity is crucial for proper membrane function and is similar to salad oil.

Membrane Proteins

  • Composition:

    • Membrane consists of a collage of diverse proteins that determine specific functions.

  • Types of membrane proteins:

    • Peripheral proteins: Bound to the membrane's surface.

    • Integral proteins: Penetrate the hydrophobic core; some are transmembrane proteins with nonpolar amino acids coiled into alpha helices.

  • Functions of membrane proteins (six major functions):

    • Transport

    • Enzymatic activity

    • Signal transduction

    • Cell-cell recognition

    • Intercellular joining

    • Attachment to cytoskeleton and extracellular matrix (ECM).

Role of Carbohydrates in Cell Recognition

  • Carbohydrates on the cell surface play a key role in recognition and binding.

  • Can be bonded to lipids (glycolipids) or proteins (glycoproteins), showing diversity across species and cell types.

Membrane Asymmetry

  • Distinct inside and outside faces of membranes due to asymmetrical distribution of proteins, lipids, and carbohydrates during the endoplasmic reticulum (ER) and Golgi apparatus assembly.

Selective Permeability and Transport Mechanisms

  • Selective Permeability:

    • Mediates material exchange with surroundings and controls molecular traffic.

  • Lipid Bilayer Permeability:

    • Hydrophobic (nonpolar) molecules pass through rapidly.

    • Polar molecules (like sugars) do not cross easily.

  • Transport Proteins:

    • Facilitate the passage of hydrophilic substances.

    • Channel Proteins: Create hydrophilic tunnels (e.g., aquaporins for water).

    • Carrier Proteins: Bind and change shape to transport molecules across the membrane.

Passive Transport

  • Definitions:

    • Passive transport involves diffusion across the membrane without energy input; molecules move down the concentration gradient.

    • Osmosis: Diffusion of water through a selectively permeable membrane from lower to higher solute concentration.

    • Tonicity:

      • Isotonic: Equal solute concentrations.

      • Hypertonic: Higher solute concentrations; water loss from cells.

      • Hypotonic: Lower solute concentrations; water gain.

Active Transport

  • Active Transport Mechanism:

    • Requires energy (typically in the form of ATP) to move solutes against their concentration gradient.

  • Notable examples:

    • Sodium-Potassium Pump: Maintains ion equilibrium by actively transporting sodium out and potassium into the cell.

Bulk Transport

  • Bulk Transport Mechanisms:

    • Involve large molecules using vesicles to transport across membranes.

    • Exocytosis: Vesicles fuse with the membrane to release contents.

    • Endocytosis: Cell uptake of macromolecules through vesicle formation, including:

      • Phagocytosis: Engulfing particles through vacuole formation.

      • Pinocytosis: Uptake of extracellular fluids in vesicles.

      • Receptor-mediated endocytosis: Ligand binding triggers vesicle formation for selective uptake.

Summary Learning Goals

  • Understand the role of transport proteins in diffusion and active transport mechanisms.

  • Explain how temperature affects membrane fluidity and the significance of membrane functions.