Cell Membrane Structure and Function

Chapter 7: Cell Membrane

Structure of Cell Membranes

• Fluid Mosaic Model

  • Cell membranes are described as fluid mosaics composed of lipids and proteins.

  • Plasma membrane acts as a gatekeeper, regulating the ingress and egress of materials through selective permeability.

• Primary Lipids

  • Phospholipids are the main type of lipid found in the plasma membrane:

  • Exhibit amphipathic properties:

    • Contain both hydrophobic (water-repelling) and hydrophilic (water-attracting) regions.

  • Structure:

    • Hydrophilic heads face the exterior (cytosol and extracellular fluid).

    • Hydrophobic tails are oriented inward, forming a bilayer.

• Historical Models of Membrane Structure

  • Sandwich Model (Davidson and Danielle):

  • Proposed phospholipid bilayer as 'peanut butter' with proteins as 'bread'.

  • Issues: Did not account for membrane proteins’ hydrophobic and hydrophilic regions.

  • Fluid Mosaic Model (Singer and Nicolson, 1972):

  • Hydrophilic regions face aqueous environments, embedding various proteins.

  • Verified through freeze fracture studies, illustrating the arrangement of proteins and phospholipid sheets.

Fluidity of Membranes

• Membranes need to be fluid for proper functioning:

  • Lateral Movement: Phospholipids can drift laterally within the bilayer approximately 10^7 times per second.

  • Flip-Flop Movement: Infrequent; occurs about once a month.

• Temperature Effect on Fluidity:

  • Membranes can solidify at low temperatures; fluidity depends on lipid composition:

  • Unsaturated Fatty Acids: Maintain greater fluidity.

  • Saturated Fatty Acids: Increase viscosity and reduce fluidity.

• Cholesterol's Role:

  • At warm temperatures, restricts phospholipid movement; at cooler temperatures, prevents solidification and packing.

Components of Cell Membranes

• Types of Membrane Proteins:

  • Peripheral Proteins: Loosely bound to the surface.

  • Integral Proteins: Span the lipid bilayer, including Transmembrane Proteins.

  • Contain hydrophobic regions primarily consisting of nonpolar amino acids coiled into alpha helices.

• Surface Markers:

  • Carbohydrates:

  • Covalently bonded to proteins (glycoproteins) or lipids (glycolipids).

  • Assist in cell recognition and signaling.

Membrane Functions and Transport

• Functions of Membrane Proteins:

  1. Transport

  2. Enzymatic Activity

  3. Signal Transduction

  4. Cell-Cell Recognition

  5. Intracellular Joining

  6. Attachment to the Cytoskeleton and Extracellular Matrix

• Transport Mechanisms:

  • Passive Transport: Molecules move without energy; includes:

  • Diffusion: Movement down concentration gradient.

    • Equilibrium occurs when molecular movement is equal in both directions.

  • Types of molecules that can diffuse passively:

    • Hydrophobic substances (small) move easily.

    • Polar molecules require facilitated diffusion using:

      • Channel Proteins: Allow specific ions/molecules to pass (e.g., aquaporins for water).

      • Carrier Proteins: Change shape to transport substances across the membrane.

  • Active Transport: Requires ATP to move substances against their concentration gradient:

  • Sodium-Potassium Pump:

    • Transports 3 sodium ions out and 2 potassium ions into the cell, critical for maintaining ion balance.

  • Electrogenic Pumps: Generate voltage across the membrane, allowing for cellular work.

• Tonicity and Water Movement:

  • Isotonic Environment: Equal solute concentrations; no net movement of water.

  • Hypertonic Environment: Higher solute concentration outside which leads to cell shrinkage.

  • Hypotonic Environment: Lower solute concentration outside which leads to cell swelling and potential lysis.

• Plant vs. Animal Cells:

  • Plant cells rely on turgor pressure when in hypotonic environments; have cell walls that prevent lysis.

  • In hypertonic conditions, plant cells undergo plasmolysis.

Bulk Transport Mechanisms

• Exocytosis: Movement of materials out of the cell via vesicles that fuse with the membrane.

• Endocytosis: Cells engulf material via vesicle formation from the membrane:

  • Phagocytosis: Large particles are engulfed, forming food vacuoles.

  • Pinocytosis: Cell takes in extracellular fluid and dissolved solutes.

  • Receptor-Mediated Endocytosis: Specific binding of ligands to receptors triggers vesicle formation.

• Comparison of Transport Types:

  • Passive Transport: No energy needed, relies on concentration gradient.

  • Active Transport: Requires energy (usually from ATP) to move against a gradient.

Summary of Membrane Dynamics

• Cell membranes are dynamic structures vital for maintaining homeostasis in cells.

• The balance of fluidity, selective permeability, and efficient transport mechanisms is crucial for cellular functions.