Chapter 7: The Cell Membrane

Overview

• Focus on cell membranes as fluid mosaics made up of lipids and proteins.
• Plasma membrane acts as a gatekeeper, allowing materials in and out with selective permeability.

Structure of Cell Membrane

• Phospholipids: Primary lipids in the plasma membrane with amphipathic properties (hydrophobic and hydrophilic regions).
• Fluid Mosaic Model: Describes cell membranes as fluid structures with embedded proteins.
  • Hydrophobic tails face inward; hydrophilic heads face cytosol and extracellular fluid.

Historical Models

• Sandwich Model (Davidson and Danielle): Proposed that proteins were like bread outside the phospholipid bilayer.
• Fluid Mosaic Model (Singer and Nicholson, 1972): Verified by freeze-fracture studies; proteins are embedded throughout, not just on surfaces.

Membrane Fluidity

• Movement of Phospholipids: Lateral movement occurs frequently; flip-flopping is rare.
• Factors Affecting Fluidity:
  • Unsaturated fatty acids increase fluidity; saturated fatty acids make it more viscous.
  • Cholesterol: Modulates fluidity by preventing packing in various temperatures.

Membrane Proteins

• Determine what can enter and exit the cell.
• Types of Proteins:
  • Peripheral proteins on membrane surface.
  • Integral proteins span the membrane (transmembrane proteins).
• Functions: Transport, enzymatic activity, signal transduction, cell recognition, intracellular joining, attachment to cytoskeleton and extracellular matrix.

Carbohydrates and Cell Recognition

• Bind to surface molecules on the extracellular side (forming glycolipids/glycoproteins).
• Vary among individuals, species, and cell types, aiding in cell recognition.
• Example: HIV entry requires specific receptors (CD4 and CCR5).

Synthesis and Sidedness of Membranes

• Asymmetrical distribution of proteins, lipids, and carbohydrates.
• ER and Golgi apparatus coordinate membrane synthesis.

Membrane Permeability

• Selective permeability allows specific molecules to diffuse passively or via transport proteins.
  • Passive Transport: Diffusion of small hydrophobic substances; facilitated by transport proteins.
  • Channel Proteins: Allow specific ions/molecules through channels.
  • Carrier Proteins: Change shape to assist molecule movement.

Passive Transport

• Diffusion: Movement from high to low concentration.
• Osmosis: Water movement across a membrane towards higher solute concentration.
• Tonicity: Impact of surrounding solution on cell water content.
  • Isotonic: No net change.
  • Hypertonic: Cell loses water.
  • Hypotonic: Cell gains water (animal cells may burst; plant cells become turgid).

Facilitated Diffusion

• Channel and Carrier Proteins: Speed up passive transport.
• Aquaporins: Specific for rapid water transport through the cell membrane.

Active Transport

• Requires ATP to move substances against concentration gradients.
• Sodium-Potassium Pump: Moves Na⁺ out and K⁺ into cells, maintaining ion gradients.
• Membrane Potential: Voltage difference across membrane due to ion distribution.
• Electrogenic Pumps: Generate voltage (e.g., sodium-potassium pump).

Bulk Transport

• Exocytosis: Vesicles release contents outside the cell.
• Endocytosis: Cells intake macromolecules via vesicle formation.
  • Phagocytosis: Engulfing particles.
  • Pinocytosis: Ingestion of fluid and ions.
  • Receptor-Mediated Endocytosis: Specific ligand binding triggers vesicle formation.