Biological Membranes - Chapter 7 MCAT Prep

Lecturer: Iman

Overview

• The plasma membrane defines the cell boundaries, controls traffic, and exhibits selective permeability.
• The lecture focuses on the structure and function of cellular membranes and the transport mechanisms across them.
• Key topics include membrane components, the fluid mosaic model, selective permeability, and membrane transport (passive, active, endocytosis, exocytosis).

Fluid Mosaic Model

• Definition: The membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.
• Components: Phospholipids & Proteins
  • Phospholipids: Amphipathic molecules (hydrophilic head, hydrophobic tail)
  • Proteins: Also amphipathic and can interact with the aqueous environment and hydrophobic interior
  • Carbohydrates: Important but less abundant than lipids and proteins
• Properties: Lipids move freely within the membrane; proteins and carbohydrates can also move but are often part of stable structures (lipid rafts).

Membrane Components

• Lipids: Primary components of membranes by mass and mole fraction
  • Triglycerides and fatty acids (phospholipid precursors)
  • Glycerophospholipids (phosphate group replaces one fatty acid)
  • Cholesterol (contributes to fluidity and stability)
  • Waxes (waterproofing and defense, mainly in plants)
• Proteins: Act as transporters, cell adhesion molecules, enzymes
  • Transmembrane proteins (receptors or channels)
  • Embedded proteins (part of catalytic complexes, cellular communication)
  • Membrane-associated proteins (recognition molecules or enzymes)
• Carbohydrates: Form protective glycoprotein coats, function in cell recognition
• Cell-Cell Junctions: Form cohesive layers, provide pathways for communication
  • Gap Junctions (Connexons): Direct cell-cell communication, movement of water and solutes
  • Tight Junctions: Prevent solute leakage, found in epithelial cells
  • Desmosomes: Bind adjacent cells, provide structural support

Selective Permeability

• Lipid Bilayer Permeability:
  • Non-polar molecules (e.g., hydrocarbons, CO2, O2) cross easily
  • Hydrophobic interior impedes ions and polar molecules
  • Transport proteins play key roles for hydrophilic substances
• Transport Proteins:
  • Channel proteins (e.g., aquaporins for water)
  • Carrier proteins (specific for substances)
• Membrane Receptors:
  • Often transmembrane proteins, regulate transporter activity
• Passive Transport:
  • Simple Diffusion: Move down concentration gradient
  • Osmosis: Water diffusion from low to high solute concentration
  • Facilitated Diffusion: Uses transport proteins for impermeable molecules
• Active Transport:
  • Move solutes against their concentration gradient, requires energy
  • Sodium-Potassium Pump: Exchanges Na+ for K+; maintains membrane potential

Bulk Transport

• Exocytosis: Vesicles secrete materials outside the cell
• Endocytosis: Cell engulfs material into vesicles
  • Pinocytosis: Ingestion of fluids and dissolved particles
  • Phagocytosis: Ingestion of large solids (e.g., bacteria)

Summary

• Membrane structure and transport are crucial for cell function.
• The fluid mosaic model helps describe the dynamic nature of membranes.
• Understanding membrane components and transport mechanisms is key for MCAT preparation.
• Next, focus on practice problems to apply these concepts.

Questions? Comment below!

Good luck and happy studying!