SC

Cell Membranes - BIOS 101

Objectives

  • Apply the Fluid Mosaic Model to membrane dynamics.
  • Understand selective permeability of membranes.
  • Distinguish passive and active transport.
  • Familiarity with endocytosis and exocytosis for molecule transport across membranes.

Fluid Mosaic Model

  • Describes cell membrane as a dynamic, two-dimensional fluid.
  • Proteins and lipids move and interact, forming a mosaic pattern.
  • Lipids and proteins are not static; certain domains can form.

Cholesterol and Fluidity

  • Cholesterol maintains intermediate fluidity of membranes.
  • Prevents solidification in low temperatures and excessive fluidity in high temperatures.

Membrane Proteins

  • Transport Proteins: Facilitate movement across membranes (passive/active).
    • Channel Proteins: Form pores for facilitated diffusion.
    • Carrier Proteins: Bind and transport molecules, potentially requiring energy.
  • Receptor Proteins: Trigger cellular responses upon binding signaling molecules.
  • Enzymatic Proteins: Catalyze reactions on or near membranes.
  • Cell Adhesion Molecules: Mediate cell interactions.
  • Structural Proteins: Support membrane integrity.

Selective Permeability

  • Controls movement of molecules across membranes, maintaining homeostasis.
  • Nutrient uptake and waste removal are key functions.

Passive Transport

  • No energy required: Moves along concentration gradients.
    • Diffusion: Molecules move from high to low concentration.
    • Facilitated Diffusion: Uses proteins for molecules needing assistance.
    • Osmosis: Water movement across a semi-permeable membrane.

Tonicity and Water Balance

  • Isotonic: Equal concentration; no net water movement.
  • Hypotonic: Lower outside solute concentration; cells may swell.
  • Hypertonic: Higher outside solute concentration; cells may shrink.

Active Transport

  • Requires energy (ATP) to move substances against concentration gradients.
  • Primary Active Transport: Direct use of ATP.
  • Secondary Active Transport (Cotransport): Utilizes electrochemical gradients.
  • Sodium-Potassium Pump: Moves Na+ out and K+ into the cell; crucial for maintaining membrane potential.

Endocytosis and Exocytosis

  • Endocytosis: Membrane engulfs materials, forming vesicles.
    • Phagocytosis: Engulfing large particles.
    • Pinocytosis: Engulfing liquids and solutes.
    • Receptor-mediated: Specific molecule uptake via receptors.
  • Exocytosis: Vesicles fuse with membrane, releasing contents; important for secretion and adding membrane components.