Passive transport mechanisms are the unsung heroes of cellular life. They move molecules across cell membranes without energy, relying on concentration gradients. Simple diffusion and osmosis are key players, allowing cells to exchange gases, nutrients, and waste.

Cells must maintain balance in the face of changing environments. Osmotic regulation mechanisms, like aquaporins and ion pumps, help cells control their volume and internal composition. These processes are crucial for cell survival and function in diverse conditions.

Passive Transport Mechanisms

Process of simple diffusion

• Movement of molecules from high concentration region to low concentration region
  • Driven by concentration gradient
  • Requires no energy input
• Molecules move randomly but net movement occurs down concentration gradient
• Crucial role in cellular transport
  • Allows small nonpolar molecules to cross cell membrane (O2, CO2)
  • Facilitates exchange of gases nutrients and waste products between cell and environment (glucose, amino acids)

Concept of osmosis

• Movement of water across selectively permeable membrane from high water potential (low solute concentration) to low water potential (high solute concentration)
  • Driven by difference in solute concentrations across membrane
  • Water moves to equalize solute concentrations on both sides
• Affects cell volume
  • In hypotonic solution (lower solute concentration than cell) water enters cell causing swelling
  • In hypertonic solution (higher solute concentration than cell) water leaves cell causing shrinkage
• Affects solute concentration
  • As water moves into or out of cell solute concentration inside cell changes
    • In hypotonic solution solute concentration decreases as water enters cell (dilution)
    • In hypertonic solution solute concentration increases as water leaves cell (concentration)

Types of osmotic solutions

• Hypotonic solution
  • Lower solute concentration than cell
  • Causes water to move into cell by osmosis
  • Results in cell swelling and potential lysis (bursting) if cell cannot regulate volume (red blood cells in water)
• Hypertonic solution
  • Higher solute concentration than cell
  • Causes water to move out of cell by osmosis
  • Results in cell shrinkage and potential crenation (shriveling) if cell cannot regulate volume (plant cells in salt water)
• Isotonic solution
  • Same solute concentration as cell
  • No net movement of water across cell membrane
  • Cell maintains normal volume and shape (blood cells in saline solution)

Factors affecting diffusion rate

1. Concentration gradient

   • Greater difference in concentration between two sides of membrane faster rate of diffusion (steep gradient)

2. Membrane permeability

   • More permeable membrane is to particular molecule faster rate of diffusion (lipid-soluble molecules)
   • Depends on size charge and polarity of molecule as well as presence of specific transport proteins (ion channels)

3. Temperature

   • Higher temperatures increase kinetic energy of molecules leading to faster diffusion rates ($Q_{10}$ effect)

4. Molecular size and shape

   • Smaller molecules diffuse faster than larger molecules (H2O vs glucose)
   • Molecules with more compact shape diffuse faster than those with more extended shape (spherical vs linear)

5. Pressure

   • Higher pressure on one side of membrane can increase rate of diffusion towards lower pressure side (gas exchange in lungs)

Cellular Osmotic Regulation

Osmotic regulation mechanisms

• Aquaporins are water channel proteins that facilitate rapid movement of water across cell membranes
  • Selectively permeable to water molecules (size exclusion)
  • Allow cells to quickly respond to changes in extracellular osmolarity (renal collecting duct cells)
• Ion pumps and transporters help maintain cell's osmotic balance
  • Na+/K+ ATPase pumps sodium ions out of cell and potassium ions into cell
    • Maintains cell's resting membrane potential ($-70$ mV)
    • Helps regulate cell volume by controlling ion concentrations (nerve cells)
  • Other ion transporters also contribute to osmotic regulation (Cl- channels, H+/K+ antiporter)