2.7 - Facilitated Diffusion

Cellular Transport Mechanisms and Membrane Dynamics

• Passive Diffusion:

  • Small Nonpolar Molecules: These molecules can easily pass through the lipid bilayer without the aid of transport proteins, utilizing passive diffusion due to their nonpolar nature.

• Facilitated Diffusion and Active Transport:

  • Facilitated Diffusion: This is a form of passive transport aided by transport proteins, allowing molecules that cannot directly pass through the lipid bilayer to move across.

  • Aquaporins: Special channel proteins that facilitate the rapid transport of water across the cell membrane.

  • Active Transport: Requires energy, typically in the form of ATP, to move molecules against their concentration gradient, essential for maintaining cellular function.

• Membrane Potential:

  • Polarization of Membranes: Cell membranes are polarized, exhibiting a voltage or electrical potential difference across the membrane. This is crucial for the movement of charged substances.

  • Membrane Potential: Often acts like a battery, influencing the diffusion of charged particles across the membrane.

  • Resting Potential: Typically around -70 mV for many cells, this potential is crucial for cellular activities.

• Electrochemical Gradients and Ion Channels:

  • Electrochemical Gradient: Both concentration and voltage differences contribute to this gradient, driving the diffusion of ions across the membrane.

  • Gated Ion Channels: These channels can open or close in response to specific stimuli, altering the membrane potential.

• Active Transport Mechanisms:

  • Sodium-Potassium Pump (Na+/K+ ATPase): This pump plays a vital role in setting the cell's resting potential by exchanging three sodium ions (Na+) out of the cell for two potassium ions (K+) into the cell using ATP.

  • Electrogenic Pumps: These are transport proteins that generate voltage across the membrane. An example is the proton pump, which actively transports hydrogen ions (H+) out of the cell.

  • Cotransport: Involves using the energy stored in a concentration gradient of one molecule to transport another molecule against its gradient. For instance, H+ ions are pumped out and then diffuse back into the cell through a sucrose cotransporter, allowing sucrose to enter the cell alongside H+ ions.