Simple Diffusion: Occurs when small, non-polar, or uncharged molecules (like or ) pass directly through the phospholipid bilayer without assistance.
Facilitated Diffusion: Required for large polar molecules (e.g., glucose) or charged ions (e.g., ) that cannot interact with the hydrophobic tails of the membrane.
Channel Proteins: Act as water-filled pores that allow specific ions to pass through; many are 'gated' and can open or close in response to specific signals.
Carrier Proteins: Bind to a specific molecule, causing the protein to change shape and release the molecule on the opposite side of the membrane.
Concentration Gradient: A steeper gradient (larger difference in concentration) results in a faster net rate of diffusion as more molecules move toward the lower concentration area.
Temperature: Higher temperatures increase the kinetic energy of particles, causing them to move faster and collide more frequently, thereby increasing the diffusion rate.
Surface Area: A larger surface area (often achieved through folding like microvilli) provides more space for molecules to cross the membrane simultaneously.
Properties of Molecules: Smaller molecules diffuse faster than larger ones because they require less energy to move; similarly, non-polar molecules diffuse faster through the lipid bilayer than polar ones.
| Feature | Simple Diffusion | Facilitated Diffusion |
|---|---|---|
| Mechanism | Direct passage through bilayer | Via transport proteins (Channel/Carrier) |
| Substances | Small, non-polar (, , lipids) | Large polar (glucose), ions (, ) |
| Specificity | Non-specific | Highly specific to one molecule/ion |
| Energy | Passive (No ATP) | Passive (No ATP) |
Unlike Active Transport, both forms of diffusion always move substances down the concentration gradient and do not require energy from the cell.
While simple diffusion rate increases linearly with concentration, facilitated diffusion can reach a saturation point where all transport proteins are occupied.
Identify the Gradient: Always check the direction of movement; if it is from high to low concentration, it is diffusion. If it is low to high, it cannot be diffusion.
Check the Molecule Type: If the question mentions an ion or a large polar molecule like glucose, you must specify facilitated diffusion rather than just diffusion.
Surface Area to Volume Ratio: Remember that as a cell grows larger, its volume increases faster than its surface area, which makes diffusion less efficient for the cell's needs.
Common Error: Do not say 'molecules stop moving' at equilibrium. They continue to move, but there is no net change in concentration.
