simple diffusion

Meaning: Simple diffusion is net movement of a solute down its concentration gradient across a membrane, driven by random thermal motion. It needs no carrier, channel or ATP, and it is spontaneous (ΔG < 0). The solute dissolves in the hydrophobic bilayer core, so small nonpolar molecules (O₂, CO₂, steroid hormones) cross readily, small uncharged polar molecules (water, urea) cross slowly, and ions and large polar molecules (glucose) cross very poorly.

Equation: J = −D(ΔC/Δx), where J is flux (mol/(m²·s)), D is the diffusion coefficient (m²/s), ΔC is the concentration difference across the membrane (mol/m³) and Δx is membrane thickness (m).

On the MCAT: Rate versus gradient is a straight line with no plateau, while facilitated diffusion curves up to a maximum. Expect questions on how lipid solubility, a larger gradient, higher temperature or a thicker membrane change the rate, and on why protein inhibitors or ATP depletion leave it unchanged.

Common confusion: Facilitated diffusion is also passive and runs down the gradient. The difference is that it uses a protein, so it is specific and saturable. Osmosis is diffusion of water, which crosses the bilayer slowly by simple diffusion and faster through aquaporins.