passive diffusion

Meaning: Passive diffusion is net movement of a solute across a membrane down its concentration gradient, directly through the lipid bilayer, with no transport protein and no ATP. Random thermal motion drives it, so ΔG is negative. Small nonpolar molecules (O₂, CO₂, steroid hormones) cross readily; ions and large polar molecules cross poorly because they cannot enter the hydrophobic core. Net movement stops at equilibrium.

Equation: J = −D(ΔC/Δx). 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). The minus sign means flow toward lower concentration.

On the MCAT: Passages compare transport types. The rate of passive diffusion rises linearly with the gradient and does not saturate, whereas carrier-mediated transport levels off at Vmax. Expect to predict that more lipid-soluble solutes diffuse faster and that a thicker membrane slows flux. A metabolic poison that depletes ATP does not directly stop it.

Common confusion: Facilitated diffusion is also passive, with no ATP and movement down the gradient, but it uses channels or carriers, so it is specific and saturable. Do not label a process passive diffusion just because it needs no ATP; check whether a protein is involved.