facilitated diffusion

Meaning: Facilitated diffusion moves a solute across a membrane through a channel or carrier protein, down its concentration (or electrochemical) gradient. Polar molecules and ions, such as glucose, Na⁺ and K⁺, cross the hydrophobic lipid core poorly, so a protein gives them a path. Water can also cross through aquaporins. No ATP is used, ΔG of transport is negative, and net movement stops when the gradient is gone. Transporters are specific for their solute.

Equation: v = Vmax[S] / (Km + [S]), where v is the transport rate (mol/s), Vmax is the maximum rate when every carrier is occupied (mol/s), [S] is the solute concentration (mol/L), and Km is the concentration giving half of Vmax (mol/L). This describes carrier-mediated transport. Simple diffusion instead follows a rate proportional to the gradient.

On the MCAT: A passage plots transport rate against solute concentration. A straight line suggests simple diffusion. A curve that plateaus suggests a carrier, with saturation and specificity. Competitive inhibitors can slow transport of the normal solute. Questions may ask what happens if ATP is depleted: facilitated diffusion continues, but active transport is affected.

Common confusion: Needing a membrane protein does not make transport active. Active transport moves solute against its gradient and requires energy, either directly from ATP or indirectly from another gradient. Facilitated diffusion only moves solute downhill.