Physioex 1: Cell Transport Mechanism and Permeability Practice Test

Simple diffusion moves molecules down their concentration gradient without the help of carrier proteins, so the rate of transfer rises as the gradient increases and does not plateau due to binding sites. This is described by Fick’s law, where flux is proportional to the concentration difference across the membrane. Since no energy is used, diffusion is a passive process, and small, nonpolar or lipid‑soluble molecules readily diffuse. The idea that diffusion would require ATP is incorrect, and saying diffusion is impossible for small molecules is also false. The statement that best fits this concept is that the rate does not saturate with higher solute concentration.