Diffusion means that the net movement of particles (molecules) is from an area of high concentration to low concentration. If the particles can move through the lipid bilayer by simple diffusion, then nothing limits the number that can fit through the membrane.

Why is passive diffusion not saturable?

This is the mechanism by which lipophilic (hydrophobic) uncharged molecules find a passage across the membrane by solubilizing within the lipids of the membrane. Passive diffusion, therefore, requires no energy expenditure by the cell; it is not saturable or subject to competition between molecules. ...

Is simple diffusion unassisted?

Passive Transport: Simple Diffusion

Diffusion across a cell membrane is a type of passive transport, or transport across the cell membrane that does not require energy. ... Therefore, simple diffusion is the unassisted passage of small, hydrophobic, nonpolar molecules from a higher concentration to a lower concentration.