The way I remember this is that simple diffusion is simple, and as such, there are no transporters involved. It is also simple in the sense that molecules may only travel passively from a high concentration to a low concentration (across a semi-permeable membrane), down its concentration gradient. Molecules that take part in simple diffusion are either uncharged molecules such as CO₂ or O₂, or lipophilic/non-polar molecules such as certain drugs entering cells from circulation. This is because the lipid bilayer of cells repels charged molecules and anything hydrophilic.Facilitated diffusion however is not so simple. This is any kind of transport that involves carrier or channel proteins in the membrane of cells. Similar to similar diffusion, this process is also passive - no ATP is required for this transport to occur directly. The need for facilitated diffusion comes from the need to transport the molecules that cannot travel by simple diffusion i.e. charged and polar molecules, for example sodium ions. These proteins facilitate the diffusion of one or several substances depending on whether it is a channel protein or a co-transporter for example.

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