∞ generated and posted on 2016.08.25 ∞
Movement of substances across lipid bilayers without protein facilitation.
This occurs, net, solely down a substances concentration gradient, from regions of high concentration to regions of low concentration. Passive diffusion can also be described as simple diffusion.
By "net" I mean that substances can pass back and forth across the membrane in both directions but, overall, more moves down the substance's concentration gradient than up that gradient and this is so simply because there is more to move from the high concentration side than from the low concentration side.
The following video provides a nice though longish overview of movement across lipid bilayers except as via active transport: