How can a substance pass through a lipid membrane in a cell through simple diffusion? In order for something to be able to go through the membrane, in simple diffusion, it must be hydrophobic, or non polar. The reason for this is that in order for it to easily interact and pass the fatty acid part of the lipid membrane, which is hydrophobic or non polar, it must also be hydrophobic and non polar. But in order for this substance to pass the phosphate part of the membrane, which is hydrophilic, or polar, doesn't it also need to be polar and hydrophilic?
Simple diffusion is the movement of solutes from an area of high solute concentation to an area of low solute concentration.
Here are a few ways molecules will make it through membrane into cell:
(A) Fuse with the membrane and enter due to like polarities i.e non-polar subtance fuses with non-polar membrane and passes through membrane
(B) Small polar molecules CAN move into cell but at a much slower rate
(C) Through transmembrane integral protein channels such as aquaporins, K+ leak channels, and other channels that DO NOT require energy, and are ALWAYS open
If you want to understand how it was determined which molecules were able to pass through the membrane look for some resources about Overton's Plasmolytic Method.
Alongside that the definition of the partition coefficient will give you some insight on how fast molecules will pass through membrane, if at all.
Good luck on the searching!
@ Ro Siv - Simple diffusion is not facilitated diffusion