[scientizscht (OP)]

Hello!

What molecules can pass freely from cell membranes?

Is there a complete list?
Or list of characteristics?

Thanks!

[evan_au]

Cell membranes block flow of many molecules, because the cell membrane often has a dual hydrophilic and a hydrophobic layer.

However, there are many "transport proteins" which cross the cell membrane and which permit (or actively encourage) molecules to cross the cell membrane.

Different types of cells express different transport proteins.

See: https://en.wikipedia.org/wiki/Membrane_transport_protein
See: https://en.wikipedia.org/wiki/Lipid_bilayer

[chiralSPO]

There is certainly NOT a complete list of molecules that can pass through membranes freely (there are almost certainly  more than 10⁴⁰ such molecules!!)

There are some rules of thumb. Lipisnki's rule of 5 is not exactly what you're looking for, but I think it is probably a good place to start https://en.wikipedia.org/wiki/Lipinski%27s_rule_of_five

It is also important to note that there are also compounds (DMSO comes to mind) that can make membranes more permeable--allowing though molecules that wouldn't normally cross easily, or increasing the crossover rate for molecules that already could cross.