a very thin piece of material lets some materials pass through while others remain inside
a very thin skin, which has pores large enough for water to pass through into the root but not large enough for dissolved salts to pass out of the root
a membrane which allows water and some solutes to pass through whilst preventing the passage of other solutes.
Typically a thin, organic film which allows the passage of some ions or materials while preventing the passage of others. Some membranes will only allow the passage of cations. (See electrodialysis.) Some membranes reject most dissolved substances, but allow the passage of water. (See reverse osmosis.)