We don't find indol in high concentrations and indol. When it collides, it cannot actually squeeze across that cell membrane because of its size and the fact that there are not too many collisions taking place between indo and the cell membrane in the first place. So three things that you have to consider to basically determine the permeability of a molecule across the cell membrane. So number one is, will it actually dissolve in that non polar core section of the phospholipid bilayer? So you have to look at the molecule's polarity. Number two is you have to look at size.