The question is what exactly determines the ability of a given molecule or atom to be able to pass across that cell membrane? Well, it turns out that two factors influence the ease with which atom our molecule or atom passes across our cell membrane. And these factors include the size of our molecule as well as the polarity of that molecule. Now, the size aspect is a bit intuitive, so the larger our molecule, the less likely it will pass through that cell membrane. Now, what about polarity? Why exactly does polarity influence the ability of the molecule to pass across the cell membrane?