And phospholipids themselves are predominantly fatty acids. So that means we have a predominant portion of the cell membrane that is non polar inside. In fact, the entire intermediate space between these heads is basically non polar. And so that means nonpolar molecules, no matter how large, will be able to move across. And that's exactly why cholesterol, which is a large molecule, has no problem moving across our cell membrane. But even a very small atom that has a full positive or full negative charge, such as the sodium atom, will not be able to move across because it is very polar.