5. Small polar molecule are not able to enter a cell through passive diffusion, but require a facilitator protein. What part of the phospholipid bilayer prevents them from doing so?
The phospholipid "tails", because they are also polar
The phospholipid "heads", because they are also polar
The phospholipid "tails", because they are non-polar
The phospholipid "heads", because they are non-polar