Question

How do the phospholipid molecules control what moves across the plasma membrane?

Answer 1

Phospholipis are polar molecules having a hydrophilc head and a hydrophobic tail. The head repels the water insoluble materials so they do not pass, and the tail repels the water soluble materials so they do not pass. It is only the gases which are very small in size that pass directly through the phospholipid bilayer. All other molecules should go through channels which are very specific in choosing what to be allowed in because of their three dimensional internal structure.