Just what is the phospholipid bilayer?
This is the two layers of phospholipids that make up the cell membrane. Understanding the structure of this organelle is extremely important to understanding how this organelle carries out its functions.
Examine the image here of the phospholipid bilayer. Notice that the red circles are labeled as hydrophilic heads, and the parts in between are labeled as the hydrophobic tails.
It is this structure with the hydrophilic heads and hydrophobic tails that makes the cell membrane selectively permeable. This means some molecules can cross the membrane while others cannot.