Every cell is covered by a membrane that separates the cell from its outside environment. A major component of a cell membrane is its lipid bilayer, which is made of phospholipids. Phospholipids consist of two hydrophobic fatty acids joined by a glycerol molecule to a hydrophilic phosphate group. In the cell membrane, the hydrophilic heads of the phospholipids face outward and the hydrophobic tails face inward. How does the structure of phospholipids help them perform their function?