Phospholipids are used by cells to create plasma membranes. A phospholipid molecule consists of two fatty acids and one phosphate group bonded to a glyceride. Fatty acids are long chains of carbon atoms. The fatty acid used to build phospholipids in Petri Dish has a 16-carbon chain and is known as palmitic acid.

The glyceride and phosphate that form the “head” of the phospholipid are both polar molecules, making the head of the phospholipid water-soluble. The long carbon chains that form the “tail” of the phospholipid are both nonpolar molecules, making the tail of the phospholipid fat-soluble. A detailed explanation of polarity and solubility can be found in our interactive, multitouch textbook, Chemistry from the Ground Up.

When mixed in water, phospholipid molecules tend to form bilayers. This is because the water-soluble head of the phospholipid is attracted to the polar water molecules, but the fat-soluble tail is not.