Now, we also have this single layer of phospholipids that protect the hydrophobic core from the hydrophilic solution found in our surrounding environment. And we also have these proteins known as apoproteins. APO simply means the protein does not have its substrate. It doesn't have its coenzyme or cofactor attached to the protein. Now, we can also call the apoprotein an apollipoprotein because the apoprotein is attached to our lipid, as shown. This is the hydrophilic section of the protein, and this is the hydrophobic section.