And as we'll see in just a moment, an epimer is basically a specific type of diaryoisomer. So let's think back to organic chemistry for just a moment. So what exactly is an isomer? Well, from organic chemistry we know that isomers are basically those molecules that contain the same exact molecular formula. So they have the same exact number of atoms and the same type of atoms, but they differ in their arrangement of atoms in the three dimensional space. Now, we have two types of isomers when it comes to sugars.