I'm reading a book concerning cell biology and have a question. Some molecules are mirror images of one another and such molecules are called optical isomers, or stereoisomers. The different stereoisomers of a molecule usually have completely different biological activities. Now in biology, function always depends on structure. Different stereoisomers of a molecule are basically the inverted form of one another (I've assumed that since its said that they are mirror images of one another) or in other words have the same structure. Why then do they perform different biological functions?