Chiral carbon

 A chiral carbon is a carbon atom which has 4 different groups attached. For each chiral carbon in a molecule there are 2 optical isomers. This increases exponentially with more chiral carbons, so a molecule with 2 chiral centers would have 4 optical isomers, and a molecule with 3 would have 8 optical isomers. This can be calcultated by 2^n where n is the number of chiral carbons. In nature often only one optical isomer is produced, for example only L isomer amino acids are produced in translation.