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 2n 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. The difference between L isomers and D isomers is the direction that they rotate plane polarised light. L isomers rotate light anti-clockwise and D isomers rotate clockwise.