A neat discussion of symmetry in living things:
The complexity of carbon chemistry means that there are many molecules that have a mirror image counterpart that is essentially different, that is, the two cannot be rotated into one another. They are in one sense chemically the same, but living things can tell the difference. The difference in the smell of oranges and lemons is caused by right and left forms of limonene. In many cases, such as with amino acids, only one stereoisomer of the pair is found in living things.
newcriterion.com/archives/25/12...
