Question

Why are optical isomers important to the cell?

Answer 1

Optical isomers are mirror-images, like a right and a left shoe.

Explanation:

Many enzymes and other receptors in the cell are made for just the right or the left shoe. The other one doesn't fit, so it won't get processed, which renders it useless.

Sometimes it's even worse:
Most people remember the Thalidomine scandal, when expectant mothers who were given this drug ended up with deformed babies. Thalidomine was given as a racemic mixture (equal amounts of right and left shoes), while only the left shoe did what was intended, and the right shoe made a mess of the unborn baby's growth.