BioCoach Activity

Concept 4: Isomers

Organic molecules exist in three-dimensional space, and the same set of atoms can be put together in many recognizably different ways, resulting in molecules called isomers.

The atoms found in a simple sugar, with the structural formula C6H12O6, can be arranged in over a dozen different ways.

Even though many isomers can theoretically exist, cells are discriminating about which ones they will synthesize and recognize. For example, the sugar glucose (C6H12O6) is very abundant and can be used by almost all organisms as a quick energy source. By contrast, an isomer of glucose called tagatose (also C6H12O6) is rare and is not useful to most forms of life—same atoms, different shapes.

Three situations can lead to the existence of isomers:

  1. Structural isomers: Variations in the position at which different atoms are joined together.
  2. Enantiomers: Left-handed and right-handed variations resulting from the tetrahedral geometry of carbon.
  3. Geometric isomers: Variations in the placement of atoms around carbon atoms joined by double covalent bonds.