New Scientist explains Saccharomyces cerevisiae, better known as brewer's yeast:
While we take yeast's brewing abilities for granted, they are in fact rather surprising. Most organisms that generate energy from sugars to use oxygen to break the molecules down into water and carbon dioxide. The energy this releases is stored in the form of adenosine triphosphate (ATP), the molecule that cells use for fuel. In this process, known as aerobic respiration, each glucose molecule yields about 36 molecules of ATP.
S. cerevisiae, however, spurns oxygen. Instead, it converts sugars into ethanol, generating a meagre two molecules of ATP per glucose molecule.
Hoist a glass and enjoy!