In biology, a description of those living 
organisms that do not require oxygen for the 
release of energy from food (carbohydrates, 
proteins, and lipids). Anaerobic organisms 
include many bacteria and yeasts. Obligate 
anaerobes such as archaebacteria cannot 
function in the presence of oxygen; but 
facultative anaerobes, like the fermenting 
yeasts and some bacteria, can function with 
or without oxygen. Anaerobic organisms 
release 19 times less of the available 
energy from their food than do aerobic
organisms. In some bacteria, instead of 
oxygen, an inorganic compound, such as 
sulphate (SO4), is the final acceptor of 
electrons stripped from food molecules during 
their breakdown. In fermentation (as
practised by yeasts) the final acceptor is an
intermediate product of the glucose molecule
being degraded.