What are facultative anaerobes?

During the process of respiration, oxygen is utilized, and carbon dioxide, water and energy are released as products. The combustion reaction requires oxygen. But some cells live where oxygen may or may not be available. Can you think of such situations (and organisms) where O₂ is not available? There are sufficient reasons to believe that the first cells on this planet lived in an atmosphere that lacked oxygen. Even among present-day living organisms, we know of several that are adapted to anaerobic conditions. Some of these organisms are facultative anaerobes, while in others the requirement for anaerobic condition is obligate. In any case, all living organisms retain the enzymatic machinery to partially oxidase glucose without the help of oxygen. This breakdown of glucose to pyruvic acid is called glycolysis.