From Academic Kids
An aerobic organism or aerobe is an organism that has an oxygen based metabolism. Aerobes, in a process known as cellular respiration, use oxygen to oxidize substrates (for example sugars and fats) in order to obtain energy.
The energy released in this equation is about 2880 kJ per mol, which is conserved in regenerating 38 ATP from 38 ADP per glucose. This is a factor of 19 times more energy per sugar molecule than the typical anaerobic reaction generates.
Notice that oxygen is used during the oxidation of glucose and water is produced.
We distinguish between obligate aerobes and facultative aerobes: obligate aerobes require oxygen, while facultative aerobes can use oxygen, but also have other options.
Almost all animals, most fungi and several bacteria are obligate aerobes. Being an obligate aerobe, although being advantageous from the energetical point of view, means also obligatory facing high levels of oxidative stress.
Yeast is an example of a facultative aerobe. Individual human cells are also facultative aerobes: they switch to lactic acid fermentation if oxygen is not available. However, for the whole organism this cannot be sustained for long, and humans are therefore obligate aerobes.