Aerobic organism
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.
A good example would be the oxidation of
glucose (a
monosaccharide) in
aerobic respiration.:C
6H
12O
6 + 6 O
2 + 38
ADP + 38 phosphate → 6 CO
2 + 6 H
2O + 38
ATPThe 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. Eukaryotic organisms (everything but bacteria) only get a net gain of 36 ATP regenerated from ADP in this process, due to an additional membrane that must be crossed by active transport.
Notice that oxygen is used during the oxidation of glucose and
water is produced.
This equation is a summary of what actually happens in three series of biochemical reactions:
glycolysis, the
Krebs cycle, and
oxidative phosphorylation.
We distinguish between
obligate aerobes and
facultative aerobes: obligate aerobes
require oxygen, while facultative aerobes can use oxygen, but also have other options, such as
nitrogen.
Microaerophiles are organisms that may use oxygen, but only at low concentration. Aerotolerant organisms can survive in the presence of oxygen, but they are anaerobic because they cannot use it.
Almost all
animals, most
fungi and several
bacteria are obligate aerobes. Most
anaerobic organisms are bacteria. 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.
Examples of Obligate Aerobic Bacteria: Nocardia (Gram positive), Pseudomonas aeruginosa (Gram negative), Mycobacterium tuberculosis (Acid Fast), and Bacillus (Gram positive).
*
Anaerobic organism*
Facultative anaerobic organism*
Fermentation*
Microaerophile
