Most organisms need oxygen to survive. They use it to metabolize sugars, proteins and lipids, but they also suffer oxidative damage, or breakdown of DNA and other cell components, if they don’t get rid of it fast enough. That is why some foods are touted as healthy because they are rich in antioxidants. When it comes to how organisms deal with this tricky element there are two camps, the aerobes and the anaerobes.
The word aerobic comes from Greek and roughly means “with air.” Aerobic organisms need oxygen to survive. All cells can get a little energy through a process called glycolysis, during which sugars are broken down. Aerobic respiration is a much more efficient way to produce energy. Aerobic organisms use oxygen in the cellular processes called the Kreb’s cycle and electron transport chain to produce 36 ATP, the molecules that transport energy within the cell. The organisms then get rid of the oxygen after they are done, usually by attaching it to a carbon atom and exporting it as carbon dioxide.
Anaerobic organisms do not use oxygen to help them break down food into energy. They use either fermentation or a process called anaerobic respiration. Anaerobic respiration is limited to glycolysis, which produces 2 ATP to fuel the cell. Fermentation can be designated either alcohol or lactic acid, depending on the by-product that they produce. It is one step beyond glycolysis and produces another 2 ATP, for a total of 4 ATP. Since these processes produce energy so much less efficiently than aerobic processes, anaerobic organisms are usually only found in environments where oxygen isn't available.
Facultative Anaerobic Organisms
Facultative anaerobes are organisms that can survive both with and without oxygen. They are aerobic when they have access to oxygen, but when they are in environments without oxygen, they switch to anaerobic respiration or fermentation. Yeast is a good example of a facultative anaerobe. In the presence of oxygen they produce carbon dioxide, which makes bread rise, but when they are deprived of oxygen, they resort to the fermentation, which produces alcohol in wines and beer.
Aerobic organisms include almost all plants and animals and many protozoa, fungi and bacteria. Anaerobic organisms are mostly microscopic, like bacteria and protozoa, but some more complex organisms that exist in harsh environments are anaerobic. An example would be a small organism called Loricifera that was recently found on the sea floor under very salty brine.
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