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Why do aerobic organisms need oxygen?
In aerobic organisms, oxygen is required as a terminal electron receptor. Whatdoes this mean? Aerobic metabolism (glycolysis and the TCA cycle) generatesreduced molecules (NADH and succinate). These are reoxidised by membrane-boundcomplexes. In eukaryotes, these complexes are found in the membranes ofmitochondria. In prokaryotes, functionally-equivalent complexes are located inthe plasma membrane. During the reoxidation reactions, electrons pass throughthe respiratory chain and protons are pumped across the membrane. Thisgenerates a which is used to synthesise ATP. Sowhat happens to those electrons that are removed from NADH/succinate? Theyneed a sink - and that sink is oxygen, which is reduced to water by thecytochrome oxidase complex.
Take a look at . It summarises the process that Idescribed - oxidative phosphorylation. You can see oxygen being reduced towater by Complex IV, cytochrome oxidase, the second from right complex.
The beauty of the chemiosmotic process is that different organisms use manyvariations of it, yet the fundamentals are the same: (1) small reducedmolecules are reoxidised by membrane bound complexes, (2) protons (or sometimesother ions) are used to generated an electrochemical gradient for ATP synthesisand (3) a terminal electron acceptor acts as an electron sink and is reduced.
Let's look at an aerobic microorganism. On the KEGG webpage, go to the selectbox at the top left and scroll down until you find an organism called, then click the "Go" button. A new diagram willbe drawn. The genes shaded light green are those that encode the respiratorycomplexes of this organism. You'll see that many of them are present,including those of cytochrome oxidase. In fact, an alpha-proteobacterium verylike is thought to be the ancestor of the eukaryoticmitochondrion ().
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How do anaerobic organisms live without oxygen?
In the absence of oxygen as a terminal electron acceptor, organisms have 2choices. First, they can use glycolysis to ferment glucose to an oxidisedcompound such as ethanol or lactate. This generates a little ATP which may beenough for survival. However, many microorganisms are capable of usingmolecules other than oxygen as a terminal electron acceptor.
If you go back to the KEGG webpage and select the organism , you'll see that it can synthesise an aerobic respiratory chainwith oxygen as terminal electron receptor, like the one we saw earlier. However, if you grow without oxygen and give it somenitrate, it will grow quite happily. It and many other bacteria are capable ofdenitrification - the use of nitrate as a terminal electron receptor instead ofoxygen. This type of metabolism is explained in detail . Scroll down the page and you'll find a diagram ofthe respiratory chain used for denitrification. You'll see that it's similarto that used for aerobic growth, except that the enzymes are used to reducenitrate to nitrite, then nitric oxide, then nitrous oxide and ultimatelynitrogen gas.
That also answers the question...
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...why are some anaerobes facultative and others obligate?
The answer is - because facultative anaerobes can synthesise an aerobicrespiratory chain or, in the absence of oxygen, rely on either (1) fermentationor (2) an alternative respiratory chain that uses a different terminal electronacceptor, such as nitrate.