Anaerobic Respiration

Paul Andersen explains the process of anaerobic respiration. This process involves glycolysis and fermentation and allows organisms to survive without oxygen. Lactic acid fermentation is used in animals and bacteria and uses lactate as an electron acceptor. Alcoholic fermentation used ethyl alcohol as an electron acceptor.

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[TonyTigerTonyTiger]

Nope. Fermentation is not anaerobic respiration, and anaerobic respiration is not fermentation: they are two very different processes.

Respiration - both aerobic and anaerobic - involves a "respiratory chain" – what is more usually referred to these days as an electron transport chain or electron transport system. The respiratory chain uses energy released by the flow of electrons from one carrier to another in a membrane in order to pump hydrogen ions unidirectionally across the membrane to form an electrochemical proton gradient. In fact, the protein complexes in the electron transport system were originally called “respiratory complexes”, and still are by many sources. The protons then flow back across the membrane through ATP synthase, which allows the enzyme to form ATP from ADP + Pi.

The key difference between aerobic respiration and anaerobic respiration is that in aerobic respiration the electron transport system's terminal electron acceptor is oxygen, whereas in anaerobic respiration the terminal electron acceptor of the electron transport chain is something other than oxygen, such as nitrate, nitrite, fumarate, DMSO, etc.

Fermentation does not use a respiratory chain or respiratory complexes, does not generate an electrochemical proton gradient across a membrane, and does not use ATP synthase to make ATP; so it is not a form of respiration. Fermentation and anaerobic respiration are two entirely different processes.


“Anaerobic Respiration
Under anoxic conditions, electron acceptors other than oxygen support respiration in certain prokaryotes. This is called anaerobic respiration. … As in aerobic respiration, anaerobic respirations require electron transport, generate a proton motive force, and employ ATP [synthase] to make ATP (Sections 3.10 – 3.12).”
(Brock Biology of Microorganisms: Fourteenth Edition, Michael T. Madigan, et al., Pearson Education, Inc., 2015, p95, 96)

“Cellular respiration, or simply respiration, is defined as an ATP-generating process in which molecules are oxidized and the final electron acceptor is (almost always) an inorganic molecule. An essential feature of respiration is the operation of an electron transport chain.

There are two types of respiration … In aerobic respiration, the final electron acceptor is O2; in anaerobic respiration, the final electron acceptor is an inorganic molecule other than O2 or, rarely, an organic molecule.”
(Microbiology: An Introduction. 9th Edition. Gerard Tortora, Berdell Funke, and Christine Case. Pearson/Benjamin Cummings. 2007. p129)

“IV Anaerobic Respirations
We examined the process of aerobic respiration in Chapter 3. As we noted there, O2 functions as the terminal electron acceptor, accepting electrons that have traversed an electron transport chain. However, we also noted that other electron acceptors can be used instead of O2, in which case the process is called anaerobic respiration. Here we consider these reactions in more detail.

13.16 Principles of Anaerobic Respiration
Bacteria that carry out anaerobic respiration have electron transport chains containing the typical electron transport proteins that we have seen in aerobic respiration, photosynthesis, and chemolithotrophy – cytochromes, quinones, iron-sulfur proteins, and the like.”
(Brock Biology of Microorganisms: Fourteenth Edition, Michael T Madigan, et al., Pearson, 2015, p410)

“ANAEROBIC RESPIRATION
In anaerobic respiration, the final electron acceptor is an inorganic substance other than oxygen (O2). Some bacteria, such as Pseudomonas and Bacillus, can use a nitrate ion (NO3-) as a final electron acceptor; the nitrate ion is reduced to a nitrite ion (NO2-), nitrous oxide (N2O), or nitrogen gas (N2). Other bacteria, such as Desulfovibrio (de-sul-fo-vib-re-o), use sulfate (SO42-) as the final electron acceptor to form hydrogen sulfide (H2S). Still other bacteria use carbonate (CO32-) to form methane (CH4). Anaerobic respiration by bacteria using nitrate and sulfate as final electron acceptors is essential for the nitrogen and sulfur cycles that occur in nature. The amount of ATP generated in anaerobic respiration varies with the organism and the pathway. Because only part of the Krebs cycle operates under anaerobic conditions, and since not all the carriers in the electron transport chain participate in anaerobic respiration, the ATP yield is never as high as in aerobic respiration.”
(Microbiology: An Introduction. 9th Edition. Gerard Tortora, Berdell Funke, and Christine Case. Pearson/Benjamin Cummings. 2007. p134)

“Anaerobic Respiration …
Fermentation and aerobic respiration occur in prokaryotes and eukaryotes. Additionally, some prokaryotes have a variation of aerobic respiration called anaerobic respiration, by which they synthesize ATP. This process … is similar to aerobic respiration; the major exception is that the terminal electron acceptor in the electron transport chain is a chemical compound other than molecular oxygen. A wide variety of substances can serve as alternate electron acceptors to oxygen (Table 6.5) [which listed nitrate, nitrite, sulfate, and fumarate].”
(Microbiology: 3rd Edition, Daniel Lim, Kendall/Hunt Publishing Company, 2002, p197)

“An important feature of ATP production from the breakdown of nutrient fuels into CO2 and H2O (see Figure 12-1, top) is a set of reactions, called respiration, involving a series of oxidation and reduction reactions called an electron-transport chain. The combination of these reactions with phosphorylation of ADP to form ATP is called oxidative phosphorylation and occurs in mitochondria in nearly all eukaryotic cells. When oxygen is available and is used as the final recipient of the electrons transported via the electron-transport chain, the respiratory process that converts nutrient energy into ATP is called aerobic oxidation or aerobic respiration. Aerobic respiration is an especially efficient way to maximize the conversion of nutrient energy into ATP because O2 is a relatively strong oxidant. If some molecule other than O2 – for example, the weaker oxidants sulfate (SO42-) or nitrate (NO3-) – is the final recipient of the electrons in the electron-transport chain, the process is called anaerobic respiration. Anaerobic respiration is typical of some prokaryotic microorganisms.”
(Molecular Cell Biology: Eighth Edition. Lodish, Berk, Kaiser, Krieger, Bretscher, Ploegh, Amon, and Martin. W. H. Freeman. 2016. p515)

“The last few sections have talked extensively about aerobic respiration. What defines it as aerobic is its use of oxygen as the terminal electron accepter. Since this is very similar to the type of respiration that humans use, our bias is obvious. Now let me fill you in on a little secret. Microbes are capable of using all sorts of other terminal electron accepters [of the electron transport chain] besides oxygen. Below we talk about a few examples of anaerobic respiration. The one thing that they all have in common is the use of an electron transport system in a membrane and the synthesis of ATP via ATP synthase. In both nitrate reduction and sulfate reduction there are two types of pathways, assimilatory and dissimilatory. …
Nitrate reduction
Some microbes are capable of using nitrate as their terminal electron accepter. The ETS used is somewhat similar to aerobic respiration, but the terminal electron transport protein donates its electrons to nitrate instead of oxygen. Nitrate reduction in some species (the best studied being E. coli) is a two electron transfer where nitrate is reduced to nitrite. Electrons flow through the quinone pool and the cytochrome b/c1 complex and then nitrate reductase resulting in the transport of protons across the membrane as discussed earlier for aerobic respiration.
N03- + 2e- + 2H+ -> N02-+ H20
Figure 1 - The reaction for nitrate reduction. N03-, nitrate; N02-, nitrite
This reaction is not particularly efficient. Nitrate does not as willingly accept electrons when compared to oxygen and the potential energy gain from reducing nitrate is less. If microbes have a choice, they will use oxygen instead of nitrate, but in environments where oxygen is limiting and nitrate is plentiful, nitrate reduction takes place.”
(©2000 Timothy Paustian, University of Wisconsin-Madison
retrieved 05/06/2018)