I mentioned in comment quite early "apparently humans don't do anaerobic respiration at all. They only do a)aerobic respiration and b)fermentation. Humans can't use nitrate or sulfite as acceptors in the electron transport chain"
I'll expand on that a bit.. https://www.khanacademy.org/science/biology/cellular-respiration-and-fermentation/variations-on-cellular-respiration/a/fermentation-and-anaerobic-respiration
"Anaerobic cellular respiration is similar to aerobic cellular respiration in that electrons extracted from a fuel molecule are passed through an electron transport chain, driving ATP synthesis. Some organisms use sulfate as the final electron acceptor at the end of the transport chain, while others use nitrate, sulfur, or one of a variety of other molecules..
What kinds of organisms use anaerobic cellular respiration? Some prokaryotes—bacteria and archaea—that live in low-oxygen environments rely on anaerobic respiration to break down fuels." (i.e. not humans)
and "Fermentation is a widespread pathway, but it is not the only way to get energy from fuels anaerobically (in the absence of oxygen). Some living systems instead use an inorganic molecule other than O2, such as sulfate, as a final electron acceptor for an electron transport chain. This process, called anaerobic cellular respiration, is performed by some bacteria and archaea."
Humans aren't able to use sulfate or nitrate(or any inorganic molecule other than o2) in the electron transport chain. So they can't do anaerobic respiration.
That is certainly the point made by khanacademy, and made by another microbiologist I spoke to, and echoed by Roland's answer.
Note- as of writing, Roland has undermined his answer somewhat.. Since the first sentence of his answer says "Humans have no anaerobic respiration" But a comment he made, says "I think these terms are defined somewhat differently depending on who you ask, and I agree with @David that it's not very interesting to argue over definitions. But my own preference would be to use the term "anaerobic respiration" only for oxidation of substrates that delivers electrons to a terminal electron acceptor other than O2, and "fermentation" to mean a process that does not result in any net oxidation. This is a major biochemical difference, so it motivates different terms, imho. With these definitions, muscle does not do anaerobic respiration"
i.e. he's saying the definition is only his preference. And that while it has good reason behind it, it's just still his preference. And so it's a question of some peoples' preferred definitions, over others.
I have spoken to a microbiologist who says all the advanced scientific texts use that definition (introductory texts aside, and human biology tests aside), i.e. it's not just some peoples' preference. I have added further re that - And what I have found strong evidence/proof that supports that there is a strict distinction made, in the non-introductory microbiology texts. He is probably correct that scientific journals such as PNAS, Nature and Science, make the distinction too.
Added
The microbiologist that I spoke to said that often biology 101 texts get it wrong so e.g. that boundless biology textbook, even the title gives away that it's a very intro level book. So no surprise that it gets it wrong. He said often introductory undergraduate books get it wrong. And books on human biology, since they don't need to distinguish between types of anaerobic metabolising, can get it wrong sometimes, because they don't need as much clarity as they aren't needing to distinguish between anaerobic processes. But the science journals such as PNAS, Science, Nature. And the advanced microbiology texts get it right. Sure it's possible for a person to use a very general definition of respiration and then to ask whether the electron transport chain is used, but the microbiology texts have no need for that question to even be asked, since they define their terms to make that distinction.
If one goes to google books and searches microbiology, I see he is correct.
For example, googling in google books for microbiology fermentation respiration
The first result is introductory so I won't look at it.
The second result is Microbiology & Plant Pathology by Dr. P.D. Sharma
"heterotrophs exhibit two basic strategies the fermentation and respiration"
"In contrast to fermentation, respiration requires an external electron acceptor"
agrees
The third result distinguishes but is by the same author so i'll skip it.
agrees
The fourth result - Principles of Microbiology - Page 530
"A comparison of aerobic respiration, anaerobic respiration, and fermentation"
And it distinguishes
agrees
The fifth result "Microbiology: A Clinical Approach, Second Edition: - Page 39" here
has a diagram that distinguishes
agrees
The sixth result Alcamo's Fundamentals of Microbiology - Page 183
"a different inorganic molecule as a final electron acceptor"
note- can't be pyruvate as final one, as pyruvate contains carbon so is organic.
He then gives the examples of a species of bacteria that uses nitrate and a species that uses sulfate
agrees
It is indeed the case that fermentation doesn't involve an electron transport chain, but also, fermentation does involve a final electron acceptor, just it's pyruvate or a pyruvate derivative.
Microbiology By Cynthia Nau Cornelissen, Richard A. Harvey, Bruce D. Fisher here
"the terminal electron acceptor in fermentation, is pyruvate or a pyruvate derivative"
And as the microbiologist I spoke to said, it's more a question of whether the final electron acceptor is internal/endogenous (i.e. produced by the organism itself), or whether it's external. (rather than whether it's organic/inorganic)
One thing boundless biology says (using the strict definition of fermentation and respiration), and consistent with that. https://www.boundless.com/microbiology/textbooks/boundless-microbiology-textbook/microbial-metabolism-5/anaerobic-respiration-49/electron-donors-and-acceptors-in-anaerobic-respiration-313-8023/
" Both inorganic and organic compounds may be used as electron acceptors in anaerobic respiration. Inorganic compounds include sulfate (SO42-), nitrate (NO3-), and ferric iron (Fe3+). Organic compounds include DMSO."
(DMSO for example is organic/carbon based, but it's an external electron acceptor, thus, still respiration when it's used)
related chat- https://web.archive.org/web/20210507000853/https://pastebin.com/raw/mkxckeqA
Conclusion-
That and all those definitions clearly distinguish respiration and fermentation, with the same strict definition, that confirms that humans do not do anaerobic respiration, they do fermentation, which is not a form of respiration.
Nevertheless, despite that, it's clearly the case that outside of advanced microbiology texts.. (And I suppose outside of scientific journals), so in introductory biology texts, and in human biology texts, and high school texts.. and among many on this website who clearly have a deep interest in biology, that general definition of respiration is still around. And in such a situation, it makes sense to pay less attention to the term but to ask regarding specific features e.g. "Does it involve the electron transport chain" / Does it use an inorganic compound such as nitrate or sulfate as an alternative to O2? if one wants to know if it's respiration, Or if one doesn't care what the term is and just wants to know what the features are so may ask that and further questions like "Does it use oxygen", or features that aren't even relevant to the term, such as what interested in commenter in how they think of the terms - "Is it being done by an animal, or not(so directly from the environment to the cell)".
Further conclusion
further clarification on the distinction..
When I asked this question to the microbiologist-
I understand respiration is usually done with an inorganic substance (O2) as terminal electron acceptor, but that it can be done with an organic substance DMSO. So, is there still an enormous chemical difference between respiration and fermentation, when respiration is done with DMSO? I don't see how the final electron acceptor being internal vs being external, would cause a major different in the chemistry.
I got a very good answer-
He said - Fermentation doesn't use a proton gradient and chemiosmotic force to generate ATP. Oxidative phosphorylation vs substrate-level phosphorylation
Looking further.. I see Glycolysis produces ATP via substrate-level phosphorylation. Looking at the distinct parts of respiration and fermentation i.e. after glycolysis. (some define respiration and fermentation as excluding glycolysis anyway). Then, (after glycolysis), Respiration uses oxidative phosphorylation. Fermentation, if it produces ATP (which it usually does), then it is done via substrate-level phosphorylation.
(Physiology and Biochemstiry of Prokaryotes https://i.sstatic.net/PkRw9.png Taking pyruvate to acetate gets you oATP. Taking pyruvate to propionate does not. It's Fermentation either way. If it produces ATP it's substrate level phosphorylation, and if it doesn't then there's no phosphorylation)
Many sources online explain how a)those two types of phosphorylation are very different chemical processes and b)oxidative phosphorylation uses the electron transport chain whereas substrate level phosphorylation doesn't. So that explains the significance/relevance/reason of the distinction between fermentation and respiration.
Added important note
An exception to this is oxidative fermentation, which does use the electron transport chain. Apparently part of the metabolic pathway doesn't use the ETC and part of it does, in such a way that it's in some way a mixture of respiration and fermentation and they decided to call it fermentation. Also see Do acetic acid bacteria use the electron transport chain when converting ethanol to acetic acid?
