Are humans capable of both anaerobic respiration, and lactic acid fermentation? And if so, when do they do each? I understand that the difference between respiration and fermentation is that respiration takes place in the electron transport chain.. I understand that aerobic respiration uses oxygen in the electron transport chain, and anaerobic respiration uses some other molecule in the electron transport chain instead of oxygen, like nitrate. Whereas fermentation doesn't use the electron transport chain at all. I know humans have an anaerobic metabolic process that produces lactic acid, but i'm not clear whether it's respiration, or fermentation, or whether it could be either, in which case when it is which? **Added** I think the answer might depend on definition.. Certainly humans don't do metabolizing involving the combination of electron transport chain + anaerobically. So sources that state the respiration involves the electron transport chain, will say that humans do not do anaerobic respiration. Other sources, that often seem to be more high school level, use a definition of respiration and fermentation that they don't specify clearly, but where anaerobic respiration and fermentation are treated like the same thing (though perhaps when given the choice of saying anaerobic respiration or fermentation, i.e. those two anaerobic metabolic processes, they use the term anaerobic respiration more in the context of animals or plants, and fermentation more in the context of non-animals e.g. yeast). Much of this question hangs on the question of what definition the high level biology books use for respiration and fermentation. Do they just use the "electron transport chain" distinction definition. Or do some use the electron transport chain definition and others a more general definition. And what is the more general definition. Some sources that state that respiration uses the electron transport chain (this link says that anaerobic respiration uses the electron transport chain - it's taking the position that respiration uses the electron transport chain) https://en.wikipedia.org/wiki/Anaerobic_respiration "Anaerobic respiration is a form of respiration using electron acceptors other than oxygen....Anaerobic respiration is used mainly by bacteria and archaea " (this link says fermentation is not respiration - it's taking the position that since respiration involves the electron transport chain and fermentation does not) https://en.wikipedia.org/wiki/Cellular_respiration "Without oxygen, pyruvate (pyruvic acid) is not metabolized by cellular respiration but undergoes a process of fermentation" https://www.reference.com/science/difference-between-fermentation-anaerobic-respiration-21e854610087c289# "Anaerobic respiration produces more energy than fermentation" https://www.khanacademy.org/science/biology/cellular-respiration-and-fermentation/variations-on-cellular-respiration/a/fermentation-and-anaerobic-respiration "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 sulfate, as a final electron acceptor for an electron transport chain. This process, called anaerobic cellular respiration, is performed by some bacteria and archaea" Those sources above are all pretty clear. Fermentation is not respiration. And so humans do not do anaerobic respiration.. including muscle cells.. which though they can metabolize anaerobically, it's fermentation, not respiration. Other sources though don't seem to distinguish fermentation and respiration clearly, and include fermentation as anaerobic respiration. This webpage seems to contradict itself https://www.boundless.com/biology/textbooks/boundless-biology-textbook/cellular-respiration-7/metabolism-without-oxygen-77/anaerobic-cellular-respiration-365-11591/ "Eukaryotes can also undergo anaerobic respiration. Some examples include alcohol fermentation in yeast and lactic acid fermentation in mammals." (so there it's stating that fermentation - which doesn't involve the electron transport chain, is a form of respiration).. but it also says "Anaerobic respiration is a type of respiration where oxygen is not used; instead, organic or inorganic molecules are used as final electron acceptors" (which would mean that fermentation isn't respiration). http://www.diffen.com/difference/Aerobic_Respiration_vs_Anaerobic_Respiration "Cellular respiration also occurs outside of macro-organisms, as chemical processes — for example, in fermentation." http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/respiration/respirationrev4.shtml (describes the conversion of glucose to lactic acid + energy) but doesn't call it fermentation, just calls it anaerobic respiration. So it takes the position that humans do anaerobic respiration.. It defines respiration quite generally. And when it talks of fermentation it doesn't talk of it in the context of a human/animal http://www.bbc.co.uk/schools/gcsebitesize/science/edexcel_pre_2011/designerproducts/foodanddrinkrev1.shtml it talks of it in the context of beer and wine. The highly voted answer by Roland did say "Humans have no anaerobic respiration" However, Roland then undermined that a little bit by stating that it's just his preference. He commented to say "I think these terms are defined somewhat differently depending on who you ask... 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" David has made the important point that definitions aren't so fundamentally important, and one can be rigorous in defining the concepts that interest us.. Is it an animal or not, does it involve oxygen or not, does it involve the electron transport chain or not.. and finding an appropriate term. He wouldn't then push his definition on anybody else, as they wouldn't always conform to any particular standard definition. I'm more interested in what the definition used in peer reviewed biology journals is. If a general definition of anaerobic respiration (to include fermentation), is ever used or is commonly used. I'd like to know more how biologists use the terms when they write in journals. Or in the high level specialist textbookx that they use. A particular person's reasoned preferences are interesting, but more secondary to me.