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According to hyperphysics.edu, and my general knowledge, anaerobic respiration occurs in the cytoplasm. ("Anaerobic respiration (both glycolysis and fermentation) takes place in the fluid portion of the cytoplasm whereas the bulk of the energy yield of aerobic respiration takes place in the mitochondria".) Is there a biochemical advantage to this, or do eukaryotic cells simply lack specialized anaerobic respiratory organelles? I could understand this; few cells use this form of respiration besides muscle cells. But even in muscle cells, it occurs in the cytoplasm. Why? Is it simply taking advantage of fortuitous chemical circumstances, or was it evolutionarily desirable to respire anaerobically in the cytoplasm and aerobically in the mitochondria? I am aware, also, of the endosymbiotic theory of mitochondria.

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  • $\begingroup$ The statement "few use this form of respiration besides muscle cells" is incorrect, as explained in a recent answer of mine to another question. (Quite what the logic of the sentence is remains unclear to me.) $\endgroup$
    – David
    May 18, 2023 at 10:16
  • $\begingroup$ @BryanKrause in this case "we usually remove them eventually" was a rather short timescale in practice, less than 24h after the comment was made. Without access to my comment (it is not in my history either) I cannot easily polish it into an answer. So this approach seems heavy handed and dismissive of my effort, in the name of site policy. If you sincerely thought it should be an answer, you don't need to delete so aggressively. $\endgroup$
    – Ryan
    May 19, 2023 at 21:12
  • $\begingroup$ @Ryan The deleted comments contained barely a single thought, there wasn't much there to preserve, just thought it'd be useful to explain the policy rather than silently removing them. The solution is to not put effort into answering in comments and to use the answer box instead, and use comments only as described in biology.stackexchange.com/help/privileges/comment $\endgroup$
    – Bryan Krause
    May 19, 2023 at 21:29

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The problems here would seem to be (1) a misconception of the biochemistry of anaerobic respiration — strictly anaerobic glycolysis or fermentation (see this Wikipedia article for explanation of the term), (2) not considering the bacteria in which these processes evolved rather than eukaryotic cells, and (3) a failure to regard the lack of organelles as the default situation.

The majority of the reactions of anaerobic fermentation constitute glycolysis, and glycolysis was probably the way in which most bacteria obtained energy before the great oxygenation occurred. The ATP is generated ‘at the substrate level’ — i.e. in the actual reactions — and there is no biochemical reason to think the pathway would need to evolve anywhere other than the bacterial cytoplasm, where the ATP would be available to reactions that need it and the various metabolites could feed in or out to other pathways. The product (lactate in mammals, but also ethanol etc. in bacteria) could easily diffuse out of the cell.

Why on earth should one imagine “specialized anaerobic respiratory organelles”? They are just not required and, for example, would impose an energetic cost for shuttling of metabolites in and out of them, as is the case for mitochondria.

The pertinent biochemical question is not, I think, “Why do eukaryotic cells lack anaerobic respiratory organelles?”, but “Why do the cells need an organelle — the mitochondrion — for aerobic respiration?”. There is a specific answer to this, which is not some vague concept of tidy subdivision of the cell.

This specific answer is, in brief, that aerobic respiration requires a compartmentalized system separated by a membrane so that a pH and charge gradient can be established, to be used subsequently to generate ATP. In Gram-negative bacteria it is the double membrane that provides this; and in eukaryotes it is the successor to the captured bacterium — the mitochondrion.

So there is a good reason for eukaryotic aerobic metabolism to require an organelle, and no reason that I can see for anaerobic metabolism to do so.

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  • $\begingroup$ An exception to this is the glycosome, a membrane-bound organelle containing the glycolytic enzymes. This organelle is present in all kinetoplastidia, and has been very well characterized in (eukaryotic) Trypanosoma brucei (see here, for example). One obvious advantage is that concentration of glycolytic intermediates and glycolytic enzymes are maintained at a higher concentration. From an evolutionary point of view, it is though that glycosomes are descended from peroxisomes $\endgroup$
    – user338907
    May 18, 2023 at 18:42
  • $\begingroup$ @user338907 — Now you mention it I remember attending a seminar on Trypanasomes (there is a group that work on them in my Uni) with mention of some odd organelles. The key question is why they evolved in this particular organism — why does it need to concentrate its glycolytic enzymes and intermediates — and if you can shed further light on that I think it would make a valuable contribution as an additional answer. $\endgroup$
    – David
    May 19, 2023 at 13:12

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