Yeap that’s the thing, before cellular respiration (taking that as the preferred metabolism) nutrients have to be broken down, so in order to keep it simple and clear to the player we could combine the both to make the “full oxidation organ” that just generates (or regenerates) ATP. But yeah all digestion is just a process of oxidation, for example proteins are broken down into amino acids by oxidazing the peptide bond.
In a sentence: “It’s the metabolic process necessary to keep glycolysis working”
I was just thinking about a player’s choice (although unlikely) not to build/have any aerobic respiration protein/organelle throughout the cellular stage, therefore at the macroscopic stage the species would not be able to do aerobic respiration.
Yeah that would do the job, though I must point out that the reactive oxygen species (ROS) are indeed a problem to consider; and too much total lung capacity (TLC - though the name would be change depending on the respiration organ’s name) leads to a lower lactic acid fermentation which ultimately leads into either a fat loss (that’d have tons of really bad physiological consequences) or a protein loss (that’d have physical consequences) as it happens with the sledge dogs, as they do not generate almost any lactic acid during physical exercise so they don’t feel tiredness at a point of oxidasing proteins to produce ATP. Maybe I got a little too far with the explanation, let me know if anything is not clear. I would also like to add that perixisomes exist hahaha.
Sure!! 
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