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I'm not a biologist so pardon any ignorance on my part. I'm working on a speculative evolution project and I'm looking to understand how the partial pressure of oxygen effects the maximum aerobic performance of muscle. This paper gives the maximum aerobic performance of 100 Watts per KG of muscle. This limit is stated to be observed for both insects and vertebrate fliers and is in turn used to calculate the aerobic flight capacities of various animals. The paper also mentions that squamate muscle tissue can achieve 450 W/kg anaerobically.

I have used the equations listed in the linked paper to calculate the size limits of my speculative world's fauna. The only problem is that this assumes an aerobic power output of 100W/kg of muscle. It is my understanding that aerobic respiration in muscle cells is driven by ATP production and that ATP production is dependant on a constant supply of oxygen. It seems logical that the maximum aerobic power output should be higher if there is more oxygen available.

For context, the atmosphere of this speculative world contains 13.5% O2 at a pressure of 12atm. This equates to a partial pressure of oxygen of 1.62 - approximately ~8x higher than our current atmosphere. In these conditions, there is 8 times the amount of oxygen available for respiration. If oxygen availability is the sole factor in ATP production (thus aerobic muscle power), then this means the aerobic power output would net 800W/kg of muscle (assuming the biology of these creatures are the same as known animals). This is almost double what lizards can produce anaerobically which doesn't seem likely.

My question is how would ATP production scale with available oxygen? I assume there comes a point where mitochondria simply cannot process ATP any faster, which would imply a maximum aerobic power output. I have read that some athletes breathe canned air to reduce fatigue. Does more oxygen equate to more power or does it simply mean that muscle cells would not fatigue as quickly? If I've made any mistakes in my reasoning then please correct them. Thanks.

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  • $\begingroup$ Welcome to Biology.SE! Please take the tour and then go through the help pages starting with How to Ask questions effectively on this site and edit your question accordingly. Your "atmospheric composition" isn't for any planet I've ever lived on — please edit your post to use the correct values for Earth or clarify where the values you are using come from and why you are choosing to use them. ——— In addition, each question should be posted separately — this improves the chances that you will get answers for each question and makes the answers more accessible for future users. Thanks! 😊 $\endgroup$ – tyersome Oct 20 '20 at 22:06
  • $\begingroup$ Thanks, I thought the description would be sufficient to explain the purpose but I'll restructure it so it's more concise. I don't think the other questions would serve much purpose individually since I don't think you could answer the title question without them. They're more like parameters wthin the overall question. I'll see if I can condense it a bit further though so thanks. $\endgroup$ – Zac Walton Oct 20 '20 at 22:35
  • $\begingroup$ I've reduced the scope of the question. Hope this makes it clearer. $\endgroup$ – Zac Walton Oct 21 '20 at 0:07
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    $\begingroup$ Carbon dioxide removal may be the limiting factor more than oxygen supply, even on Earth. $\endgroup$ – Polypipe Wrangler Oct 21 '20 at 8:14
  • $\begingroup$ @PolypipeWrangler Do you have any links available? I'd be interested in reading about this. $\endgroup$ – Zac Walton Oct 21 '20 at 13:31
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The factors limiting ATP production are the rate of fuel (glucose, fats, amino acids, etc.) metabolism and its efficiency (ATP molecules per fuel molecule).

It is possible to produce ATP with zero oxygen by the glycolytic pathway; if this were not true, there would be no anaerobic organisms on Earth (which there clearly are). Muscle can do this too for short periods of time, producing lactic acid. In fact, many researchers believe that this pathway can produce ATP the fastest of any route of glucose consumption, even though it is least efficient. In other words, the number of ATP per glucose is small (only 2), but glucose "flows" through this pathway so fast that it is still generated faster than by respiration, which produces 30-32 ATP per glucose, but can process far fewer molecules of glucose per second. The actual rate of ATP generation by mitchondria is only limited by O2 concentration, in a chemical kinetics sense, in hypoxia, i.e. when a tissue does not have adequate blood supply, as in certain tumors.

The main effect of oxygen availability is likely size scaling--if you have more O2, you can get it through a larger body more easily. It's not that the muscle cells themselves generate more energy, it's that you can supply more muscle fibers that are farther from the lungs/gills/whatever. That's why in the carboniferous period on Earth, where there was considerably more O2, many animals (particularly invertebrates) were much larger than they are today.

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  • $\begingroup$ Thank you so much! This makes a lot of sense. Just one follow up-question: You said metabolism is a limiting factor, does this mean that metabolic rate effects how quickly ATP can be produced aerobically? I'm using flying organisms as a case study as powered flight can only be sustained aerobically (otherwise it would transition to gliding/soaring). This is why I'm looking specifically at aerobic respiration, if that makes sense. Without throwing too much in the way of speculation, the speculative animals have photosynthetic capabilities so gluscose may also be more abundant. $\endgroup$ – Zac Walton Oct 23 '20 at 14:26
  • $\begingroup$ Numbers of mitochondria per cell change in response to conditions also. $\endgroup$ – Polypipe Wrangler Oct 28 '20 at 10:36
  • $\begingroup$ "Metabolic rate" usually refers to the rate of nutrient consumption in an organism as a whole, i.e. how much of the tissues (particularly muscle) are active vs. resting. The bottleneck I'm referring to concerns the rates of the enzymes themselves--no matter how much glucose (or oxygen) is available to a cell, there are only so many molecules the enzymes in a given mitchondrion can react with per second. The enzymes that break down glucose without oxygen are faster in this regard, considering how many can fit in the same volume. $\endgroup$ – biohacker Oct 30 '20 at 5:22

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