[Build Advice] Model Anaerobic/Aerboic Breathing?

[Anthony]

Originally Posted by Sunrunners_Fire

Erm. Hydrogen (from water) can be combined with carbon dioxide to produce glucose and oxygen. Where do you think the oxygen you are breathing comes from?

In an energy-negative process powered by sunlight. Doing some calorimetry, C6H12O6 (-1,271 kJ/mol) + 6O2(0) -> 6 CO2 (-393.5 * 6) + 6 H2O (-285.8*6). Left side is -1,271, right side is -4,074.6, so it takes 2,803 kJ/mol to induce this transformation.

[Ulzgoroth]

Originally Posted by Anthony

The breathing CO2 part to exhale oxygen is almost certainly energy-negative even counting in the food.

Yeah, the breathing CO2 is almost certainly energy negative unless they eat some strong oxidizers, but there's nothing precluding getting enough energy out of food to support it the energy-costly conversion.

Originally Posted by Sunrunners_Fire

Erm. Hydrogen (from water) can be combined with carbon dioxide to produce glucose and oxygen. Where do you think the oxygen you are breathing comes from?

That process costs a whole lot of energy. In the best-known example, that whole lot of energy is derived from sunlight.

[Sunrunners_Fire]

Originally Posted by Anthony

In an energy-negative process powered by sunlight. Doing some calorimetry, C6H12O6 (-1,271 kJ/mol) + 6O2(0) -> 6 CO2 (-393.5 * 6) + 6 H2O (-285.8*6). Left side is -1,271, right side is -4,074.6, so it takes 2,803 kJ/mol to induce this transformation.

Point.

Originally Posted by Ulzgoroth

Yeah, the breathing CO2 is almost certainly energy negative unless they eat some strong oxidizers, but there's nothing precluding getting enough energy out of food to support it the energy-costly conversion.

That process costs a whole lot of energy. In the best-known example, that whole lot of energy is derived from sunlight.

To the tune of 100 terrawatts.

[Anthony]

Originally Posted by Ulzgoroth

Yeah, the breathing CO2 is almost certainly energy negative unless they eat some strong oxidizers, but there's nothing precluding getting enough energy out of food to support it the energy-costly conversion.

Well, I can't think of an interaction where it wouldn't be better to just keep its mouth shut and not breathe at all.

[Sunrunners_Fire]

Originally Posted by Anthony

Well, I can't think of an interaction where it wouldn't be better to just keep its mouth shut and not breathe at all.

In this particular case? Accident of evolution. Just because it is better to do something a specific way doesn't necessarily mean that its' going to be done that way.

We've plenty examples of that in the here and now.

[Ulzgoroth]

Originally Posted by Anthony

Well, I can't think of an interaction where it wouldn't be better to just keep its mouth shut and not breathe at all.

Well, if its metabolism is set up so that it really needs to dispose of oxidizing agents and don't have anything better to oxidize than CO2...

But then the oxygen side of the metabolism stops making any sense.

Originally Posted by Sunrunners_Fire

In this particular case? Accident of evolution. Just because it is better to do something a specific way doesn't necessarily mean that its' going to be done that way.

We've plenty examples of that in the here and now.

Accident of evolution is a pretty dubious explanation for coupling your metabolism to an unrelated, costly piece of chemistry.

If it's not unrelated, then you don't have so much of a problem.

[Anthony]

Originally Posted by Sunrunners_Fire

In this particular case? Accident of evolution.

That doesn't work given the described system -- by simply keeping its mouth closed, it can take the oxygen it produced, and recombine it with food to produce energy, instead of exhaling that oxygen. It would not take evolution very long to discover that trick. The big reason to inhale CO2 and exhale oxygen is because you have surplus energy and you want to produce hydrocarbons, either because you can burn them for energy at a later point when your current energy source isn't available (plants consume oxygen at night), or because you're going to use the hydrocarbon for some other purpose. You can also use the oxygen as a convenient way of poisoning anaerobic bacteria, which may be why blue-green algae developed oxygen synthesis, but none of these fit your described creature.

[RyanW]

Originally Posted by Anthony

The big reason to inhale CO2 and exhale oxygen is because you have surplus energy and you want to produce hydrocarbons, either because you can burn them for energy at a later point when your current energy source isn't available (plants consume oxygen at night), or because you're going to use the hydrocarbon for some other purpose.

And plants do both. Plants use cellulose to make... themselves, basically.

[RyanW]

Originally Posted by Ulzgoroth

It's not violating the second law of thermodynamics that I can see. The gas exchange involved doesn't make sense, but the metabolism is burning energy all along the line.

Not violating second law, but there is an important question that suddenly occurred to me:

What is the energy source and chemistry when they are in CO2 -> O2 mode? If they are getting their energy from food like terrestrial animals, it makes little sense to oxidize hydrocarbons to turn carbon dioxide into hydrocarbons and oxygen. Most other practical energy sources I can think of are either too diffuse (plants need huge areas to gather sunlight) or too dangerous (a creature that can tolerate an oxidizer capable of cracking carbon dioxide could drink bleach like it was lemonade) to be biologically likely.

[Ashtagon]

Originally Posted by RyanW

Not violating second law, but there is an important question that suddenly occurred to me:

What is the energy source and chemistry when they are in CO2 -> O2 mode? If they are getting their energy from food like terrestrial animals, it makes little sense to oxidize hydrocarbons to turn carbon dioxide into hydrocarbons and oxygen. Most other practical energy sources I can think of are either too diffuse (plants need huge areas to gather sunlight) or too dangerous (a creature that can tolerate an oxidizer capable of cracking carbon dioxide could drink bleach like it was lemonade) to be biologically likely.

I'm confused on this point too. Afaik, the only natural organic reaction that produces O2 and consumes CO2 is photosynthesis, found in plants (which incidentally also do the O2->CO2 reaction at the same time, but in different areas). I'm not seeing anything that implies these creatures are photsynthesising.

Of course, if this is intended to be a "ignores science" racial feature, carry right on :)