That's interesting, I've always described metabolism and combustion as the opposite of photosynthesis. You even get back the water that the plant consumed, in t

That's interesting, I've always described metabolism and combustion as the opposite of photosynthesis. You even get back the water that the plant consumed, in the form of steam or condensation dripping out the car's tailpipe, or desert rodents that don't need to drink liquid water because they're so good at keeping the water they metabolize from food! We just use the atmosphere as a temporary place to store the oxygen, and keep it diluted with enough nitrogen that things don't immediately unphotosynthesize themselves. ;)