How ATP works to power cells and neurons and muscles in a human body (or likely all mammalian bodies too like in animals)

 They change fuel types: Just like a plant burns coal or gas to make electricity, mitochondria burn food and oxygen to make cellular power.

end partial quote.

So, I'm thinking the bi-product of all this might be partly carbon dioxide that we breathe out and then we are symbiotic with trees who take carbon dioxide and re-convert it back to oxygen for us to breathe.

So, this is why trees and humans are symbiotic too. Because we each give the other part of what we need to survive.