[eli5] Humans and most animals breathe in O2(dioxide) and breathe out CO2(carbon dioxide) , where does the carbon come from?

Question

[eli5] Humans and most animals breathe in O2(dioxide) and breathe out CO2(carbon dioxide) , where does the carbon come from?

In: Biology

Answer 1

Blood sugar that you’ve burned. The complete reaction goes…

C6H12O6 + 6O2 –> 6CO2 + 6H2O + 2.8MJ of energy

So glucose (produced from food you’ve digested) reacts with oxygen (which you inhaled) and the products are energy, carbon dioxide (slightly poisonous; you exhale that) and water (useful, mostly your cells hang onto it but you can pee it out if you have excess).

[Edit: I wrote the reaction in *moles*, so we’re talking 1 mole (180.18g) of glucose to produce 2.8 megajoules.]