Ok, so studying our basic glycolysis, TCA cycle and ETC, I realize that the CO2 (we breathe out) is released during TCA cycle from the oxygen that is in the glucose molecule that started glycolysis. This is to say, we often think that the oxygen we inhale gets back into the air in the form of CO2, but is this really true? Because the O2 that we breathe is used in the ETC and gets transformed into water. Right?
I agree. Krebs cycle does not need oxygen. And CO2 is released from Krebs cycle. Therefore oxygen atoms of CO2 are not the oxygen atoms we breathe. Oxygen is used to make water in the electron transport chain. One interesting point would be, where is going this water ?