O2 is the final electron acceptor. In the etc o2 accepts the electrons from the proteins and comes together with the H+ in the gradient to form h2o( water). Co2 is released during the Krebs cycle when a molecule looses a carbon atom. For example when isocitrate becomes a- ketigluterate it loses a carbon which turns to co2. This happens again with a-ketogluterate and succinyl-coA. Also co2 is released during fermentation which only happens if oxygen isn’t present. Fermentation happens after glycolysis and the sole purpose of fermentation is to recycle NAD+ to continue glycolysis since there is no oxygen. Also water is moving in and out during Krebs cycle( citric acid cycle).