Submitted by 23Silicon t3_yq9ctu in askscience
As far as I remember, the oxygen we breathe in becomes the final electron acceptor on the electron transport chain during cell respiration, and is released as water due to the electrons it accepts or something like that. I thought the CO2 came from the pyruvate decarboxylation step immediately after glycolysis. Doesn’t this mean that the CO2 we breathe out is converted from what we eat, not what we breathe?
If you think about it, the food we eat mostly ranges from glucose (CH*2O)n* to fatty acids (empirical formula CH*3(CH2)nCOOH. None of this has enough oxygen to get converted to CO2* without additional oxygen from somewhere.
It's true that it does not come directly from atmospheric oxygen, but indirectly it does. The atoms basically all get mixed around. For example, in glucose metabolism, oxygen reacts with reduced cytochrome c and H^+ to become water. Water hydrolyzes various phosphorylated substances to produce ADP and phosphate. The water and phosphate will donate this oxygen back in subsequent steps.
In glycolysis, two of the sugar's 6 oxygen atoms are eventually replaced by phosphate along the route to forming pyruvate. During decarboxylation of pyruvate, one of the lost oxygens is original to the sugar, while the other is the newly added one. So for the CO*2* molecule formed by pyruvate decarboxylation, it's half-and-half. One of the oxygens came from phosphate. The other came from the sugar.
In the citric acid cycle, another two CO*2* are produced by successive decarboxylations of isocitrate. Of course, it's a cycle, so more oxygen atoms have to be added back in. Of these, two come from water, one from phosphate, and one from pyruvate (originally glucose). So again, some of the oxygen in the CO*2* produced in the citric acid cycle comes originally from the sugar, but the majority comes from phosphate and water.
There's also eg. beta oxidation of fatty acids. That cyclically generates a double bond, hydrates it, and cleaves off, until the end of the carbon chain. The hydration can use water that came from the electron transport chain, so oxygen that was initially reduced to water can still end up in CO2.
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The carbon in the CO2 does come from food. The oxygen comes from... well, it varies, it could be food, or water you've drunk, or O2 you've breathed and that's been converted into water.
The overall reaction is (for glucose):
⅙ C6H12O6 + O2 → CO2 + H2O
but this overall reaction actually spans a large number of molecular steps. there isn't a single step where CO2 and O2 both appear
this is a similar reaction as for the burning of wood or fossil fuels. O2 reacts with reduced carbon to form CO2
Bear in mind a lot of catabolic pathways hydrate double bonds, and that water can in principle come from the electron transport chain. So that oxygen ends up attached to carbon, and can end up in CO2.
Also pyruvate decarboxylation (by pyruvate dehydrogenase) is not the only place CO2 comes from. The citric acid cycle releases two equivalents of CO2 for each equivalent of acetyl-CoA added. Pyruvate decarboxylation releases one equivalent of CO2 for each equivalent of acetyl-CoA generated, so two thirds of the CO2 comes from the citric acid cycle here (makes sense, since pyruvate is a 3-carbon unit and only one carbon is initially lost as CO2).
This is kind of similar to the reverse process in plants where CO2 is taken up and O2 is released. The oxygen in the O2 molecules is not derived from the CO2 taken up for photosynthesis, but instead from the H2O molecules that are also involved in the photosynthesis process. The oxygen molecules in the CO2 molecules are mainly used for the synthesis of sugars, while the photosynthesis process involves water being split into separate hydrogen and oxygen ions. The hydrogen ions are used as part of the process for splitting the CO2 into carbon and oxygen, while the oxygen ions are released from the plant. The origin of the released O2 from the H2O instead of the CO2 has been determined through controlled experiments where the inbound H2O and CO2 contained a different isotope of oxygen.
Break it down to a chemical equation that demonstrates the difference. C + O2 ----> (1)CO2. Hence we are adding a carbon atom to an O2 molecule, such that the body is emitting carbon bound to O2. In reality when CO2 remains disolved in blood for too long it converts to H2CO3 - carbonic acid. This is why EMS used to give sodium bicarb IV to hypercarbic patients particularly in the setting of cardiac arrest.
marieterna t1_ivrp2pz wrote
Think about it, we inhale O2 to be that terminal electron acceptor. We expel CO2 as a waste product of cellular respiration. So indirectly, we expel CO2 as waste from what we breathe, since we need O2 for cellular respiration. Also, what they may be referring to is gas exchange.