so I was studying biology today and I went over this weird question that says:

  • The presence of 6 molecules of water in this equation:
  • C6 H12 O6 + 6O2 --> 6CO2 + 6H20, Indicates that the:
  • a) Complete Glycolysis of a glucose molecule (Obviously not correct)
  • b) Occurrence of Krebs cycle Twice
  • c) Complete occurrence of oxidative phosphorylation process
  • d) Storage of energy in NADH and FADH2 molecules

Alright so this is the oxidation of one mole of glucose equation (Without the ATPs) but till now I don't exactly know the correct answer for this question, but to not create any confusion this question is related to the Aerobic respiration (Glycolysis, Krebs Cycle and Electron transport chain).

Here's how I approached this question:

(a) is obviously not correct because the products of glycloysis are 2 pyruvate molecules and 2 ATP molecules so I checked off this choice.

(b) However seems correct because the products of 2 Krebs cycle is 4 CO2 and there is already 2CO2 when the pyruvate acid formed the 2 acetyl CoA molecules so in total that's 6CO2, but still what about the 6 Water molecules?

(c) is a very debating choice because when there is a "Complete occurrence of oxidative phosphorylation process" so that means 2 Krebs cycles had already occurred and formed the 6CO2, and during the oxidative phosphorylation process Water molecules are formed. and ATPs too? I don't exactly know about the ATPs, but aren't they supposed to be in the equation's products in order for this choice to be correct?

(d) This choice indicates to Krebs cycle but the water molecules only are formed during oxidative phosphorylation only.

So basically all the choices seems very debating and confusing and if I were to choose then I'll go with (C) because it's the only choice that makes sense for the water molecules (and the question asks for water), but I want someone to please answer this question with a brief explanation to why he chose this answer,

Thanks :)

  • 3
    $\begingroup$ You have my sympathy. Even for an MCQ this is a really stupid question. Nobody cares about water molecules in biochemical pathways, nor is it generally practical (or at least the practice) to count their production. The purpose is presumably to have you look at the pathways and see where the water is generated. Forget about everything else (i.e. the important stuff). PS You could improve your writing style by not imitating informal speech patterns. "So" is not required to start a sentence, is bad style, and removing superfluous material improves readability. $\endgroup$
    – David
    Nov 13, 2019 at 12:10

2 Answers 2


This reaction only means complete oxidation of glucose to 6 molecules of carbon dioxide and 6 molecules of water.

Reaction presented in question is very generalized, but the presence of six water molecules only means complete cellular respiration. Check out the actual biochemical pathways which take place to oxidize one glucose molecule.

And other options do not represent the complete cellular respiration, so there will not be formation of six water molecules, only option C means complete oxidation of glucose. https://en.wikipedia.org/wiki/Cellular_respiration

Via: https://en.wikipedia.org/wiki/Cellular_respiration

Water molecules shown by solid blue arrows.

During glycolysis, One glucose molecule releases two water molecules. This is the 9th step of glycolytic pathway controlled by Enolase. Two water molecules are produced because two reactions are taking place. enter image description here (Image source: https://johncarlosbaez.wordpress.com/2016/01/08/glycolysis/)

During Citric acid cycle, 2 water molecules are used in two enzymatic reactions performed by Fumarase and Citrate Synthase. And Aconitase performs the isomerisation of citrate to isocitrate so it has no net effect on water molecules. So, 4 water molecules are used here. enter image description here

(Image Source: http://www.biosciencenotes.com/citric-acid-cycle/)

During Electron transport chain, 12 water molecules are produced due to oxidation of NADH (FADH2).

As electrons in this chain are passed, they move from higher to lower energy state, and release energy. Some of the energy is used to transport hydrogen ions across the membrane. This creates electrochemical gradient. And electrons are transferred to molecular oxygen to form water.

This makes net total of 10 water molecules. But 4 water molecules during cellular respiration are used in substrate level phosphorylation. Check out its explanation here (Via : https://onlinelibrary.wiley.com/doi/pdf/10.1016/0307-4412%2895%2900140-9)

enter image description here

(Image Source: https://www.khanacademy.org/science/biology/cellular-respiration-and-fermentation/oxidative-phosphorylation/a/oxidative-phosphorylation-etc)

  • $\begingroup$ Couldn't agree more, thank you for sharing your knowledge! But don't you know anything about wither they should add the ATPs to the equation or not? $\endgroup$
    – Dave
    Nov 13, 2019 at 22:20
  • $\begingroup$ The reaction catalyzed by aconitase doesn’t have any net usage or production of water (it’s an isomerization), which is actually shown in the diagram you used. Ignoring the water produced by ATP synthase, 10 molecules of water are produced by the biological oxidation of glucose, not 6. $\endgroup$
    – canadianer
    Nov 14, 2019 at 22:54
  • $\begingroup$ yes..you are right, then why don't we show 10 water molecules in this reaction. $\endgroup$ Nov 14, 2019 at 23:01
  • 1
    $\begingroup$ @canadianer I tried to improve my answer. Kindly explain me why we only show six water molecules. Thankyou $\endgroup$ Nov 14, 2019 at 23:10
  • 1
    $\begingroup$ Thanks a lot for making it clear $\endgroup$ Nov 14, 2019 at 23:38

Although I tend to agree with David on the validity of the OP example as a good MCQ question, I think that the 'required' answer is (c) Complete occurrence of oxidative phosphorylation process.

However, I think that this answer is potentially flawed (see below) and a more acceptable alternative might be something like

(c) Complete oxidation of glucose via cytochrome oxidase of the respiratory redox chain.

Glucose makes 12 pairs of electrons (all 'held' in C-C and C-H bonds) available to cellular respiration. (See, for example, the letter on TCA cycle confusion by D.E. Atkinson published in TIBS and kindly made available by David).

It takes four electrons to produce 1 water molecule in the cytochrome oxidase reaction. Therefore, if six water molecules are produced, all of 24 electrons 'supplied' by glucose have been passed to water: glucose has been 'completely' oxidized.

The reduction of oxygen to water is a four-electron transfer. A two-electron reduction produces H2O2, and a one-electron reduction produces superoxide. If either of these compounds were produced, or if electrons from glucose were 'diverted' elsewhere (thus never 'arriving' at cytochrome oxidase) the stoichiometry of water production would be less.

However, possibility (c) specifically states "Complete occurrence of oxidative phosphorylation process. This, to me, is confusing and is arguably false. There is no requirement that oxidation and phosphorylation be coupled in order for six water molecules to be produced in the respiratory redox chain. In the presence of (excess) of an uncoupler such as dinitrophenol, no ATP is produced (oxidation and phosphorylation are completely uncoupled) but this in no way precludes all 24 electrons from glucose being passed to oxygen resulting in the formation of 6 water molecules.

For example, if possibility (a) were to state 'complete oxidation of a glucose molecule', then IMO that is by far the best answer (and all others are false or ambiguous).

  • $\begingroup$ Yes exactly, this question is very inaccurate, but I guess I'll just go with (C) as it's the most suitable answer for this question, Thanks for your answer. $\endgroup$
    – Dave
    Nov 15, 2019 at 11:17

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