In the photosynthesis equation:

$$\ce{6CO2 + 6H2O ->[sunlight] C6H12O6 + 6O2}$$

The only place where we have 6 molecules of $\ce{O2}$ is in $\ce{6CO2}$. Then it reacts with $\ce{6H2O}$ to form $\ce{C6H12O6}$ and $\ce{6O2}$ that apparently comes from $\ce{CO2}$. So why do we say that the $\ce{O2}$ produced by plants comes from $\ce{H2O}$ and not $\ce{CO2}$? I don't know if I'm the one who's understanding something wrong or is it the photosynthesis formula which is wrong?

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    $\begingroup$ go through this howplantswork.com/2009/02/16/plants-dont-convert-co2-into-o2 $\endgroup$ Commented Jun 6, 2014 at 10:22
  • $\begingroup$ @The Last Word - great link. $\endgroup$
    – Alan Boyd
    Commented Jun 6, 2014 at 14:32
  • $\begingroup$ and it's still like this after the edit? $\endgroup$
    – hello all
    Commented Aug 21, 2014 at 2:23
  • $\begingroup$ It's funny that the only answer that actually addresses OP misunderstanding, i.e., the difference between the simplified and non-simplified equations, is the one with less upvotes. Again, it's not the only correct answer, but it's the only one that shows the complete (non simplified) equation. In my classroom experience, showing the non simplified equation to the students really help them to better understand that the oxygen comes from the water. $\endgroup$
    – user24284
    Commented May 3, 2017 at 5:45
  • $\begingroup$ Yeah, but the water might have come from mitochondria that devoured HCOH which came from CO2. $\endgroup$ Commented Feb 17, 2022 at 17:49

4 Answers 4


You are missing some knowledge here for sure, photosynthesis is a little complicated at A level, so I will describe it in brief.

During photosynthesis electrons and protons (A hydrogen atom without the electron) are required for a process called the electron transport chain and proton motive force. This happens during the light dependent stage of photosynthesis, (there is also a second light-independent stage called the Calvin cycle, and that is where the CO$_2$ is used), I won't go into detail about what the protons and electrons do (unless you want me to) but you need to know that these come from a water molecule, the water is split using light (photolysis, literally: cutting with light) into two hydrogens and half an oxygen molecule (or an oxygen atom). The oxygen that was released in photolysis is not required for the rest of the pathway, so it diffuses out of the cell.

For why it doesn't come from carbon dioxide, you need to consider the Calvin cycle. In the Calvin cycle, carbon dioxide is converted to glucose by enzymes, using products from the light dependent stage, so 6CO$_2$ are combined over six cycles to form one molecule of glucose. So that is where the CO$_2$ is used as well.

Hope that helped! If you want me to go into any more detail please ask!


Photosynthesis uses chlorophyll (or other pigments) for harnessing photons and water (or other compounds) as electron donor $H_2O = 1/2O_2 + 2H^+ + 2e^-$. After splitting the water it sends the electrons through the further steps of an electron transport chain and at the end it reduces $NADP^+$ into $NADPH$. Meanwhile it increases the $H^+$ concentration outside of a membrane, so it creates a $H^+$ gradient. This gradient can be used to synthesize $ATP$ from $ADP$ using a $H^+$ pump. After that $NADPH$ and $ATP$ can be used to fix $CO_2$ at any time (so that part of the photosynthesis is not light dependent).

Light-dependent reactions of photosynthesis

  • Figure 1 - Light-dependent reactions of photosynthesis - electron transport chain - source

Splitting $CO_2$ into $CO$ and $1/2O2$ is possible as well. It can be done enzymatically or due to photolysis. You need similar energy to do that as by water splitting. After that $CO$ can be used e.g. by shift reaction $CO + H_2O = CO_2 + H_2$ to create an electron donor ($H_2$) and play the same thing as normally. It is possible to reduce $CO$ with an electron donor ($_H2O$, $H_2$, $H_2S$, $NH_3$, etc...) further, and create for example ethanol or carbohydrates like glucose.

Afaik only artificial photosynthesis use $CO_2$ splitting and natural ones prefer $NADPH$ and $ATP$ creation and usage for example in the Calvin cycle, C4 pathway, CAM pathway, reverse Krebs cycle, Wood pathway, etc... I think this is because carbon fixation evolved before photosynthesis. So photosynthesis is just about plugging another energy source into the system.

(In programmer terms the carbon fixation is loosely coupled to the implementation of the energy producing process and the common interface is described with ATP and NADPH creation. In mobile phone user terms it is like charging your phone's battery (ATP and NADPH storage) from line power or using a solar charger. Both chargers provide electricity (ATP and NADPH) via USB.)

The question is good, what you meant is possible, but afaik. in natural photosynthesis the $O_2$ always comes from the water.


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    $\begingroup$ I'd suggest you cut most of the references. Nobody is going to read them all (certainly not the poster) and they discourage people scrolling to other, perhaps more succinct answers. $\endgroup$
    – David
    Commented Apr 18, 2016 at 20:19
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    $\begingroup$ @David I read them all. I think I am still somebody... $\endgroup$
    – inf3rno
    Commented Apr 18, 2016 at 20:24
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    $\begingroup$ @David Btw. I think the problem is with the site. It does not provide a way to collapse a reference list or add it as an attachment. $\endgroup$
    – inf3rno
    Commented Apr 18, 2016 at 20:25

It’s not about the oxygen!

This question indicates two misplaced concerns. One is with oxygen. I imagine that this is because of its importance to us as animals; however as far as photosynthesis is concerned oxygen is just a waste product. The other is with a chemical equation, which is as informative as the top line of a commercial balance sheet. One's focus should be on the chemical problem and what is needed to solve it.

The problem is making sugar from carbon dioxide

The purpose of photosynthesis is to make sugar from carbon dioxide. Why is this a problem?

  • The individual carbons in the carbon dioxide have to be joined together with C-C bonds. This requires energy (in the biological form of ATP).
  • In sugars most of the oxygen atoms are in OH groups. So we need to convert the C=O of the carbon dioxide to C–OH (not throw the oxygen away). This is reduction of the carbon, and requires ‘reducing power’ (in the form of the NADPH) which also requires energy input.

The sun solves the problem by supplying the energy

The energy for creating the reducing NADPH and the ATP comes from the sun in a complex series of reactions that do not involve carbon dioxide. Light of a particular wavelength is used to break a water bond so as to separate charge in a reaction that I will deliberately not balance:

(1) H2O → O2 + H

A complex series of reactions transfer the H (more strictly an electron) between different molecules, eventually reducing NADP+ to NADPH. At the same time an equally complex process builds up a H+ concentration difference across a membrane which provides the energy to convert ADP to ATP.

To reiterate, the reaction uses light energy to generate chemical energy in the form of NADPH and ATP. Oxygen is just flushed down the toilet.

Now for the Lego bricks — so easy we can do it in the dark

Having harvested the energy of the sun we have money in the bank which we can spend whenever we want — day or night — to make sugar from carbon dioxide. The overall unbalanced equation is:

(2) CO2 + NADPH + ATP → C6H12O6 + NADP+ + ADP

but this is actually a complex series of enzyme-catalysed reactions (known as the Calvin Cycle if you must know).

You still want to know the answer?

The oxygen produced in photosynthesis comes from water because this is the molecule in which a bond is broken by sunlight to separate charge allowing synthesis of molecules with reducing power and energy transfer potential. Carbon dioxide is the building block of the sugar and far from breaking a C=O bond, what is needed is for it to be converted to C-OH by reduction of the carbon.

  • $\begingroup$ @user1136 — I'm about to take a late vacation. I'll consider the chemical arguments you make regarding reduction of carbon dioxide and respond on my return. $\endgroup$
    – David
    Commented Oct 13, 2018 at 16:19
  • $\begingroup$ @user1136 — OK, I see your point. Yes, my reference to reducing the oxygen (and even a double bond) was sloppy and incorrect. I have modified my answer to make it clear that the carbon is reduced. Trust it is ok now. If so, we can tidy up the comments. $\endgroup$
    – David
    Commented Oct 29, 2018 at 13:59

I read on Wikipedia that basically 3 $O_2$'s come from water in the light reaction, and during the Calvin cycle $H_2O$ is produced from H+ ions and $CO_2$ to make more $H_2O$, which produces more $O_2$ in the light reaction.

Basically, 3 $O_2$'s come from the water and the other 3 $O_2$'s come from $CO_2$-made $H_2O$.

EDIT: The equation is $12H_2O$ + $6CO_2$ => $C_6H_1$$_2$$O_6$ + $6H_2O$ + $6O_2$ so therefore all the $O_2$ is from the water, and the $O_2$ in the sugar is from the $CO_2$, and the supposedly extra $O_2$ left over goes into some new $H_2O$.

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    $\begingroup$ It's the only answer that accounted for the discrepancy of "wait, 6 H2O only has 6 O's and so that makes 3 O2's! Also, 6 CO2 will have 3 O2's left over after making C6H12O6! Why does the missing O2 (from H2O) magically appear and the extra O2 (from CO2) magically disappear?"- well, that I could find, anyway. $\endgroup$ Commented Nov 3, 2015 at 0:54
  • $\begingroup$ Actually, I don't know if the plant reuses the CO2-generated H2O, but I know that there is H2O generated by CO2. The equation I saw later was 12H2O + 6CO2 --> C6H12O6 + 6O2 + 6H2O, but they took 6H2O from both sides to simplify the equation, but it adds confusion later when viewed without 6H2O added to each side, and the O2 is explained to come from H2O. $\endgroup$ Commented Nov 26, 2015 at 2:10

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