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In the photosynthesis equation

$$6*CO_2+6*H_2O+sunlight\rightarrow C_6H_{12}O_6+6*O_2$$

the only place where we have 6 molecules of $O_2$ is in the $6*CO_2$. Then it reacts with $H_2O$ to form $C_6H_{12}O_6$ and $6*O_2$ that apparently comes from $CO_2$. So why do we say that the $O_2$ produced by plants comes from the $H_2O$ and not the $CO_2$? 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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@The Last Word - great link. – Alan Boyd Jun 6 '14 at 14:32
Don't forget to accept answers if you find them useful! – J_mie6 Jun 9 '14 at 18:27
and it's still like this after the edit? – Greek Fellows Aug 21 '14 at 2:23
up vote 5 down vote accepted

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!

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thanks a lot for your answer, it helped me a lot. – Yuran Jun 11 '14 at 6:19

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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I read on Wikipedia that basically 3 O2's (sorry for the lack of subscripts) come from water in the light reaction, and during the Calvin cycle H2O is produced from H+ ions and CO2 to make more H2O, which produces more O2 in the light reaction.

Basically, 3 O2's come from the water and the other 3 O2's come from CO2-made H2O.

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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. – Ian Jackson Nov 3 '15 at 0:54
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. – Ian Jackson Nov 26 '15 at 2:10

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