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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

2 Answers 2

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 nucleus) 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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