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My understanding is that animal metabolism consists of exothermic reactions like

$$\ce{C6H12O6 + 6 O2 ->6 CO2 + 6 H2O + energy}$$

This makes thermodynamic sense to me. Animals need an exothermic reaction to produce the energy we need to move around, etc. However, the other half of the picture, plants and other photosynthesizers producing oxygen, has never made complete sense to me. Are plants consuming $\ce{CO2}$, a "low energy" (I think) substance, and producing higher energy $\ce{O2}$ as a waste product? Is there so much energy available for photosynthesis that plants can afford to waste some of it producing $\ce{O2}$ simply to have enough carbon atoms to store energy via sugars?

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    $\begingroup$ The purpose of photosynthesis is to convert a non storable source of energy into a usable and storable form of energy. If looked at from this perspective, it makes thermodynamic sense to produce higher potential energy substances in photosynthesis. $\endgroup$ – Brinn Belyea Feb 16 '15 at 4:02
  • $\begingroup$ I've posted a related question to get a few clarifications so I can understand the answers below. $\endgroup$ – kuzzooroo Feb 16 '15 at 19:19
  • $\begingroup$ @kuzzooroo What do you not understand in the answers? If you need clarification you should request for it using comments. Also, please accept the answers if they have provided the right answer to your posted question. $\endgroup$ – WYSIWYG Feb 17 '15 at 4:28
  • $\begingroup$ OK, thanks for the guidance, @WYSIWYG. I think maybe the clarification I should have asked for is, is it even reasonable to via photosynthesis (in its totality) as $$\ce{6 CO_2 + 6H_2O + light -> C_6 H_{12} O_6 (sugar) + 6 O_2}$$ and is it reasonable to consider the resulting oxygen as a waste product? $\endgroup$ – kuzzooroo Feb 18 '15 at 4:19
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    $\begingroup$ @kuzzooroo That reaction is not correct representation of photosynthesis. Oxygen is a byproduct but as mentioned in the answers the light reactions and production of oxygen is not directly coupled to sugar synthesis from $\ce{CO_2}$. $\endgroup$ – WYSIWYG Feb 18 '15 at 4:22
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Oxygen in photosynthesis does not come from carbon dioxide. It comes from water and this is the step that actually requires light: Photolysis of water. The proton produced in this process is used to synthesize ATP and NADPH by a chemiosmotic process similar to what happens in mitochondria. These biochemical pathways are also referred to as Light reactions.

Carbon fixation (or Calvin-Benson Cycle) requires energy (in the from of NADPH) and is therefore dependent on light reactions. The light has no direct role in carbon fixation. Oxygen production also, is not directly coupled to carbon dioxide consumption.

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This idea about photosynthesis is not correct.This reaction consists several reactions in between. O2 is produced by oxidation of H2O to get electrons which are used to revive PQ(plastoquinone) to transport electrons. Chlorophyll can easily get an electron from a Revitalizing(H2O) to transport the electron to cyt-bf and photosystem 1.

enter image description here

This reaction shows the conversion of H2O to O2:

$$\ce{2H2O ->O2 + 4H+ + 2e-}$$

As you see in this reaction. Producing O2 is useful and the plant can receive 4 H+ for ATP synthase to produce ATP and e- for producing hydrocarbons.

CO2 is fixed in this reaction of Calvin cycle:

$$\ce{6Ribulose-1,5 bisphosphate + 6CO2 + 6 H2O ->12 phosphoglycyrate}$$

enter image description here

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    $\begingroup$ @WYSIWYG I agree.so I changed it. $\endgroup$ – user12874 Feb 17 '15 at 12:10
  • $\begingroup$ you can say that but the primary objective of calvin cycle is to fix CO₂. By saying that it is used in a certain reaction you are actually understating that fact. $\endgroup$ – WYSIWYG Feb 17 '15 at 12:12

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