Why do plants release excess oxygen, rather than consuming it entirely given it is an excellent energy source?

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    $\begingroup$ I guess because plants don't walk around collecting food and it doesn't rain carbohydrates to burn oxygen with. But this gives me ideas for a sci-fi story involving farting trees. $\endgroup$ – Nikolaj-K Oct 29 '15 at 12:59
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    $\begingroup$ One thing worth noting is that, at night, most plants 'breathe' oxygen and use it to metabolize stored sugars, just like animals. However, they don't want to keep extra oxygen around during the day, when they're photosynthesizing, as oxygen actually competes with carbon dioxide during the photosynthetic process, slowing it down. $\endgroup$ – Dan Bryant Oct 29 '15 at 16:18
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    $\begingroup$ oxygen is not energy, its needed for energy, like when you want fire, you need oxygen. $\endgroup$ – Djeroen Oct 29 '15 at 18:15
  • $\begingroup$ As wonderful and abundant oxygen is, it is also highly reactive (consider rusted metal), and can contain ozone and oxygen singlets (a free radical). Free radicals are highly mutagenic and lead to double-stranded breaks in DNA. When cells die by programmed cell death they lyse their mitochondria, possibly to hasten the process. Finally, the enzyme super oxide dismutase exists to safely remove these free radicals from cells. $\endgroup$ – mdperry Oct 30 '15 at 10:51

Short answer
Plants release O2, because it is not an energy source. Instead, it is used to free energy from energy-rich organic compounds.

Plants are solar powered. They release oxygen as a waste product during carbon fixation. Carbon fixation is basically the storage of solar power into carbon-bonds in glucose, a process referred to as photosynthesis or the Calvin Cycle.

To release the solar energy from glucose it has to be burned (oxidized). Oxidation of glucose can be realized via the the citric acid cycle (or Krebs cycle) and electron transport chain to generate ATP. O2 only comes in in the very last step by acting as the final electron acceptor to produce H2O (Fig. 1.)

Fig. 1. Electron transport chain in mitochondria. Electrons are sent through a chain of proteins that drive hydrogen transport out of the mitochondrial space. ATP-synthase uses the proton gradient to generate ATP. Source: Davis University College

Hence, O2 is not the source of energy, it is merely used to free it from energy rich compounds such as glucose. Because it is so abundant in the atmosphere, it doesn't make sense to store it.

- Berg et al., Biochemistry. 5th edition. New York: W H Freeman (2002)

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    $\begingroup$ "Because it is so abundant in the atmosphere, it doesn't make sense to store it." this makes only sense today, it made no sense when the first photosynthesis was done. $\endgroup$ – PlasmaHH Oct 29 '15 at 16:14
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    $\begingroup$ @plasma perhaps early plants (/weed/phytoplankton/...) did retain oxygen and released it when out of storage space. You could also drop evolution for intelligent design and be done with it. $\endgroup$ – John Dvorak Oct 29 '15 at 18:53
  • $\begingroup$ @PlasmaHH - defintely true. But the fact that oxygen levels only increased when plant life became abundant still supports it being a waste product. $\endgroup$ – AliceD Oct 29 '15 at 22:38
  • $\begingroup$ @AliceD: of course it is a waste product, but I do also throw away my waste because it is waste, not because it is abundant on the streets... $\endgroup$ – PlasmaHH Oct 30 '15 at 9:05
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    $\begingroup$ Another reason that oxygen is not stored could be that it would result in Warburg effect i.e, reduction in photosynthesis due to decreased functioning of RUBisCO due to increased O2 conc. and also due to photooxidation of chlorophyll due to same reasons. $\endgroup$ – ajitanshu singh Mar 28 '16 at 6:42

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