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This question came up as I work my way through the answers to another question I just asked.

My understanding is that plant photosynthesis looks something like this (though with lots more going on under the hood):

$$\ce{6 CO_2 + 6H_2O + light -> C_6 H_{12} O_6 (sugar) + 6 O_2}$$

(source) and that the primary functions of the sugar are to store and transport energy. If that's true, then there's something fishy about the familiar view that plants need CO2 to breathe and produce O2, while animals need O2 and produce CO2. Doesn't a plant need just as much O2 as an animal would when it (the plant) gets around to consuming the sugar it has produced? Doesn't consumption of the sugar take you back to the original reactants?

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Plants need sugar not only for energy but also for synthesizing essential metabolites and building blocks including cellulose (used in cell wall), glycoproteins and 5-carbon sugars for nucleotides.

As already mentioned by canadianer, plants respire by oxidizing sugars. This happens mostly during the night. However photosynthesis also produces ATP and therefore plants are not strictly dependent on respiration for energy. Moreover the fixed carbon in the form of starch makes up an energy reserve (like fats and glycogen in animals).

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