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I recently asked the question, "Do plants need O2 to consume energy they've stored via sugar?" to which @canadianer responded, "Yes, plants require oxygen to generate ATP from sugar. However, they generate far more sugar (and O2) than they use." Why do plants produce all that extra sugar, especially given that sugar is thermodynamically expensive?

One reason, I believe, is to produce fruit and so recruit animals to help with reproduction. But are there others?

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    $\begingroup$ Plants store sugar as starch and use it to make cellulose (for their cell wall). It's also important to realize that sugar is only one of the products of photosynthesis. The Calvin cycle produces 3 carbon molecules which can feed into other metabolic pathways such as for amino acid or nucleoside synthesis. Basically it's the source of all carbon in plants. $\endgroup$
    – canadianer
    Commented Feb 16, 2015 at 20:41
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    $\begingroup$ I just realized my other comment may have been misleading. Of course they use all of the sugar they produce, just not as energy. $\endgroup$
    – canadianer
    Commented Feb 16, 2015 at 20:50
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    $\begingroup$ @canadianer, you write, "They use all of the sugar they produce, just not as energy." Why use such an energy-expensive substance for cell walls? Doesn't the high energy content make it more appealing for animals to eat the plant? $\endgroup$
    – kuzzooroo
    Commented Feb 16, 2015 at 23:13
  • $\begingroup$ possible duplicate of Do plants need O₂ to consume energy they've stored via sugar? $\endgroup$
    – WYSIWYG
    Commented Feb 17, 2015 at 6:44
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    $\begingroup$ @kuzzooroo Building anything is energy expensive. Anabolic processes are energy expensive but they are essential for survival. Cellulose cannot be digested by all animals. Herbivores got the ability to metabolize cellulose in animals much later than the development of cellulose cell walls in plants, in evolution. (Actually herbivores rely on bacteria to do that) $\endgroup$
    – WYSIWYG
    Commented Feb 17, 2015 at 7:26

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