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I am studying a level chemistry. I have come across this fact that organic matter generally undergo endothermic reactions. I can't find an explanation for this anywhere online.

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    $\begingroup$ Organic reactions are mostly exothermic. Of course photosynthetic reactions may be endothermic if you don't take the photon energy into account. $\endgroup$
    – Maurice
    Apr 21 at 12:27
  • $\begingroup$ As an A-level your greatest experience with organic reactions is the reactions happening in your own body. Step back and observe; are the reactions in your body exothermic, releasing heat, keeping you warm and toasty, and enabling work, or endothermic making you cold and torpid? Read, temper your reading with careful observations, and avoid generalizations. Most organic reactions have high activation energies and relatively low energy changes, and every exothermic reaction is endothermic in the reverse direction. $\endgroup$
    – jimchmst
    Apr 21 at 20:26

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Catalysts for the opposite reactions are too large to randomly form. Energy saved in molecules is the only way for organisms to use it on demand. Information about setting up the right conditions for the endothermic reactions has depleted the biosphere of all the atoms suitable for saving any information at all. If something does the opposite, it decays faster and is therefore less noticeable in nature.

Another reason is more obvious: to get the high-energy compounds necessary for animals to accelerate, multiple reactions are necessary (due to limited available photon energy per step) whereas for these compounds to decay into low-energy molecules, less steps can be taken. Indeed, we count the number of endothermic reactions to be larger.

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  • $\begingroup$ Catalysts work on forward and reverse reactions. $\endgroup$
    – jimchmst
    Apr 21 at 20:06
  • $\begingroup$ I wanted to emphasize that it takes some non-random (at the microscopic level) process to initiate a reaction going directly from high to low energy. For example, a tree does not spontaneously react with oxygen despite having enough energy. Is "catalyst" a totally irrelevant word here? @jimchmst $\endgroup$
    – Paul Kolk
    Apr 22 at 10:43

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