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Originally Posted by Big Dave  You forgot another main one: carbon dioxide. If sunlight and water are also present, that's what allows photosynthesis in plants to occur. |
Well, I did not mention it on purpose in my example, because you could argue that it contains an O[2] molecule whereas methane CH[4] and nitrogen N[2] doesn't.
It might be possible that in some kind other evolved plants and animals could use methane and chlorine instead of carbon dioxide and oxygen for exothermic chemical reactions.