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Why do plants have certain chemicals?

  1. Jan 13, 2005 #1
    There are a whole bunch of plants on this planet which produce an array of chemicals. I'm sure there are chemicals in the plants which had developped as a result of natural selection; some of them are used to help the plant survive certain conditions/predators. But I'm sure there are a lot of chemicals which don't seem to have any value to the survival/reproduction of the plant. As far as I know, these chemicals (like aspirin, for example) are more useful to humans/other creatures than to the plant itself. Does each and every chemical a plant produce have a specific function for the plant itself?

    I have a few thoughts on this..

    A friend of mine suggested that these chemicals might not necessarily have use for the plant, but it could have been created as a by product of some other reaction within the plant. This could be true, but, as far as my knowledge in chemistry goes, the chemical reaction would need to involve a chemical that's similar in structure to what is being produced. Morphine, for example, has a specific structure and requires a reactant of a somewhat similar structure to be produced. So it still seems to bring it back to the initial question; the plant would have to produce a morphine related structure, but what would be the purpose of doing this?

    Another thought I had, assuming certain chemicals don't have a reason in being in the plant, was that these chemicals are produced to help other species survive. This would imply that the plant, in evolving, had intentions of assisting the survival of future species. In some ways I'd like to believe that there is an overall awareness within evolution, an arranged plan.

    The plants are realllllllly old, AFAIK, one of the first forms of life (well, maybe not that close, but they're earlier than the insects). So the chemicals they had produced through evolution at the time weren't designed to fight off any predators, because there weren't any predators there to eat it. Okay, I must add that I'm speculating here, I really don't know very much about plants; I'm sure there were predators, but surely all plants had more advantage in survival then than they do now. I'm guessing the determing factor at that point was reproduction/surviving the elements...So anyways, these plants made the chemicals many, many years ago and only now do we really see how useful they are for ourselves and most certainly other species.

  2. jcsd
  3. Jan 14, 2005 #2
    I think broadly, every life form has an immense amount of "junk".. that which appears to serve no purpose. Some, like the role of untranslated regions (does not encode proteins) of our own genome, have only relatively recently been found to have a purpose..

    On the otherhand, there are proteins that appears to have absolutely no "point" at all, scientists can remove it and the animal/plant and all its progeny appear normal. In that case you could assume that it is a by product of mutation.. it is a protein that use to have a function, but was perhaps partially transposed (where only a small part of a gene is copied to another part of the genome) and thus its original function is distupted.

    But such a waste of resources would have been cleaned up by natural selection, but as you surmised, these defunct proteins may assit in the organisms survival in the future.. so I suspect this is possibly why there is no real "cleanup" feature in any organisms where only used genes are copied to the next generation. Mess, i suppose is advantageous.

    On another note, plants and animals DID evolve (i think) from an original ancestor.. there are many analogues between us. So it's not impossible to believe that a protein in a plant would have an effect in an animal...
  4. Jan 20, 2005 #3
    I think part of the problem is that plant biochemistry - in some regards - is not well enough understood that we can understand things in the same sort of detail that exists for mammals and other medically important organisms (for example, pathogenic bacteria and such). To be only slightly cynical, it seems that if you can't bother to get some new anticancer drug, no one wants to study it. For example, salicylic acid does have a role in plant physiology (regulating flowering, plant growth & development, and controlling the redox state of a cell).

    However, I have a case study which vaguely supports an idea or two that is not totally unrealistic. The Pacific yew tree has in its bark a naturally occurring chemical that is known as taxol. Taxol is also a rather potent antitumor compound. In the early days of its study, the researchers would require enormous amounts of tree bark and other taxol-containing parts because it was not in large concentrations. Perhaps plants keep producing these compounds which seem to be secondary metabolites since it is not that metabolically expensive and/or because the chemistry required to make it exists anyway in other pathways. For example, to make secondary metabolite compound 1, one needs to do somewhat complicated and specific reactions T, U, V, and W to finish the job from some unexciting looking precursor. The thing is that reactions T, U, V, and W are done no matter what in the course of two or three other metabolic pathways. Given the chances of all of these reactions occurring frequently enough to produce the rare secondary metabolite, there isn't that much of this metabolite in a cell. The other possibility is that it is worth the plant's effort, but we simply don't know what it is due to our lack of knowledge. For example, I don't think there is any real idea what taxol's role in yew trees is - or, at the very least, I've never read of one since it's all about the anticancer story in everything I've come across.
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