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From a E-book written by Hungary scientists, the reason is $\ce{N2}$ is inert.

  • Is this correct?
  • How does the chemical properties of gas species ($\ce{SO2}$, $\ce{O2}$, $\ce{NH3}$, etc) influence the assimilation process by organisms?
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    $\begingroup$ You can read about nitrogen fixing: en.wikipedia.org/wiki/Nitrogen_fixation. Most plants that "fix" nitrogen are symbiotically associated with nitrogen-fixing bacteria (Rhizobia, en.wikipedia.org/wiki/Rhizobia). Also some diatoms (algae), Rhopalodia gibba, are able to fix nitrogen as they have cyanobacteria in their cytoplams. $\endgroup$
    – aretxabaleta
    Commented Jan 26, 2016 at 15:32
  • $\begingroup$ Thanks for your reply! I have read about the content of nitrogen fixing. I just curious about the chemical nature why assimilation process select certain gas species. $\endgroup$
    – Han Zhengzu
    Commented Jan 26, 2016 at 16:48
  • $\begingroup$ By the way I think this does not have a direct relation with geochemistry or atmospheric chemistry, e.g. asking about how these processes affect the atmosphere or Earth, or things like the whole nitrogen cycle. Rather you are focusing on the specific processes that occur in the organisms (and nothing about environmental interaction), that's biochemistry. That's all just IMHO tho, you can ignore me if you don't agree :-) . $\endgroup$
    – busukxuan
    Commented Jan 27, 2016 at 5:30
  • $\begingroup$ Thanks for your insight. The tag I set is inappropriate. $\endgroup$
    – Han Zhengzu
    Commented Jan 27, 2016 at 8:09
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    $\begingroup$ But since we're already here: "inert" simply means that it does not readily react with other molecules, i.e., that the activation energy is very large before it reacts with anything else. Plants and animals have simply not evolved the very strong reagents you need to break the N-N triple bond. $\endgroup$
    – Wolfgang Bangerth
    Commented Jan 27, 2016 at 14:43

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As @Wolfgang Bangerth commented, inertness just means that nitrogen gas doesn't undergo reactions because the NN triple-bond has a very high bond energy and the activation energy for reactions that break the bond (even though thermodynamically favourable) is therefore high. So I suppose that you could say that a major chemical factor in the reactivity of gases is their bond energies.

The key reaction of biological interest for nitrogen is reduction to ammonia, as referenced by the comment of @aretxabaleta. Stryer makes a couple of points that seem chemically relevant (http://www.ncbi.nlm.nih.gov/books/NBK22522/). He says that the reaction pathway for the reduction of nitrogen involves unstable intermediates, and he also mentions that the nitrogenase enzyme in Rhizomium is extremely sensitive to inhibition by oxygen. I am no expert, but I presume that the instability of the intermediates is related to their ease of oxidation. Leguminous plants maintain a very low concentration of free oxygen in their root nodules by synthesising the protein, leghaemoglobin, which mops oxygen up.

So the answer to the question in the title — why animals (at least) don't fix nitrogen — could well be because as aerobic organisms they would find it very difficult to provide an oxygen-free environment to do this. And the fact that animals can get their metabolizable nitrogen second- or third-hand means that there is no evolutionary pressure to develop such a mechanism. (Most bacteria and plants manage without too.)

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