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  3. For the past forty years it has been known that the ammonia produced

For the past forty years it has been known that the ammonia produced

游客2024-01-11  69
问题            For the past forty years it has been known that the ammonia produced by
       the rhizobia bacteria in soybean roots diffuses through membranes surrounding
       bacterial cells into the soybean tissue. Interestingly, these membranes are
Line     highly impermeable to organic molecules such as amino acids. Once in the plant
(5)      tissue, ammonia can be incorporated into the amino acids glutamine and
       glutamate, which are used to synthesize other amino acids and nitrogen-
       containing molecules the plant needs, such as proteins, nucleotides and nucleic
       acids.
           But it has recently been discovered that the ammonium or ammonia formed
(10)     by nitrogen fixation is rapidly converted into the amino acid alanine, which is
       then transported out of the bacterium; thus little if any fixed nitrogen leaves the
       bacteria as ammonia. While researchers remain perplexed by the means through
       which alanine actually passes the membranes, they speculate that the process
       may keep the newly fixed nitrogen separate from the everyday nitrogen
(15)     metabolism of the bacteria themselves, thus ensuring the rapid export of most
       of it to the plant. [br] The passage mentions that at various points, scientists have believed all of the following to be essential for the nitrogen fixation process EXCEPT
选项 A、The presence of ammonia
B、The permeability of cell membranes
C、The presence of rhizobia
D、The synthesis of amino acids
E、Nitrogen-containing molecules like nucleotides
答案 E
解析
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