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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  79
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] It can be inferred from the passage that scientists are "perplexed by the means" (line 12) for which of the following reasons?
选项 A、They are unsure of how alanine remains separate from the everyday nitrogen metabolism of the plant itself.
B、They have yet to establish what function the alanine serves for the soybean plants.
C、They have thought alanine to exist only in very small quanties in rhizobia.
D、They have long considered the membranes of the soybean plant impermeable to amino acids.
E、They are uncertain how such a rapid export of fixed nitrogen is possible through the soybean membranes.
答案 D
解析
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