SECTION 3Directions: Each passage in this group is followed by questions based on its cont

SECTION 3

Directions: Each passage in this group is followed by questions based on its content. After reading a passage, choose the best answer to each question. Answer all questions following a passage on the basis of what is stated or implied in the passage.

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.

The passage suggests which of the following about scientists' conception of the function of the amino acid alanine in the rhizobia's nitrogen fixation process？

A．They have proposed at least one hypothesis that might account for the function of the immediate conversion of ammonia into alanine.

B．They have established the relationship between alanine and the production of nitrogen-containing molecules beneficial to soybean plants.

C．They continue to believe that ammonia plays an integral role in the symbiosis between rhizobia and soybean plants.

D．They remain uncertain as to how glutamine and glutamate can be produced in the absence of ammonium or ammonia.

E．They have largely rejected the notion that ammonia or ammonium plays any role in nitrogen fixation.