题目内容
How any fetus survives gestation has baffled scientists for decades: the embryo's tissues
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British political scientists once had difficulty understanding how the unbalancing effects
These so-called facts are the product of considerable______in the analysis and exposition
SECTION 1
Directions: Each sentence below has one or two blanks, each blank indicating that something has been omitted. Beneath the sentence are five lettered or sets of words. Choose the word or set of words for each blank that best fits the meaning of the sentence as a whole.
In contemporary usage, continents are understood to be large, continuous, ______ masses of land, for they are almost always separated by expanses of water.
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？
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