2. 外语
  3. It was once believed that the brain was independent of metabolic processes occurring elsewhere in the body. In recent studies, h

It was once believed that the brain was independent of metabolic processes occurring elsewhere in the body. In recent studies, h

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问题     It was once believed that the brain was independent of metabolic processes occurring elsewhere in the body. In recent studies, however, we have discov- ered that the production and release in brain neurons of the neurotransmitter serotonin(neurotransmitters are compounds that neurons use to transmit signals to other cells)depend directly on the food that the body processes.
    Our first studies sought to determine whether the increase in serotonin observed in rats given a large injection of the amino acid tryptophan might also occur after rats ate meals that change tryptophan levels in the blood. We found that, immediately after the rats began to eat, parallel elevations occurred in blood tryptophan, brain tryptophan, and brain serotonin levels. These findings suggested that the production and release of serotonin in brain neurons were normally coupled with blood-tryp-tophan increases. In later studies we found that injecting insulin into a rat’s bloodstream also caused parallel elevations in blood and brain tryptophan levels and in serotonin levels. We then decided to see whether the secretion of the animal’s own insulin similarly affected serotonin production. We gave the rats a carbohydrate-containing meal that we knew would elicit insulin secretion. As we had hypothesized, the blood trypto- phan level and the concentrations of tryptophan serotonin in the brain increased after the meal.
    Surprisingly, however, when we added a large amount of protein to the meal, brain tryptophan and serotonin levels fell. Since protein contains tryptophan, why should it depress brain tryptophan levels? The answer lies in the mechanism that provides blood tryp- tophan to the brain cells. This same mechanism also provides the brain cells with other amino acids found in protein, such as tyrosine and leucine. The consumption of protein increases blood con- centration of the other amino acids much more, proportionately, than it does that of tryptophan. The more protein in the meal, the lower is the ratio of the resulting blood-tryptophan concentration to the concentration of competing amino acids, and the more slowly is tryptophan provided to the brain. Thus the more protein in a meal, the less serotonin subsequently produced and re- leased.
The authors’ discussion of the "mechanism that provides blood tryptophan to the brain cells"(lines 44-45)is meant to
选项 A、stimulate further research studies.
B、summarize an area of scientific investigation.
C、help explain why a particular research finding was obtained.
D、provide supporting evidence for a controversial scientific theory.
E、refute the conclusions of a previously mentioned research study.
答案C
解析 作者为什么要讨论“色氨酸进入脑细胞的机制”?A．激励继续研究。无。B．总结科学研究。无。C．正确。解释为何会有一特殊观测结果。这一特殊结果即第三段一开始surprisingly的内容。D．支持一有争议论点。无。E．驳斥一个前面的研究结论。第三段举一特例并不是为了将前面观点驳倒，只是给一个补充说明而已。
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