GMAT综合阅读精解之三十六

2022-06-07 18:00:09

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  It was once believed that the brain was independent

  of metabolic processes occurring elsewhere in the body.

  In recent studies, however, we have discovered that the

  production and release in brain neurons of the neuro-

  (5) transmitter 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 injec-

  (10)tion 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 find-

  (15) ings suggested that the production and release of sero-

  tonin in brain neurons were normally coupled with

  blood-tryptophan increases. In later studies we found

  that injecting insulin into a rat’s bloodstream also caused

  parallel elevations in blood and brain tryptophan levels

  (20) 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 secre-

  tion. As we had hypothesized, the blood tryptophan

  (25) 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,

  (30) 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

  (35) of protein increases blood concentration 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

  (40) acids, and the more slowly is tryptophan provided to

  the brain. Thus the more protein in a meal, the less

  serotonin subsequently produced and released.

  1. Which of the following titles best summarizes the

  contents of the passage?

  (A) Neurotransmitters: Their Crucial Function in

  Cellular Communication

  (B) Diet and Survival: An Old Relationship Reexamined

  (C) The Blood Supply and the Brain: A Reciprocal

  Dependence

  (D) Amino Acids and Neurotransmitters: The

  Connection Between Serotonin Levels and Tyrosine

  (E) The Effects of Food Intake on the Production and

  Release of Serotonin: Some Recent Findings

  2. According to the passage, the speed with which

  tryptophan is provided to the brain cells of a rat varies

  with the

  (A) amount of protein present in a meal

  (B) concentration of serotonin in the brain before a meal

  (C) concentration of leucine in the blood rather than on

  the concentration of tyrosine in the blood after a

  meal

  (D) concentration of tryptophan in the brain before a

  meal

  (E) number of serotonin-containing neurons present in

  the brain before a meal

  3. According to the passage, when the authors began their

  first studies, they were aware that

  (A) they would eventually need to design experiments

  that involved feeding rats high concentrations of

  protein

  (B) tryptophan levels in the blood were difficult to

  monitor with accuracy

  (C) serotonin levels increased after rats were fed meals

  rich in tryptophan

  (D) there were many neurotransmitters whose

  production was dependent on metabolic processes

  elsewhere in the body.

  (E) serotonin levels increased after rats were injected

  with a large amount of tryptophan

  4. According to the passage, one reason that the authors

  gave rats carbohydrates was to

  (A) depress the rats’ tryptophan levels

  (B) prevent the rats from contracting diseases

  (C) cause the rats to produce insulin

  (D) demonstrate that insulin is the most important

  substance secreted by the body

  (E) compare the effect of carbohydrates with the effect

  of proteins

  5. According to the passage, the more protein a rat

  consumes, the lower will be the

  (A) ratio of the rat’s blood-tryptophan concentration to

  the amount of serotonin produced and released in the

  rat’s brain

  (B) ratio of the rat’s blood-tryptophan concentration to

  the concentration in its blood of the other amino

  acids contained in the protein

  (C) ratio of the rat’s blood-tyrosine concentration to its

  blood-leucine concentration

  (D) number of neurotransmitters of any kind that the rat

  will produce and release

  (E) number of amino acids the rat’s blood will contain

  6. The authors’ discussion of the “mechanism that provides

  blood tryptophan to the brain cells” (lines 31-32) is

  meant to

  (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

  7. According to the passage, an injection of insulin was

  most similar in its effect on rats to an injection of

  (A) tyrosine

  (B) leucine

  (C) blood

  (D) tryptophan

  (E) protein

  8. It can be inferred from the passage that which of the

  following would be LEAST likely to be a potential

  source of aid to a patient who was not adequately

  producing and releasing serotonin?

  (A) Meals consisting almost exclusively of protein

  (B) Meals consisting almost exclusively of

  carbohydrates

  (C) Meals that would elicit insulin secretion

  (D) Meals that had very low concentrations of tyrosine

  (E) Meals that had very low concentrations of leucine

  9. It can be inferred from the passage that the authors

  initially held which of the following hypotheses about

  what would happen when they fed large amounts of

  protein to rats?

  (A) The rats’ brain serotonin levels would not decrease.

  (B) The rats’ brain tryptophan levels would decrease

  (C) The rats’ tyrosine levels would increase less quickly

  than would their leucine levels

  (D) The rats would produce more insulin.

  (E) The rats would produce neurotransmitters other than

  serotonin.

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