GMAT考试阅读资料(七)b

Passage 38

　　It was once believed that the brain was independent　of metabolic¹ 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 elevations² 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 decided³ to see whether the secretion⁴ of the animal’s own insulin similarly affected⁵　serotonin production. We gave the rats a carbohydrate-containing meal that we knew would elicit⁶ 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 largeamount 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 mechanism⁷ 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 Cellular⁸ Communication
　　　（B） Diet and Survival： An Old Relationship Reexamined
　　　（C） The Blood Supply and the Brain： A Reciprocal Dependence⁹
　　　（D） Amino Acids and Neurotransmitters： The Connection Between Serotonin Levels and Tyrosine
　　　（E） The Effects of Food Intake¹⁰ 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 carbohydrates¹¹ 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 secreted¹² 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） stimulate¹³ further research studies
　　　（B） summarize an area of scientific investigation¹⁴
　　　（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 previously¹⁵ 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 initially¹⁶ 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.