求助 阅读详解

echo0709

PASSAGE 38
* 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 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 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.

 

PASSAGE 38

3. According to the passage, when the authors began theirfirst studies, they were aware that
(A) they would eventually need to design experimentsthat involved feeding rats high concentrations ofprotein
(B) tryptophan levels in the blood were difficult tomonitor with accuracy
(C) serotonin levels increased after rats were fed mealsrich in tryptophan
(D) there were many neurotransmitters whoseproduction was dependent on metabolic processeselsewhere in the body.
(E) serotonin levels increased after rats were injectedwith a large amount of tryptophan

4. According to the passage, one reason that the authorsgave 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 importantsubstance secreted by the body
(E) compare the effect of carbohydrates with the effectof proteins

5. According to the passage, the more protein a ratconsumes, the lower will be the
(A) ratio of the rat's blood-tryptophan concentration tothe amount of serotonin produced and released in therat's brain
(B) ratio of the rat's blood-tryptophan concentration tothe concentration in its blood of the other aminoacids contained in the protein
(C) ratio of the rat's blood-tyrosine concentration to itsblood-leucine concentration
(D) number of neurotransmitters of any kind that the ratwill produce and release
(E) number of amino acids the rat's blood will contain

7. According to the passage, an injection of insulin wasmost 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 thefollowing would be LEAST likely to be a potentialsource of aid to a patient who was not adequatelyproducing and releasing serotonin?
(A) Meals consisting almost exclusively of protein
(B) Meals consisting almost exclusively ofcarbohydrates
(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

PASSAGE 38

3. According to the passage, when the authors began theirfirst studies, they were aware that
(A) they would eventually need to design experimentsthat involved feeding rats high concentrations ofprotein
(B) tryptophan levels in the blood were difficult tomonitor with accuracy
(C) serotonin levels increased after rats were fed mealsrich in tryptophan
(D) there were many neurotransmitters whoseproduction was dependent on metabolic processeselsewhere in the body.
(E) serotonin levels increased after rats were injectedwith a large amount of tryptophan

4. According to the passage, one reason that the authorsgave 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 importantsubstance secreted by the body
(E) compare the effect of carbohydrates with the effectof proteins

5. According to the passage, the more protein a ratconsumes, the lower will be the
(A) ratio of the rat's blood-tryptophan concentration tothe amount of serotonin produced and released in therat's brain
(B) ratio of the rat's blood-tryptophan concentration tothe concentration in its blood of the other aminoacids contained in the protein
(C) ratio of the rat's blood-tyrosine concentration to itsblood-leucine concentration
(D) number of neurotransmitters of any kind that the ratwill produce and release
(E) number of amino acids the rat's blood will contain

7. According to the passage, an injection of insulin wasmost 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 thefollowing would be LEAST likely to be a potentialsource of aid to a patient who was not adequatelyproducing and releasing serotonin?
(A) Meals consisting almost exclusively of protein
(B) Meals consisting almost exclusively ofcarbohydrates
(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