Reading Comprehension

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 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 biood-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 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 tryptophan level and
the concentrations of tryptophan and of 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 tryptophan 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 concentration of the other amino acids much
more, proportionately, than it does that of tryptophan.

The more protein is in a 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 released.

78) 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

79) 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 with 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

80) 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

81) 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

82) 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) ratioof 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

83) The authors' discussion of the "mechanism that provides blood tryptophan to the brain cells" (lines 34-35) 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

84) 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

85) It can be inferred from the passage that which of the following would be LEAST likelyto 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

86) 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.

78: E
79: A
80: E
81: C
82: B
83: C
84: D
85: A
86: A

78) 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

The gist of the passage is captured in the second sentence of the first paragraph: In recent studies, however, we have discovered 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.


E is a paraphrase of this sentence.

79) 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 with 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

This info can be gleaned from the first sentence of the final paragraph: The more protein is in a 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.

Captured in A

80) 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

Here is the first sentence of paragraph 2: 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 see here that when they conducted their first studies, they already knew that an injection of tryptophan would lead to an increase in serotonin levels in rats. The answer is E

81) 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

The answer can be found in the first sentence of paragraph three: We gave the rats a carbohydrate-containing meal that we knew would elicit insulin secretion.

The answer is C.

82) 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

The answer is in the first sentence of the last paragraph: The more protein is in a meal, the lower is the ratio of the resulting blood-tryptophan concentration to the concentration of competing amino acids,

B is a paraphrase of this.

83) The authors' discussion of the "mechanism that provides blood tryptophan to the brain cells" (lines 34-35) 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

This line shows up in a paragraph relating the surprising relationship between protein consumption and both tryptophan and serotonin levels. (As protein consumption increases, tryptophan and serotonin levels drop.) The discussion of the mechanism by which tryptophan is delivered to the brain helps explain this phenomenon. Captured in C