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OG It was once believed that the brain was independent

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OG It was once believed that the brain was independent

Post Tue Aug 29, 2017 2:49 pm
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    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

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    Post Wed Aug 30, 2017 3:01 pm
    Quote:
    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.

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    Post Wed Aug 30, 2017 3:05 pm
    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

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    Post Wed Aug 30, 2017 3:08 pm
    Quote:
    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

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    Post Wed Aug 30, 2017 3:10 pm
    Quote:
    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.

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    Post Wed Aug 30, 2017 3:14 pm
    Quote:
    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.

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    Post Wed Aug 30, 2017 3:19 pm
    Quote:
    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

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