OG RC Question #80

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OG RC Question #80

by happyface101 » Mon Jul 06, 2015 8:33 pm
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.

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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

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The answer is C, however, I'm confused why the answer it's B. Understand that B is probably not perfect either because it's not summarizing an entire area of scientific investigation...but C isn't quite right either. It's not explaining why a research finding was "obtained", it's just explaining a research finding, no? Thanks!

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by MartyMurray » Tue Jul 07, 2015 6:21 pm
The answer is C, however, I'm confused why the answer it's B. Understand that B is probably not perfect either because it's not summarizing an entire area of scientific investigation...but C isn't quite right either. It's not explaining why a research finding was "obtained", it's just explaining a research finding, no? Thanks!
Ok, A is out, because that's just not what the author's purpose is.
B is out, because that one sentence is definitely not summarizing any area of scientific investigation.
D is out, because that sentence is not supporting anything controversial.
E is out, because nothing is being refuted.

So that leaves us with C.

In lines 27-29 results of an experiment are described. Those results were "obtained" by doing that experiment. Why were such results obtained? They were obtained because the mechanism that provides blood tryptophan to brain cells works in a certain way.

So C works. Maybe C could simply say that the sentence explains the results, but anyway, as it stands it does work.
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