It was once believed that the brain was independent of

Question

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
 (5)  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
 (10)  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
 (15)  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
 (20)  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.
 (25)  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.

(30)  Surprisingly, however, when we added a large
amount of protein to the meal, brain tryptophan
and serotonin levels fell. Since protein contains
tryptophan, whyshould it depress brain tryptophan
levels? The answer lies in the mechanism that
 (35)  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
 (40)  more, proportionately, than it does that of tryptophan.
The more protein is in a meal, the lower is the ratio
of the resulting blood-tryptophanconcentration to
the concentration of competing amino acids, and
the more slowly is tryptophan provided to the brain.
 (45)  Thus the more protein in a meal, the less serotonin
subsequently produced and released.   1 
 1.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

2 
 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 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

3 
 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 
 4. 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

5 
 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 
 6. 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

7 
 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 
 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 
 9. 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.

raiprem123 · Accepted Answer

Here are the answers with explanations:

1. Accordingto the passage, the more protein a rat consumes, the lower will be the
Ans : (E) The Effects of Food Intake on the Production and Release of Serotonin: Some Recent Findings
Mentioned in the very first passage.

2. According to the passage, the speed with which tryptophan is provided to the brain cells of a rat varies with the
Ans : (A) amount of protein present in a meal
The variation in the speed with which tryptophan is provided to the brain cells is correlated with amount of protein present in meal in lines 30 to 32.

3. According to the passage, when the authors began their first studies, they were aware that
Ans : (E) serotonin levels increased after rats were injected with a large amount oftryptophan
From lines 10 to 14 it is clear that scientists were already aware that serotonin levels increased after rats were injected with a large amount of tryptophan and they wanted to verify if the same happens after rats eat meals that change tryptophan levels in the blood.

4. According to the passage, one reason that the authors gave rats carbohydrates was to
Ans : (C) cause the rats to produce insulin >
Clearly mentioned in lines 25-26/

5. According to the passage, the more protein a rat consumes, the lower will be the
Ans : (B) ratioof the rat's blood-tryptophan concentration to the concentration in its blood of the other amino acids contained in the protein
Mentioned in lines 38 to 40

6. The authors' discussion of the "mechanism that provides blood tryptophan to the brain cells" (lines 34-35) is meant to
Ans : (c) help explain why a particular research finding was obtained
Scientists found that when rats are fed with protein, the brain tryptophan and serotonin levels fell. In order to explain this particular finding they discussed about the mechanism which provides blood tryptophan to the brain cells.

7. According to the passage, an injection of insulin was most similar in its effect on rats to an injection of
Ans : (D) tryptophan
Lines 10 to 11 indicate that injecting amino acid tryptophan increased the serotonin level in brain. Similarly from lines 19 to 22 it is clear that injecting rats with insulin increased tryptophan which in turn increased the serotonin level.

8. 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?
Ans : (A) Meals consisting almost exclusively of protein
All other options mention source of aid desired for patient who was not adequately producing and releasing serotonin.

9. Itcan 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?
Ans : (A) The rats' brain serotonin levels would not decrease.
All the other options don't make sense. Though, as scientists had fed large amounts of protien to rats they must have expected the tryptophan level to increase. In answer options only option A comes in line with this. They must have thought that at least the tryptophan level shall not decrease.

MartyTargetTestPrep · Answer

The key thing to understand is that experiments are either the same as studies or parts of studies. So, studies would not come before experiments. Studies and experiments would occur at the same time.

Furthermore, notice that question asks what they were "aware" of when they began their studies, not what they hypothesized before beginning their studies. So, regardless of what they may have hypothesized, we need something that they were aware of, something that they would have learned through discovering it.

So, (D) is not supported.

aragonn · Answer

P1 - a believed theory provved wrong.
P2 - proof of theory in P1.
P3 - more evidence but in reverse direction.
Main point - a theory was discarded, multiple evidences are given to prove that.

Which of the following titles best summarizes the contents of the passage?

(A) Neurotransmitters: Their Crucial Function in Cellular Communication - no 
(B) Diet and Survival: An Old Relationship Reexamined - no 
(C) The Blood Supply and the Brain: A Reciprocal Dependence - no 
(D) Amino Acids and Neurotransmitters: The Connection between Serotonin Levels and Tyrosine - p3
(E) The Effects of Food Intake on the Production and Release of Serotonin: Some Recent Findings - to the point

--------------------------------------------
According to the passage, the speed with which tryptophan is provided to the brain cells of a rat varies with the

when we added a large amount of protein to the meal, brain tryptophan and serotonin levels fell.

(A) amount of protein present in a meal
--------------------------------------------

According to the passage, when the authors began their first studies, they were aware that

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.
They don't already know it.They are hoping to finding out if serotonin levels increased after rats were fed meals rich in tryptophan.

(E) serotonin levels increased after rats were injected with a large amount of tryptophan

---------------------------------------------

According to the passage, one reason that the authors gave rats carbohydrates was to
lines to read - We gave the rats a carbohydrate-containing meal that we knew would elicit insulin secretion.
(C) cause the rats to produce insulin 
------------------------------------------------
According to the passage, the more protein a rat consumes, the lower will be the

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. (45) Thus the more protein in a meal, the less serotonin subsequently produced and released.

(B) ratioof the rat's blood-tryptophan concentration to the concentration in its blood of the other amino acids contained in the protein

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

PT - no doubt this is explanation for the previous findings.
(C) help explain why a particular research finding was obtained -

-------------------------------------------------

According to the passage, an injection of insulin was most similar in its effect on rats to an injection of 
injecting insulin into a rat's bloodstream also caused parallel elevations in blood and brain tryptophan levels

(D) tryptophan - 
-------------------------------------------

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?

when we added a large amount of protein to the meal, brain tryptophan and serotonin levels fell.

(A) Meals consisting almost exclusively of protein - straight A

---------------------------------------------

Itcan 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?

Thus the more protein in a meal, the less serotonin subsequently produced and released.

(A) The rats' brain serotonin levels would not decrease.

moropa2018 · Answer

1. Which of the following titles best summarizes the contents of the passage?

(A) Neurotransmitters: Their Crucial Function in Cellular Communication
 This option talks about neurotransmitters in general but in the passage we are talking about a specific kind of neurotransmitter- Serotonin. 
 
(B) Diet and Survival: An Old Relationship Reexamined.  Completely out of scope.  
 No where in the passage survival is talked about

(C) The Blood Supply and the Brain: A Reciprocal Dependence.
 This passage is about the release of serotonin and tryptophan in brain with respect to various types of food. Hence this option is out of scope.

(D) Amino Acids and Neurotransmitters: The Connection between Serotonin Levels and Tyrosine
 The Connection between Serotonin Levels and Tyrosine is used as a case in point for the main point of the passage.
 
(E) The Effects of Food Intake on the Production and Release of Serotonin: Some Recent Findings
 Exactly the gist of the passage.

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
 
 The above line talks about speed in the context of protein meal. Hence A is the answer.

(B) concentration of serotonin in the brain before a meal 
 The author says that the concentration of serotonin increases after a carbohydrate meal but nothing about its concentration before a mean and its speed.

(C) concentration of leucine in the blood rather than with the concentration of tyrosine in the blood after a meal
  The author mentions leucine and tyrosine as examples of a mechansim he was explaining. Tyrosine and leucine concentration have nothing to do with the speed with which tryptophan is provided to the brain cells of a rat.

(D) concentration of tryptophan in the brain before a meal
 Again concentration of tryptohan in the brain before a meal was never implied in the passage.

(E) number of serotonin-containing neurons
 This number is never mentioned in the passage let alone in the context of speed with which tryptophan is provided to the brain cells of a rat.

3. According to the passage, when the authors began their first studies, they were aware that

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.
Since authors already know that with the injection of tryptophan, serotonin increases, they are hoping to finding out if the same happens in the case of food.

(A) they would eventually need to design experiments that involved feeding rats high concentrations of protein
 Nothing about protein is mentioned in this part of the passage.

(B) tryptophan levels in the blood were difficult to monitor with accuracy
 Nothing about difficulty of monitoring.

(C) serotonin levels increased after rats were fed meals rich in tryptophan.
 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.
They don't already know it.They are hoping to finding out if 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.
 Completely out of context.

(E) serotonin levels increased after rats were injected with a large amount of tryptophan
 Correct. According to the above quoted text from the passage, we can infer this.

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

The author says since protein contains tryptophan and when one consumes protein, the tryptophan levels should increase in the brain. But that is not the case and he questions why. In the next line, he answers his question and says that 'due to the mechanism', this happens. So, he is mentioning 'mechanism' to explain why tryptophan levels are decreasing.

(A) stimulate further research studies
 Nothing about further research was mentioned in the passage.

(B) summarize an area of scientific investigation.
 Authors are not summarizing.

(C) help explain why a particular research finding was obtained
 Yes. correct answer.

(D) provide supporting evidence for a controversial scientific theory
 Nothing controversial was mentioned in the passage.

(E) refute the conclusions of a previously mentioned research study
 The authors by saying that tryptophan levels decrease in the brain when protein rich meal is consumed, are not refuting the fact that tryptophan levels increase in the brain when meals elicit insulin secretion (mentioned in the previous passage). He is merely explaining the effect of protein on tryptophan production in brain.

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
 
 Hence the right answer.

(B) Meals consisting almost exclusively of carbohydrates
 This is likely suitable. Hence , not the answer.

(C) Meals that would elicit insulin secretion
 This is most likely suitable. Hence , not the answer.

(D) Meals that had very low concentrations of tyrosine
 Out of scope. We do not know the effects of tyrosine in production and release of sorotonin.

(E) Meals that had very low concentrations of leucine
 Out of scope. We do not know the effects of leucine in production and release of sorotonin.

mSKR · Answer

Hi  GMATNinja   CrackVerbalGMAT   MartyTargetTestPrep   mikemcgarry   ExpertsGlobal5   GMATGuruNY

Could you please give your suggestion why the answer can not be D . I can understand why is it C.

My reasoning: 
 In recent studies,  however, we have discovered that the production and release (5) 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.

Thanks in advance!!

Were the recent studies performed after first experiment. I believe recent studies should exist first and then experiment happen to confirm what is said in studies. 
With this thought I think answer could also be D.

Please give your opinion.

saswat1987 · Answer

I find this passage as one of the most challenging RC passage.

One has to read this passage very carefully and invest more time on understanding the flow and context of the passage rather than understanding the contents, since the questions are based on author's point of view, why the experiments were done, whether the result were positive / negative, etc.,

This is real OLD IS GOLD type passage. I made a lot of mistakes when I did this passage the 1st time but now after changing the way to approach a RC passage, I made 1 mistake (silly one).

nikitamaheshwari · Answer

(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

Can you explain why ans choice C and D are wrong?  GMATNinja   VeritasKarishma

GMATNinja · Answer

Question 3 
 
Let's consider the following key points from the passage:

It was "once believed that the brain was independent of metabolic processes occurring elsewhere in the body."  
 The authors of the passage recently performed studies showing this was not the case (i.e. some neurotransmitters are dependent on food processed by the body).  The authors recently conducted a study to see how eating affected tryptophan levels in rats. 
 This study was based on the  previous observation  that rats given "a large injection of the amino acid tryptophan" experienced an increase in serotonin.

Now let's consider (C):
 
This is exactly what the authors found  in their first study . But the question asks what the authors were aware of WHEN they began their first studies. In other words -- the right answer should be something the authors were  already  aware of BEFORE their first study. Since the authors weren't aware of this until AFTER they performed their study, (C) is wrong.

Let's consider (D):
 
Again, this is something the authors found during their study. Since they weren't aware of this fact BEFORE their study, (D) is out.

Now take a look at (E):
 
According to the passage, the authors were aware that "a large injection of the amino acid tryptophan" caused serotonin levels to increase in rats.  Based on this fact , their study sought to confirm whether eating would also cause serotonin to increase in rats.

Since they were aware of this fact BEFORE their study, (E) is correct.

I hope that helps!

Sneha2021 · Answer

GMATNinja   MartyTargetTestPrep   KarishmaB

Hi Experts,
6. The authors' discussion of the "mechanism that provides blood tryptophan to the brain cells" (lines 34-35) is meant to - 
(C) help explain why a particular research finding was obtained
(D) provide supporting evidence for a controversial scientific theory

Why D is incorrect?
The scientific theory is that large amount of protein will also increase serotonin level but the result was different. To support this controversial finding, the lines 34-35 provide supporting evidence by introducing "mechanism" that helps to explain this finding.

Why C is correct?
C says mechanism helps explain why we did protein experiment. I believe the mechanism helps to explain the result of the protein experiment.

Please help to understand how C is correct.

Thanks for your time!

KarishmaB · Answer

Sneha2021

Note how the previous paragraph ends and the new paragraph begins.

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. 
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 previous paragraph talks about expected results obtained in the study. This paragraph starts with how an unexpected result was obtained (highlighted). The next line explains why this result was obtained - why a large amount of protein in the meal made brain tryptophan and serotonin levels fall. It tells us that the same mechanism provides other amino acids too and tryptophan loses is the competition.

6. 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

Hence, what is the purpose of this discussion? Why does he mention the mechanism? To explain why we got the surprising result. Hence (C) works. He does mention it to explain a finding.

(D) is not correct because no controversial theory has been discussed. He open the first paragraph with what was once believed (brain was
independent of metabolic processes) but now has been shown to be incorrect (brain is impacted by metabolic processes). Then he goes on to explain how the brain is impacted (the results established by studies). Then he gives a surprising impact of metabolic processes on the brain and then goes on to explain why it is so. There is no discussion on 'controversial theory'. A controversial theory would a belief held by some while others would not agree etc.

VerbalBot · Answer

Automated notice from GMAT Club VerbalBot:

A member just gave Kudos to this thread, showing it’s still useful. I’ve bumped it to the top so more people can benefit. Feel free to add your own questions or solutions.

This post was generated automatically.

It was once believed that the brain was independent of

Prep Toolkit

Top 5 GMAT Debrief Videos

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards

GMAT Reading Comprehension (RC) Questions

It was once believed that the brain was independent of

Prep Toolkit

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards