Caffeine, the stimulant in coffee, has been called “the most widely used psychoactive substance on Earth.” Snyder, Daly, and Bruns have recently proposed that caffeine affects behavior by countering the activity in the human brain of a naturally occurring chemical called adenosine. Adenosine normally depresses neuron firing in many areas of the brain. It apparently does this by inhibiting the release of neurotransmitters, chemicals that carry nerve impulses from one neuron to the next. Like many other agents that affect neuron firing, adenosine must first bind to specific receptors on neuronal membranes. There are at least two classes of these receptors, which have been designated A1 and A2. Snyder et al. propose that caffeine, which is structurally similar to adenosine, is able to bind to both types of receptors, which prevents adenosine from attaching there and allows the neurons to fire more readily than they otherwise would. For many years, caffeine’s effects have been attributed to its inhibition of the production of phosphodiesterase, an enzyme that breaks down the chemical called cyclic AMP. A number of neurotransmitters exert their effects by first increasing cyclic AMP concentrations in target neurons. Therefore, prolonged periods at the elevated concentrations, as might be brought about by a phosphodiesterase inhibitor, could lead to a greater amount of neuron firing and, consequently, to behavioral stimulation. But Snyder et al. point out that the caffeine concentrations needed to inhibit the production of phosphodiesterase in the brain are much higher than those that produce stimulation. Moreover, other compounds that block phosphodiesterase’s activity are not stimulants. To buttress their case that caffeine acts instead by preventing adenosine binding, Snyder et al. compared the stimulatory effects of a series of caffeine derivatives with their ability to dislodge adenosine from its receptors in the brains of mice. “In general,” they reported, “the ability of the compounds to compete at the receptors correlates with their ability to stimulate locomotion in the mouse; i.e., the higher their capacity to bind at the receptors, the higher their ability to stimulate locomotion.” Theophylline, a close structural relative of caffeine and the major stimulant in tea, was one of the most effective compounds in both regards. There were some apparent exceptions to the general correlation observed between adenosine receptor binding and stimulation. One of these was a compound called 3-isobutyl-1-methylxanthine (IBMX), which bound very well but actually depressed mouse locomotion. Snyder et al. suggest that this is not a major stumbling block to their hypothesis. The problem is that the compound has mixed effects in the brain, a not unusual occurrence with psychoactive drugs. Even caffeine, which is generally known only for its stimulatory effects, displays this property, depressing mouse locomotion at very low concentrations and stimulating it at higher ones. 1 D 71. A. The passage discusses a current investigation, not one planned for the future. B. The passage examines two explanations, but the earlier theory is discussed only to expose its weakness and the diff erences between the explanations are not reconciled. Most of the passage is devoted to the more recent hypothesis. C. Only two theories are presented in the passage. D. Correct. Th e recent hypothesis provides an alternative to an earlier one and is supported by evidence and arguments. E. Lines 32–37 do pose such a challenge to the earlier theory; however, the challenge is a small part of the whole passage. Similarly, in the final paragraph, an exception to the more recent theory is introduced, only to be dismissed as an unimportant concern. The correct answer is D. 1. The primary purpose of the passage is to (A) discuss a plan for investigation of a phenomenon that is not yet fully understood (B) present two explanations of a phenomenon and reconcile the differences between them (C) summarize two theories and suggest a third theory that overcomes the problems encountered in the first two (D) describe an alternative hypothesis and provide evidence and arguments that support it (E) challenge the validity of a theory by exposing the inconsistencies and contradictions in it 2 D 72. A. The passage does not suggest that adenosine stimulates behavior. B. While the final paragraph reveals that caffeine displays mixed eff ects, the passage does not state that adenosine has a stimulatory effect. C. Increasing cyclic AMP concentrations is part of the earlier theory, not that of Snyder et al. D. Correct. Lines 17–21 explain that caffeine binds to the receptors, releasing neurotransmitters, whereas adenosine hinders that release. E. Inhibiting the production of phosphodiesterase is discussed in the earlier theory, not in the work of Snyder et al. The correct answer is D. 2. According to Snyder et al., caffeine differs from adenosine in that caffeine (A) stimulates behavior in the mouse and in humans, whereas adenosine stimulates behavior in humans only (B) has mixed effects in the brain, whereas adenosine has only a stimulatory effect (C) increases cyclic AMP concentrations in target neurons, whereas adenosine decreases such concentrations (D) permits release of neurotransmitters when it is bound to adenosine receptors, whereas adenosine inhibits such release (E) inhibits both neuron firing and the production of phosphodiesterase when there is a sufficient concentration in the brain, whereas adenosine inhibits only neuron firing 3 A 73. A. Correct. The results of one experiment can be explained by mixed eff ects in the brain, which Snyder et al. say may occur with psychoactive drugs. B. This response refers back to the earlier theory, not to Snyder et al.’s response concerning IBMX experiment results. C. Caffeine is only included within the broad category of psychoactive drugs. D. This effect is attributed to one compound,IBMX, not to all psychoactive drugs. E. This ability is not discussed in the passage. The correct answer is A. 3. In response to experimental results concerning IBMX, Snyder et al. contended that it is not uncommon for psychoactive drugs to have (A) mixed effects in the brain (B) inhibitory effects on enzymes in the brain (C) close structural relationships with caffeine (D) depressive effects on mouse locomotion (E) the ability to dislodge caffeine from receptors in the brain 4 E 74. A. Lines 54–55 state that IBMX binds to receptors. B. Lines 17–19 state that caff eine binds to receptors. C. Lines 13–14 state that adenosine binds to receptors. D. Lines 46–50 state that theophylline binds to receptors. E. Correct. The passage includes no evidence that phosphodiesterase binds to receptors. The correct answer is E. 4. According to Snyder et al., all of the following compounds can bind to specific receptors in the brain EXCEPT (A) IBMX (B) caffeine (C) adenosine (D) theophylline (E) phosphodiesterase 5 B 75. A. Phosphodiesterase is discussed in an entirely diff erent context in lines 22–25. B. Correct. Lines 17–19 suggest that caffeine’s structural similarity to adenosine may be responsible for its ability to bind to A1 and A2 receptors. C. Caff eine acts on neurotransmitters; it is not structurally similar to them. D. Caffeine’s ability to stimulate behavior results from, rather than causes, this process. E. The passage does not discuss the natural occurrence of these compounds. The correct answer is B. 5. Snyder et al. suggest that caffeine’s ability to bind to A1 and A2 receptors can be at least partially attributed to which of the following? (A) The chemical relationship between caffeine and phosphodiesterase (B) The structural relationship between caffeine and adenosine (C) The structural similarity between caffeine and neurotransmitters (D) The ability of caffeine to stimulate behavior (E) The natural occurrence of caffeine and adenosine in the brain 6 B 76. A. The quotation explains results of an experiment, not assumptions about a theory. B. Correct. The quotation summarizes the experiment with mice and reports a major finding in support of the hypothesis. C. The quotation generalizes on the basis of the experiment; it does not limit the finding to mice. D. Specific, not general, correlations were made between the ability to bind to receptors and to stimulate locomotion. E. The passage includes no such objections; therefore no refutations are needed. The correct answer is B. 6. The author quotes Snyder et al. highlighted ] in lines 43–48 most probably in order to (A) reveal some of the assumptions underlying their theory (B) summarize a major finding of their experiments (C) point out that their experiments were limited to the mouse (D) indicate that their experiments resulted only in general correlations (E) refute the objections made by supporters of the older theory

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28 May 2024, 12:11

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Question 6

nikitathegreat

Can someone explain Q6? GMATNinja

Question 6 asks why the author included the following quotation from Snyder et al. (which just means "Snyder and others") in the passage:

Quote:

"In general,” they reported, “the ability of the compounds to compete at the receptors correlates with their ability to stimulate locomotion in the mouse; i.e., the higher their capacity to bind at the receptors, the higher their ability to stimulate locomotion.”

To understand why the author includes this quotation, first think through the structure of the passage as a whole. In the first two paragraphs, the author explains Snyder's idea about how caffeine works. In the third paragraph, the author discusses a different, older idea about how caffeine works.

The quotation that we care about happens in the fourth paragraph. In this paragraph, the author gives Snyder et al. a chance to explain why they are right and the older idea is wrong. Snyder performed an experiment in mice, and from this experiment concluded something that is true "in general." In other words, in this quotation, they show the broader finding that they extrapolated out of their experiment.

So, which answer choice captures this best? Does the quotation:

Quote:

A. reveal some of the assumptions underlying their theory

Nope, the quotation reveals a finding of an experiment, NOT an assumption underlying the theory. (A) is out.

Quote:

B. summarize a major finding of their experiments

This seems to be on track! The quotation is there to quickly explain what Snyder et al. found in their experiments. Keep (B) for now.

Quote:

C. point out that their experiments were limited to the mouse

Sure, the experiments were done on mice -- but this quotation is actually a broader statement about how caffeine works in general. The purpose of the quote isn't to point out limitations of the experiments, but rather to explain a finding of the experiments. Eliminate (C).

Quote:

D. indicate that their experiments resulted only in general correlations

The word "only" stands out here. Is the author trying to minimize the experiments by saying, "well, they ONLY resulted in general correlations..."?

Not at all! The words "in general" here actually makes these experiments more applicable to situations outside the mice. The author includes the quotation to help Snyder et al. make their case, not to minimize the importance of the experiments. (D) is out.

Quote:

E. refute the objections made by supporters of the older theory

Snyder et al. certainly disagree with an older theory. But we haven't seen any "objections" from people who support the old theory -- they haven't been given a chance to weigh in on Snyder's new theory at all. So, the quotation can't be refuting such objections.

We can eliminate (E), and (B) is the correct answer to question 6.

I hope that helps!

arthurchichester

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GMATNinja

kivalo

Please add timers to the OP.

How do you approach passage like this one on the first read?
Do you just understand the structure and then move on to the questions or do you try to understand the relationships immediately? I thought I could skim over details but the questions proved me wrong

Check out the Ultimate RC Guide for Beginners for tips on how to approach RC passages like this one!

Could you please explain a strategy on how to go about question 1. I solved the passage in under 6mins having only the first one wrong, I was so confused in the language of the last 3 options that I marked one (E) randomly and moved ahead.

I do get stuck in main point questions but only in RC and not in CR.

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29 Jul 2024, 15:33

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arthurchichester

For tips on identifying the primary purpose of this passage, check out this video, which analyzes this passage in detail and goes through several of the questions, including question 1.

For general pointers on how to identify the primary purpose, check out our RC guide for beginners or this RC video playlist.

I hope that helps a bit!

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23 Dec 2024, 04:01

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according to me its 705 above level

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