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Part of New RC Series- Please check this link for more questions 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. 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
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.
(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
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.
(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
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.
(A) IBMX
(B) caffeine
(C) adenosine
(D) theophylline
(E) phosphodiesterase
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.
(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
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.
(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
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.
(A) At very low concentrations in the human brain, both caffeine and theophylline tend to have depressive rather than stimulatory effects on human behavior.
(B) The ability of caffeine derivatives at very low concentrations to dislodge adenosine from its receptors in mouse brains correlates well with their ability to stimulate mouse locomotion at these low concentrations.
(C) The concentration of cyclic AMP in target neurons in the human brain that leads to increased neuron firing can be produced by several different phosphodiesterase inhibitors in addition to caffeine.
(D) The concentration of caffeine required to dislodge adenosine from its receptors in the human brain is much greater than the concentration that produces behavioral stimulation in humans.
(E) The concentration of IBMX required to dislodge adenosine from its receptors in mouse brains is much smaller than the concentration that stimulates locomotion in the mouse.
The passage suggests that Snyder et al believe that if the older theory concerning caffeine’s effects were correct, which of the following would have to be the case?I. All neurotransmitters would increase the short-term concentration of cyclic AMP in target neurons.
II. Substances other than caffeine that inhibit the production of phosphodiesterase would be stimulants.
III. All concentration levels of caffeine that are high enough to produce stimulation would also inhibit the production of phosphodiesterase.
(A) I only
(B) I and II only
(C) I and III only
(D) II and III only
(E) I, II, and III
The last paragraph of the passage performs which of the following functions?(A) Describes a disconfirming experimental result and reports the explanation given by Snyder et al in an attempt to reconcile this result with their theory.
(B) Specifies the basis for the correlation observed by Snyder et al and presents an explanation in an attempt to make the correlation consistent with the operation of psychoactive drugs other than caffeine.
(C) Elaborates the description of the correlation observed by Snyder et al and suggests an additional explanation in an attempt to make the correlation consistent with the older theory.
(D) Reports inconsistent experimental data and describes the method Snyder et al will use to reanalyze this data.
(E) Provides an example of the hypothesis proposed by Snyder et al and relates this example to caffeine’s properties.
1.
D is the best answer. This question requires you to identify the primary concern of the passage as a
whole. The first paragraph presents a recent hypothesis about how caffeine affects behavior. The
second paragraph describes an earlier and widely accepted hypothesis about how caffeine affects
behavior, and then presents evidence that is not consistent with that hypothesis. The third and
fourth paragraphs return to the newer hypothesis introduced in the first paragraph and provide
“evidence and arguments” that support this alternative hypothesis.
2.
D is the best answer.
Lines 11-23 state that adenosine “depresses neuron firing” by binding to specific receptors on
neuronal membranes, which in turn inhibits the release of neurotransmitters. Lines 27-35 describe
Snyder et al’s hypothesis about caffeine. They propose that caffeine binds to specific receptors on
neuronal membranes, which prevents adenosine from binding to those receptors and “allows the
neurons to fire more readily that they otherwise would”. Therefore, according to Snyder et al,
caffeine differs from adenosine in that caffeine permits neurotransmitter release when it is bound
to adenosine receptors, whereas adenosine inhibits neruotransmitter release.
3.
A is the best answer. The effects of IBMX are discussed in the last paragraph of the passage.
IBMX apparently binds to adenosine-specific receptors on neuronal membranes, but, in contrast to
the other caffeine derivatives that Snyder et al experimented with, IBMX depresses rather than
stimulates mouse locomotion. Snyder et al respond to this experimental result by stating that
IBMX has “mixed effects in the brain, a not unusual occurrence with psychoactive drugs”(line
104-107)
4.
E is the best answer. This question asks you to identify which compound, according to Snyder et
al, does NOT bind to specific receptors in the brain. Phosphodiesterase, identified as an “enzyme
that breaks down the chemical called cyclic AMP”(lines 40-42), is the only compound that is not
identified as one that binds to specific receptors in the brain.
5.
B is the best answer.
This question asks you to identify information that is suggested rather than directly stated in the
passage. To answer it, first look for the location in the passage of the information specified in the
question. The A1 and A2 receptors are mentioned in lines 23-26. Lines 27-35 go on to describe
Snyder et al’s hypothesis about the effects of caffeine on behavior. They propose that caffeine,
“which is structurally similar to adenosine,” is able to bind to A1 and A2 receptors in the brain, the
same receptors that adenosine normally binds to. Thus, the passage suggests that the structural
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relationship between caffeine and adenosine may be partially responsible for caffeine’s ability to
bind to A1 and A2 receptors.
6.
B is the best answer.
This question asks you to identify the function of a quotation in the third paragraph of the passage.
The third paragraph provides evidence for Snyder et al’s hypothesis by discussing experiments
they conducted on mice. The quotation in lines 74-84 “summarizes” the findings of these
experiments. Snyder et al found that a number of caffeine derivatives are able to bind to specific
receptors in the brains of mice just as adenosine does, and that the derivatives that are most
successful at stimulating locomotion are also the most successful in competing with adenosine in
binding at the receptors. This finding is “major” in that it supports their hypothesis that the
simulative effects of caffeine are a result of its ability to compete with adenosine.
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Updated on: 06 Apr 2020, 21:27
45%
(03:27) wrong 
45%
(medium)
Here is my understanding of the passage. 
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