请教哪位牛牛给讲解一下GMAT OG阅读里的一篇文章？愁...
今天做了这片阅读以后，自己都掉眼泪了...太惨了，竟然6道题错了5道... 想请教哪位牛牛能受累讲讲你的分析过程，小女子叩谢了！！！就是下面这道caffeine的题：
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-l-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
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. 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. 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. 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. The author quotes Snyder et al in lines 38 - 43 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 今天太困了，明天中午以前把解题过程贴出来~上XDF第一篇阅读题，这么久了还记得挺深刻的~lz不必担心，刚做G阅读都是这样的;d: 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(Q2: adenosine的作用是inhibiting the release of neurotransmitters,但是caffeine会countering adenosine的activity.所以caffeine permits release of neurotransmitters). 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(Q5:因为caffeine跟adenosine在结构上的相似性,使得caffeine能bind to at least two classes of these 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(Q4的D选项,先提出理论,然后以theophylline为例进一步说明).(Q6:in general一词,本身就带有了总结的含义.在前面的文章里,提到了Snyder et al将mice进行实验的内容,这里一句in general然后引出一句总结性的话, the ability of the compounds to compete at the receptors correlates with their ability to stimulate locomotion in the mouse.就是总结他在实验中的一个发现) 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-l-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(Q3:IBMX在大脑里有mixed effects,这个mixed effects在psychoactive drugs中是not unusual, not unusual换个说法也可以是not uncommon). 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(文章讨论的就是一个对相关问题的研究) (B) present two explanations of a phenomenon and reconcile the differences between them(文章确实给出了两个方面的解释,但是并没有在两者之间进行比较) (C) summarize two theories and suggest a third theory that overcomes the problems encountered in the first two(没有出现第三种理论) (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(文章并没有对某一理论的真实性等进行挑战,是针对一个问题先提出了当前的研究结果,然后提出在此之前针对该问题的一个研究结论,但是文章花了更多的笔墨用事实来支撑现在对于该问题的研究情况和最终结论,不存在去挑战旧的研究成果的问题) 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. 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. According to Snyder et al, all of the following compounds can bind to specific receptors in the brain EXCEPT (A) IBMX(文章最后一段) (B) caffeine(文章第一段,both adenosine和caffeine都bind to receptors in the brain) (C) adenosine(文章第一段,both adenosine和caffeine都bind to receptors in the brain) (D) theophylline(解析内容在文章中) (E) phosphodiesterase (文中对phosphodiesterase是否与receptors相连没有做出说明) 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?(题目的大意是,caffeine的bind to A1,A2的能力来源于下列哪个方面的原因,或者是基于下列什么方面的原因,使caffeine能bind to A1,A2) (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. The author quotes Snyder et al in lines 38 - 43 most probably in order to (A) reveal some of the assumptions underlying their theory(没有some of assumptions吧,只是对实验情况做出了一个总结,而且理论也是包含在总结里的,先前根本没有提出理论部分) (B) summarize a major finding of their experiments(见原文处) (C) point out that their experiments were limited to the mouse(文中没有提及limit) (D) indicate that their experiments resulted only in general correlations(不是only in general correlations,而是针对实验结果的一个总结性的归纳) (E) refute the objections made by supporters of the older theory(文中一直没有提到objection的问题,谈何refute?)
红色为涉及该问题的原文和答案在文章中的体现,蓝色字体为解析,选项中的黑体为正确选项
[ 本帖最后由 Tyliayss 于 2008-4-10 11:26 编辑 ]
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