GMAT考试阅读资料(二)d

Passage 10
　　Caffeine， the stimulant¹ in coffee， has been called　 “the most widely used psychoactive substance on Earth .”　 Synder， Daly and Bruns have recently proposed that　 caffeine affects behavior by countering the activity in（5） the human brain of a naturally occurring chemical called　adenosine. Adenosine normally depresses neuron firing　in many areas of the brain. It apparently² does this by　inhibiting⁴ the release of neurotransmitters， chemicals　that carry nerve impulses from one neuron to the next.（10） Like many other agents that affect neuron firing，adenosine must first bind⁵ to specific receptors onneuronal membranes⁶. There are at least two classesof these receptors， which have been designated A1 andA2. Snyder et al propose that caffeine， which is struc-（15）　turally similar to adenosine， is able to bind to both typesof receptors， which prevents adenosine from attachingthere and allows the neurons to fire more readily thanthey otherwise would.
　　For many years， caffeine‘s effects have been attri-（20）　buted to its inhibition of the production of phosphodi-esterase， an enzyme⁷ that breaks down the chemicalcalled cyclic AMP.A number of neurotransmitters exerttheir effects by first increasing cyclic AMP concentra-tions in target neurons. Therefore， prolonged periods at（25）　the elevated concentrations， as might be brought aboutby a phosphodiesterase inhibitor， could lead to a greateramount of neuron firing and， consequently， to behav-ioral stimulation⁸. But Snyder et al point out that thecaffeine concentrations needed to inhibit³ the production（30）　of phosphodiesterase in the brain are much higher thanthose that produce stimulation. Moreover， other com-pounds that block phosphodiesterase’s activity are notstimulants.
　　To buttress¹⁰ their case that caffeine acts instead by pre-（35）　venting¹¹ adenosine binding¹²， Snyder et al compared thestimulatory effects of a series of caffeine derivatives¹⁴ withtheir ability to dislodge adenosine from its receptors inthe brains of mice. “In general，” they reported， “the ability of the compounds to compete at the receptors（40） correlates with their ability to stimulate¹⁵ locomotion¹⁶ in　 the mouse； i.e.， the higher their capacity to bind at thereceptors， the higher their ability to stimulate locomo-tion.” Theophylline， a close structural¹⁷ relative of caffeineand the major stimulant in tea， was one of the most（45） effective compounds in both regards.
　　There were some apparent exceptions to the generalcorrelation observed between adenosine-receptor bindingand stimulation. One of these was a compound called　3-isobuty1-1-methylxanthine（IBMX）， which bound very（50） well but actually depressed¹⁹ mouse locomotion. Snyderet al suggest that this is not a major stumbling block totheir hypothesis. The problem is that the compound hasmixed effects in the brain， a not unusual occurrence withpsychoactive drugs. Even caffeine， which is generally（55） known only for its stimulatory¹³ effects， displays thisproperty， depressing mouse locomotion at very low　concentrations and stimulating²⁰ it at higher ones.

　　1. The primary purpose of the passage is to
　　　（A） discuss a plan for investigation²¹ of a phenomenon　　 that is not yet fully²² 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. Which of the following， if true， would most weaken the　 theory proposed by Snyder et al？
　　　（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　　 phosphodi esterase 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 stimulates²³　　 locomotion in the mouse.

　　3. According so 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 inhibits²⁴ 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

　　4. In response to experimental results concerning IBMX，　Snyder et al contended that it is not uncommon²⁵ for　psychoactive drugs to have
　　　（A） mixed effects in the brain
　　　（B） inhibitory effects on enzymes²⁶ 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

　　5. 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 stimulants⁹.
　　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） Ⅰand Ⅲ only
　　　（D） Ⅱ and Ⅲ only
　　　（E） Ⅰ，Ⅱ，and Ⅲ

　　6. 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

　　7. Snyder et al suggest that caffeine‘s ability to bind to A1 and A2 receptors can be at least partially²⁷ 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

　　8. The author quotes Snyder et al in lines 38-43 most probably in order to
　　　（A） reveal some of the assumptions underlying²⁸ 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 correlations²⁹
　　　（E） refute the objections made by supporters of the older theory

　　9. 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） Specifies³⁰ the basis for the correlation¹⁸ 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.