GMAT阅读资料第10篇
F*** many years, caffe***e's effects h**e been attri- (20) but*** *** its ***hibition of the production of phosphodi- esterase, an enzy*** that breaks down the chem***al call*** cyc***c AMP.A number of ***urotransmitters exert their effects by first ***creas***g cyc***c AMP ***ncentra- tions *** target ***urons. Theref***e, prolong*** periods at (25) the elevat*** ***ncentrations, as might be brought about by a phosphodiesterase ***hibi***r, ***uld lead *** a greater amount of ***uron fir***g and, ***nsequently, *** beh**- i***al stimulation. But Snyder et al po***t out that the caffe***e ***ncentrations ********* *** ***hibit the production (30) of phosphodiesterase *** the bra*** are much higher than those that produce stimulation. M***eover, other ***m- pounds that block phosphodiesterase's acti***ty are not stimulants.
To buttress their case that caffe***e acts ***stead by pre- (35) vent***g adenos***e b***d***g, Snyder et al ***mpar*** the stimula***ry effects of a series of caffe***e derivatives with their abi***ty *** dislodge adenos***e from its recep***rs *** the bra***s of m***e. "In ge***ral," they rep***t***, "the abi***ty of the ***mpounds *** ***mpete at the recep***rs (40) ***rrelates with their abi***ty *** stimulate lo***motion *** the mouse; i.e., the higher their capacity *** b***d at the recep***rs, the higher their abi***ty *** stimulate lo***mo- tion." Theophyl******, a close structural relative of caffe***e and the maj*** stimulant *** tea, was o*** of the most (45) effective ***mpounds *** both regards.
There were so*** apparent exceptions *** the ge***ral ***rrelation observ*** between adenos***e-recep***r b***d***g and stimulation. O*** of these was a ***mpound call*** 3-isobuty1-1-***thylxanth***e(IBMX), wh***h bound very (50) well but actually depress*** mouse lo***motion. Snyder et al suggest that this is not a maj*** stumb***ng block *** their hypothesis. The problem is that the ***mpound has mix*** effects *** the bra***, a not unusual o***urrence with psychoactive drugs. Even caffe***e, wh***h is ge***rally (55) known o***y f*** its stimula***ry effects, displays this property, depress***g mouse lo***motion at very low ***ncentrations and stimulat***g it at higher o***s.
1. The primary purpose of the passage is ***
(A) discuss a plan f*** ***vestigation of a pheno***non that is not yet fully unders***od
(B) present two explanations of a pheno***non and re***ncile the differences between them
(C) summarize two the***ies and suggest a third the***y that over******s the problems en***unter*** *** the first two
(D) describe an a***ernative hypothesis and pro***de e***dence and argu***nts that supp***t it
(E) challenge the va***dity of a the***y by expos***g the ******nsistencies and ***ntrad***tions *** it
2. Wh***h of the follow***g, if true, would most weaken the the***y propos*** by Snyder et al?
(A) At very low ***ncentrations *** the human bra***. both caffe***e and theophyl****** tend *** h**e depressive rather than stimula***ry effects on human beh**i***.
(B) The abi***ty of caffe***e derivatives at very low ***ncentrations *** dislodge adenos***e from its recep***rs *** mouse bra***s ***rrelates well with their abi***ty *** stimulate mouse lo***motion at these low ***ncentrations
(C) The ***ncentration of cyc***c AMP *** target ***urons *** the human bra*** that leads *** ***creas*** ***uron fir***g can be produc*** by several different phosphodi esterase ***hibi***rs *** addition *** caffe***e.
(D) The ***ncentration of caffe***e requir*** *** dislodge adenos***e from its recep***rs *** the human bra*** is much greater than the ***ncentration that produces beh**i***al stimulation *** humans.
(E) The ***ncentration of IBMX requir*** *** dislodge adenos***e from its recep***rs *** mouse bra***s is much smaller than the ***ncentration that stimulates lo***motion *** the mouse.
3. A******d***g so Snyder et al, caffe***e differs from adenos***e *** that caffe***e
(A) stimulates beh**i*** *** the mouse and *** humans, whereas adenos***e stimulates beh**i*** *** humans o***y
(B) has mix*** effects *** the bra***, whereas adenos***e has o***y a stimula***ry effect
(C) ***creases cyc***c AMP ***ncentrations *** target ***urons, whereas adenos***e decreases such ***ncentrations
(D) permits release of ***urotransmitters when it is bound *** adenos***e recep***rs, whereas adenos***e ***hibits such release
(E) ***hibits both ***uron fir***g and the production of phosphodiesterase when there is a suff***ient ***ncentration *** the bra***, whereas adenos***e ***hibits o***y ***uron fir***g
4. In response *** experi***ntal resu***s ***ncern***g IBMX, Snyder et al ***ntend*** that it is not un***mmon f*** psychoactive drugs *** h**e
(A) mix*** effects *** the bra***
(B) ***hibi***ry effects on enzy***s *** the bra***
(C) close structural relationships with caffe***e
(D) depressive effects on mouse lo***motion
(E) the abi***ty *** dislodge caffe***e from recep***rs *** the bra***
5. The passage suggests that Snyder et al be***eve that if the older the***y ***ncern***g caffe***e's effects were ***rrect, wh***h of the follow***g would h**e *** be the case?
Ⅰ.All ***urotransmitters would ***crease the ***rt-term ***ncentration of cyc***c AMP *** target ***urons.
Ⅱ.Substances other than caffe***e that ***hibit the production of phosphodiesterase would be stimulants.
Ⅲ.All ***ncentration levels of caffe***e that are high enough *** produce stimulation would also ***hibit the production of phosphodiesterase.
(A) Ⅰ o***y
(B) Ⅰ and Ⅱ o***y
(C) Ⅰand Ⅲ o***y
(D) Ⅱ and Ⅲ o***y
(E) Ⅰ,Ⅱ,and Ⅲ
6. A******d***g *** Snyder et al, all of the follow***g ***mpounds can b***d *** specif*** recep***rs *** the bra*** EXCEPT
(A) IBMX
(B) caffe***e
(C) adenos***e
(D) theophyl******
(E) phosphodiesterase
7. Snyder et al suggest that caffe***e's abi***ty *** b***d *** A1 and A2 recep***rs can be at least partially attribut*** *** wh***h of the follow***g?
(A) The chem***al relationship between caffe***e and phosphodiesterase
(B) The structural relationship between caffe***e and adenos***e
(C) The structural similarity between caffe***e and ***urotransmitters
(D) The abi***ty of caffe***e *** stimulate beh**i***
(E) The natural o***urrence of caffe***e and adenos***e *** the bra***
8. The ***th*** quotes Snyder et al *** ******s 38-43 most probably *** ***der ***
(A) reveal so*** of the assumptions underly***g their the***y
(B) summarize a maj*** f***d***g of their experi***nts
(C) po***t out that their experi***nts were ***mit*** *** the mouse
(D) ***d***ate that their experi***nts resu****** o***y *** ge***ral ***rrelations
(E) refute the objections made by supp***ters of the older the***y
9. The last paragraph of the passage perf***ms wh***h of the follow***g functions?
(A) Describes a dis***nfirm***g experi***ntal resu*** and rep***ts the explanation given by Snyder et al *** an attempt *** re***ncile this resu*** with their the***y.
(B) Specifies the basis f*** the ***rrelation observ*** by Snyder et al and presents an explanation *** an attempt *** make the ***rrelation ***nsistent with the operation of psychoactive drugs other than caffe***e.
(C) Elab***ates the description of the ***rrelation observ*** by Snyder et al and suggests an additional explanation *** an attempt *** make the ***rrelation ***nsistent with the older the***y.
(D) Rep***ts ******nsistent experi***ntal data and describes the ***thod Snyder et al will use *** reanalyze this data.
(E) Pro***des an example of the hypothesis propos*** by Snyder et al and relates this example *** caffe***e's properties.
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