咖啡因第2题它怎么就weaken了啊?

anjjule

如题, 大全里第10篇咖啡因的那个文章,好像还是OG10第一篇阅读, 第2题weaken题搜遍了CD没人问过, 求牛牛们帮忙给解释一下,D怎么就weaken了S等人的理论了呢?
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 affect 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 suggests 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.

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 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.这个是正确答案.

(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.

貌似D选项就是上面画线部分的改写阿!!!!
万分感谢您的关注和点拨!

小桃仁cherry

感谢nn分享，多读几遍真的理解了！

jianghxsir

其实，这题答案的weaken手法已经在原文中出现过，即‘Snyder et al point out that the caffeine concentrations needed to inhibit the production of phophodiesterase in the brain are much higher than those that produce stimulation’.和答案D的逻辑都是一个套路，即：还没达到你提出理论假设的条件时，就已经有刺激作用产生了，说明刺激的产生与该假设无关。

wjtwenzi

anjjule 发表于 2011-7-20 22:37
看到XDF课堂笔记上说"太难了,不要求做", 直接厥倒! 本来就做题做得愁苦满怀,还整个不要求做,看来什么事都不 ...

这题目我总结的时候都花了比一般题目两倍的功夫，XDF直接来一句太难了什么意思！有考虑过GMAT的感受吗？

wandering_wind

这篇真的好难 ls说的这个act instead by 是对第二段的回应，第二段说 caffeine's effect have been attribute to its inhibition of the production of phosphodiesterase. 然后第三段说 并不是inhibiting the production of phosphodiesterase 而是preventing adenosine binding

隽宝宝

我想问问文章中间的的To buttress their case that caffeine acts instead by preventing adenosine binding，是不是文章开头的内容意思说反了？CSnyder, Daly and Bruns have recently proposed that caffeine affect behavior by countering the activity in the human brain of a naturally occurring chemical called adenosine. 。。。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.
文章开头说这些人提出了种新观点caffeine通过和receptors结合阻碍adenosine和receptors结合，从而起到刺激作用，但是后面又说支持他们的case：咖啡因作用并非阻碍A的binding作用。这里是不是矛盾？

HQBDPP

太强大了。看了中文也不行。。不懂啊。。

birbird11

佩服佩服佩服

dayuyunhai

读懂文章最重要，一切套路和思路都是浮云。

-- by 会员 izhuimeng (2011/7/22 9:23:25)

说得好！不懂的文章正是进步的阶梯，抓住机会多读几遍，不要轻易放过。

powerlgx

终于明白了，cd上nn真是多，谢谢谢！！！