NO4.3 sectionA长阅读求助

ecustdreamerUID: 3396543

It has long been known that the rate of oxidative metabolism (the process that uses oxygen to convert food into energy) in any animal has a profound effect on its living patterns. The high metabolic rate of small animals, for example, gives them sustained power and activity per unit of weight, but at the cost of requiring constant consumption of food and water. Very large animals, with their relatively low metabolic rates, can survive well on a sporadic food supply, but can generate little metabolic energy per gram of body weight. If only oxidative metabolic rate is considered, therefore, one might assume that smaller, more active, animals could prey on larger ones, at least if they attacked in groups. Perhaps they could if it were not for anaerobic glycolysis, the great equalizer.
Anaerobic glycolysis is a process in which energy is produced, without oxygen, through the breakdown of muscle glycogen into lactic acid and adenosine triphosphate (ATP), the energy provider. The amount of energy that can be produced anaerobically is a function of the amount of glycogen present—in all vertebrates about 0.5 percent of their muscles’ wet weight. Thus the anaerobic energy reserves of a vertebrate are proportional to the size of the animal. If, for example, some predators had attacked a 100-ton dinosaur, normally torpid, the dinosaur would have been able to generate almost instantaneously, via anaerobic glycolysis, the energy of 3,000 humans at maximum oxidative metabolic energy production. This explains how many large species have managed to compete with their more active neighbors: the compensation for a low oxidative metabolic rate is glycolysis.
There are limitations, however, to this compensation. The glycogen reserves of any animal are good, at most, for only about two minutes at maximum effort, after which only the normal oxidative metabolic source of energy remains. With the conclusion of a burst of activity, the lactic acid level is high in the body fluids, leaving the large animal vulnerable to attack until the acid is reconverted, via oxidative metabolism, by the liver into glucose, which is then sent (in part) back to the muscles for glycogen resynthesis. During this process the enormous energy debt that the animal has run up through anaerobic glycolysis must be repaid, a debt that is proportionally much greater for the larger vertebrates than for the smaller ones. Whereas the tiny shrew can replace in minutes the glycogen used for maximum effort, for example, the gigantic dinosaur would have required more than three weeks. It might seem that this interminably long recovery time in a large vertebrate would prove a grave disadvantage for survival. Fortunately, muscle glycogen is used only when needed and even then only in whatever quantity is necessary. Only in times of panic or during mortal combat would the entire reserves be consumed.


对比以下2题
20.        The primary purpose of the passage is to
错选A，答案E
文中并未提及misconception，而E选项是主线，20题理解答案没问题，但是将以下两题对比之后觉得有矛盾。
也在矛盾之下随即开始YY，导致错选A
(A) refute a misconception about anaerobic glycolysis
(B) introduce a new hypothesis about anaerobic glycolysis
(C) describe the limitations of anaerobic glycolysis
(D) analyze the chemistry of anaerobic glycolysis and its similarity to oxidative metabolism
(E) explain anaerobic glycolysis and its effects on animal survival
27.        Which of the following best states the central idea of the passage?
错选C，答案A
(A) The disadvantage of a low oxidative metabolic rate in large animals can be offset by their ability to convert substantial amounts of glycogen into energy.
(B) The most significant problem facing animals that have used anaerobic glycolysis for energy is the resynthesis of its by-product, glucose, into glycogen.
(C) The benefits to animals of anaerobic glycolysis are offset by the profound costs that must be paid.
(D) The major factor ensuring that a large animal will triumph over a smaller animal is the large animal’s ability to produce energy via anaerobic glycolysis.
(E) The great differences that exist in metabolic rates between species of small animals and species of large animals can have important effects on the patterns of their activities.
理由：本文set 一个background即有氧呼吸和无氧糖酵解是相平衡的，目的是为了阐述无氧呼吸及其自身的平衡方法即，无氧呼吸之后造成乳酸的堆积，使动物vulnerable。故20选E是符合大意的，但较之27题，似乎27说的文章大意却将background里的平衡作为大意，有矛盾啊。想不通。至少觉得27题不对。
求大大们指教。

crazyrobinUID: 2368627

如果你对主旨题很拿不准的话那么给每个段落总结大意，然后看权重。

第一段介绍了oxidative metabolism对于小型动物的优势，大型动物劣势，同时引出anaerobic glycolysis这个概念

第二段anaerobic glycolysis作用原理并分析anaerobic glycolysis对大型动物的劣势进行补偿

第三段anaerobic glycolysis这种补偿的限制性并指出这种情况发生的可能性并不大。

结合看一下，二三段都是 anaerobic glycolysis 所以权重是2， oxidative metabolism权重是1

2段讲了 anaerobic glycolysis，所以anaerobic glycolysis是重点，anaerobic glycolysis有限制性，又可以起到offset作用，

C选项

The benefits to animals of anaerobic glycolysis are offset by the profound costs that must be paid

是第三自然段的内容
所以是错误的

ecustdreamerUID: 3396543

crazyrobin 发表于 2013-1-24 11:55
如果你对主旨题很拿不准的话那么给每个段落总结大意，然后看权重。

第一段介绍了oxidative metabolism对 ...

C选项确实有点located的感觉，文中是在说明oxidative metabolism 和anaerobic glycolysis的平衡，而后文为了补充说明details about anaerobic glycolysis讲anaerobic glycolysis的自身平衡所以C是locate的意思，而global是A，所以第一段并不是起到background的作用，引出下文，而是总领全文，而本身文章就是在阐述这两种代谢之间平衡，这样的理解对吗？

crazyrobinUID: 2368627

ecustdreamer 发表于 2013-1-24 12:15
C选项确实有点located的感觉，文中是在说明oxidative metabolism 和anaerobic glycolysis的平衡，而后文为 ...

第一段可以算作是背景，但是第一段讲了oxidative metabolism的机理和对大型动物的影响，所以在主旨里这应该也会写出来。

反映在A选项的就是The disadvantage of a low oxidative metabolic rate