老G阅读一篇，求助central idea和primary purpose的区别在哪呢，请不吝指教！！

无敌浩克OneUID: 3605802

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.

27.        Which of the following best states the central idea of the passage?
Look at the structure sentences of second and third paragraphs.
The passage first talks about Anaerobic glycolysis, then talks about the limitation and compensation for this limitation.
(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.

(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.

第一段上来就说犹豫新陈代谢速度不一样所以生活方式不同，于是第一段是为了引出新陈代谢速率低的大型动物是如何弥补自己的这个缺陷（第三段）的吗？
可是这篇阅读第一题，20.        The primary purpose of the passage is to
(E) explain anaerobic glycolysis and its effects on animal survival
又选的E啊。。。
central idea和primary purpose的区别在哪呢？？我感觉确实不可能考两个一模一样的点，可是区别又在哪呢？

yukie小丑猪UID: 3517895

举个栗子。。。比如我现在用一段很长很长的话回答了你的问题，然后有个人没耐心看帖了，于是直接问你我这个回帖的primary purpose是什么，你会说“她回答了我的问题”，然后别人再问这个回帖的central idea是什么，你会说“她解释了primary purpose和central idea的区别是什么”。。。

说的有点混乱啊，反正感觉就是，primary purpose是高度概括写作目的（解释现象，提出质疑，比较两个东西。。。），是一个关于“why有这篇文章”的问题；central idea相当于文章的中心论点，是在问“这篇文章说了what”。大概是这种的感觉吧。。。

无敌浩克OneUID: 3605802

yukie小丑猪 发表于 2015-5-20 14:17
举个栗子。。。比如我现在用一段很长很长的话回答了你的问题，然后有个人没耐心看帖了，于是直接问你我这个 ...

理解了！！谢谢！！
一个侧重purpose or aim
一个侧重idea or  content

第八题是问文章具体说了什么，两种代谢→第一题是问说这些的目的是什么，就是解释为什么会有差别