GRE閱讀題型分類——復(fù)述題

    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.
21. According to the author, glycogen is crucial to the process of anaerobic glycolysis because glycogen
　　(A) increases the organism’s need for ATP
　　(B) reduces the amount of ATP in the tissues
　　(C) is an inhibitor of the oxidative metabolic production of ATP
　　(D) ensures that the synthesis of ATP will occur speedily
　　(E) is the material from which ATP is derived
　　22. According to the author, a major limitation of anaerobic glycolysis is that it can
　　(A) produce in large animals more lactic acid than the liver can safely reconvert
　　(B) necessitate a dangerously long recovery period in large animals
　　(C) produce energy more slowly than it can be used by large animals
　　(D) consume all of the available glycogen regardless of need
　　(E) reduce significantly the rate at which energy is produced by oxidative metabolism
　　23. The passage suggests that the total anaerobic energy reserves of a vertebrate are proportional to the vertebrate’s size because
　　(A) larger vertebrates conserve more energy than smaller vertebrates
　　(B) larger vertebrates use less oxygen per unit weight than smaller vertebrates
　　(C) the ability of a vertebrate to consume food is a function of its size
　　(D) the amount of muscle tissue in a vertebrate is directly related to its size
　　(E) the size of a vertebrate is proportional to the quantity of energy it can utilize

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