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 gen- erate little metabolic energy per gram of body weight. If only oxidative metabolic rate is considered， there- fore， 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 glcolysis is a process in which energy is produced， without oxygen， through the breakdown of muscle glycogen into lactic acid and adenosine tri- phosphate (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 verte- brate 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 compensa- tion. 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 tthe 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 resyn- thesis. 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 inter- minably 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.
1. What is the text mainly about?。
[A] refute a misconception about anaerobic glycolysis.
[B] introduce a new hypothesis about anaerobic glycolysis.
[C] describe the limitations of anaerobic glycolysis.
[D] explain anaerobic glycolysis and its effects on animal survival.
2. According to the author， glycogen is crucial to the process of anaerobic glyrolysis 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] is the material form which ATP is derived.
3. It is implied that the total anaerobic energy reserves of a vertebrate are proportional to its size because
[A] larger vertebrate 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.
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