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  3. Cells and Temperature Cells cannot remain alive outside certain limits of temperature, and much narrower limits mark the bou

Cells and Temperature Cells cannot remain alive outside certain limits of temperature, and much narrower limits mark the bou

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问题     Cells and Temperature
    Cells cannot remain alive outside certain limits of temperature, and much narrower limits mark the boundaries of effective functioning. Enzyme (酶) systems of mammals and birds are most efficient only within a narrow range around 37℃; a departure of a few degrees from this value seriously impairs their functioning. Even though cells can survive wider fluctuations, the integrated actions of bodily systems are impaired. Other animals have a wider tolerance for changes of bodily temperature.
    For centuries it has been recognized that mammals and birds differ from other animals in the way they regulate body temperatures. Ways of characterizing the difference have become more accurate and meaningful over time, but popular terminology still reflects the old division into "warm-blooded" and "cold-blooded" species; warm-blooded included mammals and birds, whereas all other creatures were considered cold-blooded. As more species were studied, it became evident that this classification was inadequate. A fence lizard or a desert iguana (鬣蜥)—each cold-blooded—usually has a body temperature only a degree or two below that of humans and so is not cold. Therefore the next distinction was made between animals that maintain a constant body temperature, called homotherms(同温动物) ,and those whose body temperature varies with their environment, called poikilotherms (变温动物). But this classification also proved inadequate, because among mammals there are many that vary their body temperatures during hibernation. Furthermore, many invertebrates (无脊椎动物) that live in the depths of the ocean never experience a change in the chill of the deep water, and their body temperatures remain constant.
    The current distinction is between animals whose body temperature is regulated chiefly by internal metabolic processes and those whose temperature is regulated by, and who get most of their heat from, the environment. The former are called endotherms (恒温动物) ,and the latter are called ectotherms (外温动物). Most ectotherms do regulate their body temperature, and they do so mainly by locomoting to favorable sites or by changing their exposure to external sources of heat. Endotherms (mainly mammals and birds) also regulate their temperature by choosing favorable environments, but primarily they regulate their temperatures by making a variety of internal adjustments.  
The passage mainly discusses______.
选项 A、body temperatures of various animals
B、the newest research on measuring temperature
C、methods of temperature reduction
D、the classification of animals by temperature regulation
答案D
解析 主旨大意题。本题问本文主要讨论了什么问题。从第二段和第三段的第一句可以看出，这两段讨论的都是如何划分动物的，而且这两段中出现频率最高的词都与differ或distinction有关。第一段则出现了诸多有关温度的语句。A只说到了主旨的一部分；B文中虽提到有关体温测量的研究，但未说是最新研究；C说降温的诸多方法，似乎也不妥当。D的概括性最强，故选D。
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