公共英語四級閲讀文章《細胞與温度》

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Cells cannot remain alive outside certain limits of temperature, and much narrower limits mark the boundaries of effective functioning. Enzyme 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 temperature. 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 —— 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 homeotherms, and those whose body temperature varies with their environment, called poikilotherms. But this classification also proved inadequate, because among mammals there are many that vary their body temperatures during hibernation. Furthermore, many invertebrates 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.

細胞只能在一定的温度範圍內存活，而進一步保證它們有效工作的温度範圍就更小了。哺乳動物和鳥類的酶系統只能在 37℃左右的很小範圍內才能有效工作。與此相差僅幾度的 温度都會大大削弱它們的工作效率。儘管温度變化更大時細胞仍能存活，但機體系統的整 體運行能力卻被削弱了。其它動物對體温的變化有更強的適應性。幾個世紀以來，人們就認識到哺乳動物和鳥類調節體温的方式與其它動物不同。隨着時間的推移，人們對這種差 異的描述越來越精確和有意義，但是"暖血動物"和"冷血動物"這一古老的分類方式至今仍在 大眾詞彙中有所反映。暖血動物包括哺乳動物和鳥類，其它動物統統被視為冷血動物。但是對更多物種進行的研究表明這種分類顯然是不適當的。美洲一種小型蜥蜴和沙漠鬣蜥同 屬冷血動物，但實際上它們的體温通常只比人類的.體温低 1～2 度，因此並不是真正的冷血。因此又出現了恆温動物(即保持恆定體温的動物)和變温動物(即體温隨外界環境的變化而改變的動物)這一區分方式。但這種分類也不恰當。因為有不少哺乳動物在冬眠期間會改變 體温，而許多生活在深海的無脊椎動物在寒冷的深海水域中體温並不變化，而是恆定的。