背诵美文——细胞与温度

Cells and Temperature

eas 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.

细胞只能在肯定的温度范围内存活，而进一步保证它们有效工作的温度范围就更小了。哺乳动物和鸟类的酶系统只能在37℃左右的非常小范围内才能有效工作。与此相差仅几度的温度都会大大削弱它们的工作效率。 尽管温度变化更大时细胞仍能存活，但机体系统的整体运行能力却被削弱了。 其它动物对体温的变化有更强的适应性。 几个世纪以来，大家就认识到哺乳动物和鸟类调节体温的方法与其它动物不同。 伴随时间的推移，大家对这种差异的描述愈加精确和有意义，但"暖血动物"和"冷血动物"这一古老的分类方法到今天仍在大众词语中有所反映。 暖血动物包含哺乳动物和鸟类，其它动物统统被视为冷血动物。 但对更多物种进行的研究表明这种分类显然是不适合的。 美洲一种小型蜥蜴和沙漠鬣蜥同属冷血动物，但事实上它们的体温一般只比人类的体温低 1～2 度，因此并非真的的冷血。因此又出现了恒温动物和变温动物这一区别方法。 但这种分类也不适合。 由于有不少哺乳动物在冬眠期间会改变 体温，而很多生活在深海的无脊椎动物在寒冷的深海水域中体温并不变化，而是恒定的。

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