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1、新东方优美背诵短文50篇 Unit50:CellsandTemperature50. Cells and Temperature 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 37C;a depar
2、ture 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
3、 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 mamm
4、als 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 iguanaeach cold-blooded-usually has a body temperature only a degree or two below that of humans and so is not c
5、old. Therefore the next distinction was made between animals that maintain a constant body temperature, called home0therms, and those whose body temperature varies with their environments, called poikilotherms. But this classification also proved inadequate, because among mammals there are many that
6、 vary their body temperatures during hibernation. Furthermore, many invertebrates that live in the depths of the ocean never experience change in the depths of the ocean never experience change in the chill of the deep water, and their body temperatures remain constant. 细胞与温度细胞只能在一定的温度范围内存活，而进一步保证它们
7、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左右的很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，人们就认识到哺乳动物和鸟类调节体温的方式与其它动物不同。随着时间的推移，人们对这种差异的描述越来越精确和有意义，但是暖血动物和冷血动物这一古老的分类方式至今仍在大众词汇中有所反映。暖血动物包括哺乳动物和鸟类，其它动物统统被视为冷血动物。但是对更多物种进行的研究表明这种分类显然是不适当的。美洲一种小型蜥蜴和沙漠鬣蜥同属冷血动物，但实际上它们的体
8、温通常只比人类的体温低12度，因此并不是真正的冷血。因此又出现了恒温动物(即保持恒定体温的动物)和变温动物(即体温随外界环境的变化而改变的动物)这一区分方式。但这种分类也不恰当。因为有不少哺乳动物在冬眠期间会改变体温，而许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。?新东方优美背诵短文50篇 Unit49:Sleep49 Sleep Sleet is part of a persons daily activity cycle. There are several different stages of sleep, and they too occur in cycl
9、es. If you are an average sleeper, your sleep cycle is as follows. When you fist drift off into slumber, your eyes will roll about a bit, you temperature will drop slightly, your muscles will relax, and your breathing well slow and become quite regular. Your brain waves slow and become quite regular
10、. Your brain waves slow down a bit too, with the alpha rhythm of rather fast waves 1 sleep. For the next half hour or so, as you relax more and more, you will drift down through stage 2 and stage 3 sleep. The lower your stage of sleep. slower your brain waves will be. Then about 40to 69 minutes afte
11、r you lose consciousness you will have reached the deepest sleep of all. Your brain will show the large slow waves that are known as the delta rhythm. This is stage 4 sleep. You do not remain at this deep fourth stage all night long, but instead about 80 minutes after you fall into slumber, your bra
12、in activity level will increase again slightly. The delta rhythm will disappear, to be replaced by the activity pattern of brain waves. Your eyes will begin to dart around under your closed eyelids as if you were looking at something occurring in front of you. This period of rapid eye movement lasts
13、 for some 8 to 15 minutes and is called REM sleep. It is during REM sleep period, your body will soon relax again, your breathing will slip gently back from stage 1 to stage 4 sleep-only to rise once again to the surface of near consciousness some 80 minutes later.睡眠睡眠是人每天日常活动循环的一部分。人的睡眠分几个阶段，而这些阶段也
14、是循环发生的。如果你是一个正常的睡眠者，你的睡眠循环会这样进行。在你开始昏昏入睡时，你的眼睛会滚动几下，体温略有下降，肌肉放松，呼吸变得缓慢而有节奏。除了开始几分钟比较快的节奏外，脑电波也稍有减缓。这被称为第一阶段睡眠。在随后约半小时内，你进一步放松，进入第二和第三阶段睡眠。睡眠越深入，脑电波就越缓慢。大约在开始睡眠后的40到60分钟，你将进入沉睡状态。这时的脑电波表现为巨大的缓波，被称为节奏。这就是第四阶段睡眠。但你并不是整夜都保持这种沉睡状态。入睡后约80分钟左右，你的大脑运动水平会再度略有提高。节奏消失，并被脑电波的运动图形取代。你的眼睛会在闭着的眼睑下迅速转动，就好象你在看着眼前发生的
15、什么事情。这种迅速的眼球运动持续约815分钟，这一阶段睡眠被称之为快速眼动(REM)睡眠。在REM睡眠阶段，你的肢体会很快再度放松，呼吸也再次放慢并变得有节奏，你会轻松地从第一阶段滑入第四阶段睡眠-直到大约80分钟后重新接近清醒状态。?新东方优美背诵短文50篇 Unit48:Bacteria 48 Bacteria Bacteria are extremely small living things. While we measure our own sizes in inches or centimeters, bacterial size is measured in microns. O
16、ne micron is a thousandth of a millimeter: a pinhead is about a millimeter across. Rod-shaped bacteria are usually from two to four microns long, while rounded ones are generally one micron in diameter. Thus if you enlarged a rounded bacterium a thousand times, it would be just about the size of a pinhead. An adult human magnified by the same amount would be over a mile(1.6 kilometer) tall. Even w
