Board logo

标题: Cells and Temperature [打印本页]

作者: 幽幽草    时间: 2007-4-18 17:23     标题: 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 37Ca 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 intowarm-bloodedandcold-bloodedspecies 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 iguanaeach 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 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 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.




欢迎光临 英语听力论坛 (http://bbs.tingroom.com./) Powered by Discuz! 7.2