sample essays of IELTS

50 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 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 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.