细胞和温度(Cells and Temperature)

cellscannotremainaliveoutsidecertainlimitsoftemperatureandmuchnarrowerlimitsmarktheboundariesofeffectivefunctioning.enzymesystemsofmammalsandbirdsaremostefficientonlywithinanarrowrangearound37c;adepartureofafewdegreesfromthisvalueseriouslyimpairstheirfunctioning.

eventhoughcellscansurvivewiderfluctuationstheintegratedactionsofbodilysystemsareimpaired.otheranimalshaveawidertoleranceforchangesofbodilytemperature.

forcenturiesithasbeenrecognizedthatmammalsandbirdsdifferfromotheranimalsinthewaytheyregulatebodytemperature.waysofcharacterizingthedifferencehavebecomemoreaccurateandmeaningfulovertime,butpopularterminologystillreflectstheolddivisioninto"warm-blooded"and"cold-blooded"species;warm-bloodedincludedmammalsandbirdswhereasallothercreatureswereconsideredcold-blooded.

asmorespecieswerestudied,itbecameevidentthatthisclassificationwasinadequate.afencelizardoradesertiguana-eachcold-blooded----usuallyhasabodytemperatureonlyadegreeortwobelowthatofhumansandsoisnotcold.

thereforethenextdistinctionwasmadebetweenanimalsthatmaintainaconstantbodytemperature,calledhome0therms,andthosewhosebodytemperaturevarieswiththeirenvironments,calledpoikilotherms.butthisclassificationalsoprovedinadequate,becauseamongmammalstherearemanythatvarytheirbodytemperaturesduringhibernation.

furthermore,manyinvertebratesthatliveinthedepthsoftheoceanneverexperiencechangeinthedepthsoftheoceanneverexperiencechangeinthechillofthedeepwater,andtheirbodytemperaturesremainconstant.