新概念第四册+Lesson+68+Bacteria+讲义.docxVIP

新概念第四册lesson69讲义

Bacteria

Bacteriaareextremelysmalllivingthings.Whilewemeasureourownsizesininchesorcentimeters,bacterial1sizeismeasuredinmicrons.Onemicronisathousandthofamillimeter:apinheadisaboutamillimeteracross.Rod-shapedbacteriaareusuallyfromtwotofourmicronslong,whileroundedonesaregenerallyonemicronindiameter.Thusifyouenlargedaroundedbacterium2athousandtimes,itwouldbejustaboutthesizeofapinhead.Anadulthumanmagnifiedbythesameamountwouldbeoveramile(1.6kilometer)tall.

Evenwithanordinarymicroscope,youmustlookcloselytoseebacteria.Usingamagnificationof100times,onefindsthatbacteriaarebarelyvisibleastinyrodsordots.Onecannotmakeoutanythingoftheirstructure.Usingspecialstains,onecanseethatsomebacteriahaveattachedtothemwavy-looking“hairs”calledflagella3.Othershaveonlyoneflagellum.Theflagellarotate,pushingthebacteriathroughthewater.Manybacterialackflagellaandcannotmoveaboutbytheirownpower,whileotherscanglide4alongoversurfacesbysomelittle-understoodmechanism5.

Fromthebacteriapointofview,theworldisaverydifferentplacefromwhatitistohumans.Toabacteriumwaterisasthickasmolassesistous.Bacteriaaresosmallthattheyareinfluencedbythemovementsofthechemicalmolecules6aroundthem.Bacteriaunderthemicroscope,eventhosewithnoflagella,oftenbounceaboutinthewater.Thisisbecausetheycollidewiththewatery7moleculesandarepushedthiswayandthat.Moleculesmovesorapidlythatwithinatenthofasecondthemoleculesaroundabacteriahaveallbeenreplacedbynewones;evenbacteriawithoutflagellaarethusconstantlyexposedtoachangingenvironment.

细菌细菌是极其微小的生物体。我们用英寸或厘米来测量自己的大小，而测量细菌却要用微米。一微米等于千分之一毫米。针头直径大约一毫米。

棒状细菌通常有2～4微米长，而圆形细菌的直径一般只有1微米。因此，即使你把一个圆形细菌放大1000倍，它也不过一个针头那么大。

可是如果把一个成年人放大1000倍，就会变成1英里(或1.6公里)多高。用一般的显微镜观察细菌时，你必须仔细观察才能看见它们。使用

100倍的显微