解放军文职招聘考试Bacteria

  Bacteria

Bacteria are extremely small  living things. While we  measure our  own

sizes in  inches or centimeters, bacterial size is measured in microns. One

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 kilometers) tall.

Even  with  an  ordinary  microscope,  you   must  look  closely  to  see 

bacteria.  Using  a magnification of 100 times, one    finds that bacteria are

barely visible as tiny rods or dots. One cannot make out anything of their

structure. Using special stains, one can see that some bacteria have

attached to them wavy-looking "hairs" called flagella. Others have only one

flagellum. The flagella rotate, pushing the bacteria through the water.

Many bacteria lack flagella    and cannot move  about   by  their   own 

power,  while  others   can  glide   along  over  surfaces  by  some

little-understood    mechanism.

From the bacterial point of view, the world is a very different place from

what it is to humans. To a bacterium   water is as thick as  molasses is to

us. Bacteria are so small that they are influenced  by  the  movements  of 

the    chemical  molecules  around  them.  Bacteria  under  the microscope,

even those with no flagella, often bounce about in       the water. This is

because they collide with the water molecules and are pushed this way and

that. Molecules move      so rapidly that within a tenth of a second the

molecules around a bacterium have all been replaced by new ones;       even

bacteria without flagella are thus constantly exposed to a changing

environment.

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 倍的显微镜 时，你会发现细菌不过是隐约可见的小细棒或小点点，而它们的结构你却根本看不出来。 使

用特殊的着色剂后，你会发现有的细菌上长着不少波状的"毛发"即鞭毛，而有的细菌只有一 根鞭毛。  鞭毛的旋转可以推动细菌在水中行进。 

不少细菌没有鞭毛，因而不能自己行进。 还有些细菌却能通过某些鲜为人知的机制沿物体表面滑动。  我们所熟知的世界在细菌眼中 完全是另一个样子。 

对于细菌来说，水就同糖浆之于人类一样稠密。  细菌是如此的微小， 周围化学分子的一举一动都会对它们产生影响。  在显微镜下，细菌，甚至包括那些没有鞭

毛的细菌，经常在水中跳来跳去。  这是因为它们与水分子相撞后，被弹向各个方向。  分子 移动很迅速，仅  0.1 

秒之隔，一个细菌周围的分子就会完全更新。  因此，即使是没有鞭毛 的细菌也暴露在一个不断变化的环境中。