高频再现指数☆☆☆☆
阅读时长预计：8分钟
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 tour microns long, while rounded ones are generally one micron in diameter Thus if you enlarged a founded 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 though 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 Molecules move so rapidly that within a tenth of a second the molecules around a bacterium have all been replaced by new ones even bacteria without flagella are thus constantly exposed to a changing environment.
1.Which of the following is the main topic of the passage?
A.The characteristics of bacteria
B.How bacteria reproduce
C.The various functions of bacteria
D.How bacteria contribute to disease
2.Bacteria are measured in
A.inches
B.centimeters
C.microns
D.millimeters
3.Which of the following is the smallest?
A.A pinhead
B.A rounded bacterium
C.A microscope
D.A rod-shaped bacterium
4.According to the passage, someone who examines bacteria using only a microscope that maanifies 100 times would see
A.tiny dots
B.small "hairs"
C.large rods
D.detailed structures
5.The relationship between a bacterium and its flagella is most nearly analogous to which of the following?
A.A rider jumping on a horse's back
B.A ball being hit by a bat
C.A boat powered by a motor
D.A door closed by a gust of wind
6.In line 1 6, the author compares water to molasses, in order to introduce which of the following topics?
A.The bacterial content of different liquids
B.What happens when bacteria are added to molasses
C.The molecular structures of different chemicals
D.How difficult it is for bacteria to move through water
参考答案 A C B A C D
参考翻译：
细菌是极小的生物。我们的尺寸通常以英寸或厘米来衡量，而细菌的大小以微米为单位。一微米等于千分之一毫米，一个针尖大约有一毫米宽。杆状细菌通常长约两到四微米，而圆形的细菌通常直径约为一微米。因此，如果将一个圆形细菌放大一千倍，它的大小将大约与一个针尖相当。同样放大一千倍的成年人将会高达一英里（1.6公里）。
即使通过普通显微镜，你也必须仔细观察才能看到细菌。使用100倍放大率，你会发现细菌只能看到微小的杆状或点状。你无法分辨出它们的结构。使用特殊染色剂，你可以看到一些细菌表面附着着一些波浪状的“毛”，称为鞭毛。其他一些只有一个鞭毛。这些鞭毛会旋转，推动着细菌穿过水。许多细菌缺乏鞭毛，无法靠自身力量移动，而其他一些则可以通过一些尚不为人了解的机制在表面滑动。
从细菌的角度来看，世界与人类所看到的截然不同。对于细菌来说，水就像糖浆对我们一样浓稠。细菌如此之小，以至于它们受到周围化学分子运动的影响。在显微镜下观察细菌时，即使那些没有鞭毛的细菌，也经常在水中跳动。这是因为它们与水分子碰撞，并被推动到各个方向。分子运动如此迅速，以至于在十分之一秒内，细菌周围的分子就全部被新的分子所取代，即使没有鞭毛的细菌也因此不断暴露在不断变化的环境中。
