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 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 Molecules move so rapidly that within a tenth of a second the molecules around a bacterium have all been replaced by new ones; even bacteria without flagella are thus constantly exposed to a changing environment.
战争下的孤儿
乡村四季
一张旧照片
乡村生活(2)
一个真实的故事
生命是一首美丽的歌
大嘴惹祸 印高官“享受强奸”言论招致批评
田园的清晨
乡村生活(1)
《深化改革决定》要点双语对照(二)
美国男子不满被戴“绿帽” 搬进前妻隔壁竖中指雕像
感悟生命
我的傻瓜爸爸
来自天堂的一封信
废墟中的孩子
青少年艾滋病毒感染者十年上升33%
生命本是风景
战争与孩子
国际英语资讯:Coronavirus fatalities in French hospitals rise by 541 to 7,632
《深化改革决定》要点双语对照(三)
战争中的孩子
可爱的小虎子
我的哥哥
战争是魔鬼
幸运的战争孤儿
缩小贫富差距是当务之急
乡村景色(3)
加州一家四口失踪3年 遗体惊现沙漠
国际英语资讯:Spotlight: Major cities in Japan appear quiet on 1st day in state of emergency
生命的献礼
不限 |
英语教案 |
英语课件 |
英语试题 |
不限 |
不限 |
上册 |
下册 |
不限 |