曰韩免费_91久久精品国产亚洲_一区二区成人影院_九一视频在线免费观看_91国视频_亚洲成人中文在线

GRE写作范文——Bacteria

雕龙文库 分享 时间: 收藏本文

GRE写作范文——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 bacteria 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 watery molecules and are pushed this way and that. Molecules move so rapidly that within a tenth of a second the molecules round a bacteria have all been replaced by new ones; even bacteria without flagella are thus constantly exposed to a changing environment.

  

  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 bacteria 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 watery molecules and are pushed this way and that. Molecules move so rapidly that within a tenth of a second the molecules round a bacteria have all been replaced by new ones; even bacteria without flagella are thus constantly exposed to a changing environment.

  

主站蜘蛛池模板: 欧美性大战xxxxx久久久 | 免费a级毛视频 | 午夜影院免费看 | 欧美激情性做爰免费视频 | 久久久国产99久久国产一 | 国精产品一二三区精华液 | 欧美啊v在线 | 久久午夜福利无码1000合集 | 欧美精品免费观看二区 | 免费一级毛片视频 | 成人天堂在线 | 国产精品毛片在线大全 | 午夜小视频在线播放 | 国产一二三四区中 | 色94色欧美sute亚洲线 | 综合在线播放 | 亚洲午夜精品一区二区 | 永久免费av网站可以直接看的 | 天堂√最新版中文在线地址 | 久久久久无码精品国产h动漫 | 91久久福利国产成人精品 | 久久久久亚洲av无码专区桃色 | 国产97在线视频观看 | 91九色视频| 狠狠色成人综合网 | 亚洲系列 | 国产高清自产拍av在线 | 无码av天堂一区二区三区 | 小明精品国产一区二区三区 | 国产三级免费观看 | 国产精品9999久久久久 | 又黄又爽又色视频 | 香蕉eeww99国产精选播放 | 丰满人妻一区二区三区免费视频 | 在线观看做爰视频成人 | 四虎精品成人免费影视 | 国内精品久久久久久西瓜色吧 | 国产精品无打码在线播放9久 | 色综合天天综合网国产国产人 | 99国产超薄丝袜足j在线观看 | 国产精品国产亚洲区艳妇糸列短篇 |