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

大学英语六级考试拓展阅读-背诵美文48

雕龙文库 分享 时间: 收藏本文

大学英语六级考试拓展阅读-背诵美文48

  48 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 Molecules move so rapidly that within a tenth of a second the molecules around a bacterium have all been replaced by new ones; even bacteria without flagella are thus constantly exposed to a changing environment.

  

  48 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.

  

主站蜘蛛池模板: 久久精品国产亚洲av果冻传媒 | 狠日狠干日日射 | 日本一道本中文字幕 | 波多野结衣一区二区三区高清 | 国产美女精品一区二区三区 | 久久久久亚洲av无码a片软件 | 亚洲欧美伦理 | 在线观看日韩精品 | 久久久久女教师免费一区 | 亚洲成人视 | www色网| 久久久久这里只有精品 | 无码高潮爽到爆的喷水视频app | 欧美日韩色综合网站 | 国产真人做爰视频免费 | 精品国产av最大网站 | 又爽又黄又无遮挡的视频 | 粗大的内捧猛烈进出少妇 | 在线播放国产视频 | 久久精品国产一区二区小说 | 久久综合五月丁香久久激情 | 77788色淫视频免费观看 | 高清激情小视频在线观看 | 一本大道熟女人妻中文字幕在线 | 99热久久只有精品6国产32 | 久久66热re国产毛片基地 | 蜜臀国产在线视频 | 上司揉捏人妻丰满双乳电影 | 亚洲国产成人精品小蝌蚪 | 一级网站在线观看 | 久久频精品99香蕉国产 | 成人久久18网站 | 国产精伦| 亚洲99爱 | 精品亚洲成a人在线观看青青 | 国产成人精品免费视频软件 | 欧美日韩一区二区在线观看 | 高清免费毛片 | 国产精品无码制服丝袜 | 国产av一区二区三区最新精品 | 免费观看欧美一级特黄 |