2. 外语
  3. Bacteria are microscopic organisms which live on the surface of objects. They are one of the most ancient living things, which e

Bacteria are microscopic organisms which live on the surface of objects. They are one of the most ancient living things, which e

admin2011-02-08  75
问题     Bacteria are microscopic organisms which live on the surface of objects. They are one of the most ancient living things, which exist on this planet for nearly 4, 000 million years. Do you know the size of bacteria? It can only be measured in microns. Maybe you have not a standard in your mind how long a micron is. One micron is a thousandth of a millimeter, which equals about the diameter of a pinhead. Therefore, even if we enlarged the rounded bacterium a thousand times, it would only be the size of a pinhead. We barely see bacteria by a magnification an ordinary microscope of 100 times, even if we try, we cannot make out anything of their structure. There are normally millions of them together, for they can multiply really fast.
    Scientists have found that some bacteria have attached to wavy-looking "hairs" called flagella. The flagella rotate, pushing the bacteria through the water. Others can glide along over surfaces by some little-understood mechanism. Bacteria are so small that they are influenced by the movements of the chemical molecules around them. They are active all the time. Even the bacteria without flagella often bound about in the water. They are pushed here and there, colliding with the watery molecules.
    Bacteria cannot be detected because they don’t produce bad odor or change the color or texture of the food. Therefore, when people eat the food with many bacteria, they are likely to get hepatitis A, acute gastroenteritis and a host of other illnesses. Many households have refrigerators to prevent from bacteria. Of course, freezing food slows or stops-the growth of bacteria, however, when food is thawed, the bacteria will become reactivated. Bacteria can not be totally destroyed before the food is thoroughly cooked. We need to know that not all bacteria are in connection with illness. Just some bacteria can cause disease. They are called pathogenic bacteria. Fortunately our immune system can protect us from them.
    Bacteria are prokaryotes (single cells that do not contain a nucleus). It may seem weird to classify organisms according to such details, but with or without a nucleus is not trivial at all. The division between prokaryotes and eukaryotes (all organisms with a nucleus inside their cells) is of extreme importance in biology, and is the result of a major evolutionary breakthrough. Visit our program tomorrow if you want to know more, thank you.
What can be inferred from the last paragraph?
选项 A、Whether or not to have a nucleus is important in biology.
B、The speaker will tell us how to distinguish bacteria tomorrow.
C、Evolution will be the main topic of the program next time.
D、Eukaryotes develop from prokaryotes.
答案C
解析 根据最后一段第二句It may…“but with or without a nucleus is not trivial at all．”以及第三句“The division between prokaryotes and eukaryotes…is of extreme importance in biology．”可见，是否有细胞核这种区分对于生物学是很重要的，A项直接体现了文意，而不是推断出来的，故不选。根据该段倒数第二句“The division between prokaryotes…and
转载请注明原文地址:https://kaotiyun.com/show/omxd777K
本试题收录于： 公共英语四级笔试题库公共英语（PETS）分类
0

公共英语四级笔试

公共英语（PETS）

相关试题推荐
随机试题
最新回复(0)