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.
Which of the following is true according to the first paragraph?
A.Bacteria are as big as a pinhead.
B.Bacteria can not be seen by an ordinary microscope.
C.Bacteria are hard to see because they often live together.
D.Bacteria are very small so they can only be measured in microns.
第2题
X射线机设备应能对等效原子序数小于10的有机物赋予()。
A.橙色
B.蓝色
C.绿色
D.红色
第6题
A.红色——非常厚、X射线穿不透的物体。
B.橙色——无机物(如炸药、毒品、塑料等)、危险物品(原子序数10以内的物质)。
C.绿色——混合物,即有机物与无机物的重叠部分。
D.蓝色——有机物,重金属(原子序数大于10的物质)。
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