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Bacteria （细菌) are extremely small living things. While we measure our own sizes in inche

Bacteria (细菌) 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 (1.6 kilometers) 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 (糖蜜) 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this way and that molecules move so rapidly that within a tenth of a second the molecules around a flagellum have been replaced by new ones. Even bacteria without flagella are thus constantly exposed to a changing environment.

Which of the following is the main topic of the passage？

A．The characteristics of bacteria.

B．How bacteria reproduce.

C．The various functions of bacteria.

D．How bacteria contribute to disease.

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更多“Bacteria （细菌) are extremely sm…”相关的问题

第1题

Milk was one of the main foods for people long before history was written.Maybe【21】will re

Milk was one of the main foods for people long before history was written. Maybe【21】will remain an important food as long as there are【22】that give milk. The old word "milk"【23】Sanskrit(梵文), one of the oldest【24】known to man. A very old picture of milking,【25】was drawn five thousand years ago, has been found.

In the years long ago, people got their milk【26】their own animals. But【27】modern times new inventions made the milk industry(产业) a big business.【28】1851 Gall Borden found a way to【29】some of the water out from milk. This made milk keep longer. Four years later, Louis Pasteur【30】 a way to kill the bacteria(细菌) in milk. Then a special milk bottle was made. This was followed【31】 the invention of machines that could fill bottles with milk. The discoveries had a great effect(影响) on the milk industry. They meant that milk could be kept longer.

Some people believe that milk drinking will become less popular【32】 it has been. But remember how long milk has been an important food【33】 think of the many ways【34】 it is useful. It seems【35】 to say that the milk industry will always be important.

(36)

A．it

B．they

C．he

D．she

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第2题

A. decade B. aggressive C. ancestor D. bacteria

A. decade

B. aggressive

C. ancestor

D. bacteria

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第3题

完成下列各题 D Forty—thousand of the world’ s you

ng children die every day．Many of these deaths can be prevented．In fact，experts say，techniques now exist to save the lives of half of these children．The techniques are medically effective(有效的)，simple to use and low cost.The biggest task has been to inform. parents about these life-saving techniques． Diarrhea(腹泻)is the main cause of death among children in developing countries．Diarrhea results when the body tries to clear itself of harmful bacteria(细菌)．The body does this by getting our liquids，sometimes uncontrollably．If too much water，sugar and salt ale lost in this way，the body cannot work normally．The child may die． The Uni ted Nations International Children’s Fund，UNICF，introduces a technique．It uses a simple mixture of sugar and salt in water．The mixture does not stop diarrhea．But if used often as needed，it keeps fluid(体液)levels normal until the body has fought off the harmful bacteria． In Egypt，for example，the treatment was used in a children’s health care program in the province of Alexandria．It reduced by one—half the number of deaths caused by diarrhea.The technique has now been used throughout the country． According to the passage，a large number of Children can be saved only when________．

A.the life—saving ways are medically effective

B.their parents are rich enough for the life—saving techniques

C.rite life—saving techniques are easy to use

D.their parents learn the life—saving techniques

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第4题

In paragraph 3, sentence 2, the author compares water to molasses in order to introduce wh

ich of the following topics？

A．The bacterial content of different liquids.

B．What happens when bacteria are added to molasses.

C．The molecular structures of different chemicals.

D．How difficult it is for bacteria to move through water.

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第5题

A．areaB．appealC．bacteriaD．cafeteria

A.area

B.appeal

C.bacteria

D.cafeteria

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第6题

The university has launched a research center to develop new ways of ______ bacteria which

have become resistant to drug treatments.

A．regulating

B．halting

C．interrupting

D．combating

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第7题

When the mixture is used, it is important to______.A．keep sugar and salt in waterB．drink i

When the mixture is used, it is important to______.

A．keep sugar and salt in water

B．drink it as often as necessary

C．take it as much as possible

D．use it to kill the bacteria

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第8题

Why does cream go bad faster than butter？ Some researchers think they find the answer, and

it comes down to the structure of the food, not its chemical composition—a finding that could help rid some processed foods of chemical preservatives.

Cream and butter contain pretty much the same substances, so why cream should sour much faster has been a mystery. Both are emulsions—tiny globules(小球) of one liquid evenly distributed throughout another. "The difference lies in what's in the globules and what's in the surrounding liquid", says Brocklehurst, who led the investigation. In cream, fatty globules drift about in a sea of water. In butter, globules of a watery solution are locked away in a sea of fat. The bacteria which make the food go bad prefer to live in the watery regions of the mixture. "This means that in cream, the bacteria are free to grow throughout the mixture", he says.

When the situation is reversed, the bacteria are locked away in compartments(密封仓) buried deep in the sea of fat. Trapped in this way, individual colonies cannot spread and rapidly run out of nutrients. They also slowly poison themselves with their waste products. "In butter, you get a self-limiting system which stops the bacteria growing", says Brocklehurst. The researchers are already working with food companies keen to see if their products can be made resistant to bacterial attack through alterations to the food's structure. Brocklehurst believes it will be possible to make the emulsions used in salad cream, for instance, more like that in butter. The key will be to do this while keeping the salad cream liquid and not turning it into a solid lump.

The significance of Brocklehurst's research is that ______.

A．it suggested a way to keep some foods fresh without preservatives

B．it discovered tiny globules in both cream and butter

C．it revealed the secret of how bacteria multiply in cream and butter

D．it found that cream and butter share the same chemical composition

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第9题

保险需求的交叉弹性的公式表示()。

A.Ex=△D/D△Pg/Pg

B.Ex=△Pg/Pg△D/D

C.Ex=△D/P△Pg/D

D.Ex=△Pg/D△D/Pg

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第10题

矿用一般型电气设备的符号是（)A、EXB、MAC、KY

A.EX

B.MA

C.KY

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第11题

His eyes shone brightly when he finally received the magazine he ______.A．had long been ex

His eyes shone brightly when he finally received the magazine he ______.

A．had long been expected

B．had long expected

C．has long expected

D．was long expected

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