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A．knew B．concluded C．saw D．thought
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Eat More, Weigh Less, Live Longer
Clever genetic detective work may have found out the reason why a near-starvation diet prolongs the life of many animals.
Ronald Kahn at Harvard Medical School in Boston, US, and his colleagues have been able to extend the lifespan （寿命） of mice by 18 per cent by blocking the rodent's （啮齿动物） increase of fat in specific cells. This suggests that thinness--and not necessarily diet--promotes long life in "calorie （热量卡） restricted" animals.
"It's very cool work," says aging researcher Cynthia Kenyon of the University of California, San Francisco. "These mice eat all they want, lose weight and live longer. It's like heaven."
Calorie restriction dramatically extends the lifespan of organisms as different as worms and rodents. Whether this works in humans is still unknown, partly because few people are willing to submit to such a strict diet.
But many researchers hope they will be able to trigger the same effect with a drug once they understand how less food leads to a longer life. One theory is that eating less reduces the increase of harmful things that can damage cells. But Kahn's team wondered whether the animals simply benefit by becoming thin.
To find out, they used biology tricks to disrupt the insulin （胰岛素） receptor （受体） gene in lab mice--but only in their fat cells. "Since insulin is needed to help fat cells store fat, these animals were protected against becoming fat," explains Kahn.
This slight genetic change in a single tissue had dramatic effects. By three months of age, Kahn's modified mice had up to 70 per cent less body fat than normal control mice, despite the fact that they ate 55 per cent more food per gram of body weight.
In addition, their lifespan increased. The average control mouse lived 753 days, while the thin rodents averaged a lifespan of 887 days. After three years, all the control mice had died, but one-quarter of the modified rodents were still alive.
"That they get these effects by just manipulating the fat cells is controversial," says Leonard Guarente of the Massachusetts Institute of Technology, who studies calorie restriction and aging.
But Guarente says Kahn has yet to prove that the same effect is responsible for increased lifespan in calorie-restricted animals. "It might be the same effect or there might be two routes to long life," he points out, "and that would be very interesting."
第 41 题 Ronald Kahn and his colleagues can make mice live longer by.
Keeping Cut Flowers
1．While everybody enjoys fresh cut flowers around their house, few people know how to keep them for as long as possible. This may be done by keeping in mind a few simple facts.
2．An important thing to remember about cut flowers is that they are sensitive to temperature. For example, studies have shown that cut carnations （康乃馨） retain their freshness eight times longer when kept at 12OC than when kept at 26OC． Keeping freshly harvested flowers.at the right temperatures is probably the most important aspect of flower care.
3．Flowers are not intended by nature to live very long. Their biological purpose is simply to attract birds or insects, such as bees, for pollination （授粉）. After that, they quickly dry up and die. The process by which flowers consume oxygen and produce carbon dioxide （二氧化碳）, called respiration （呼吸）, generates the energy the flower needs to give the flower its shape and colour. The making of seeds also depends on this energy. While all living things respire, flowers have a high level of respiration. A result of all this respiration is heat,and for flowers the level of heat relative to the mass of the flower is very high. Respiration also brings about the eventual death of the flower. Thus the greater the level of respiration, the sooner the flower dies.
4．How, then, to control the rate at which flowers die By controlling respiration. How is respiration controlled By controlling temperature. We know that respiration produces heat, but the reverse is also true. Thus by maintaining low temperatures, respiration is reduced and the cut flower will age more slowly.
5．Another vital factor in keeping cut flowers is the quality of the water in which they are placed. Flowers find it difficult to "drink" water that is dirty or otherwise polluted. Even when water looks and smells clean, it almost certainly contains harmful substances that can endanger the flowers. To rid the water of these unwanted substances, household chlorine bleach （含氯漂白剂） can be used in small quantities. It is recommended that 15 drops of chlorine bleach （at 4% solution） be added to each litre of water. The water and solution should also be replaced each day.
第 23 题 Paragraph 2______
A．resort B．refer C．turn D．attend
A．on B．to C．by D．with
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