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游客2024-01-03  8

问题 [br] According to the researchers in the roundworm study, how did a calorie-restricted diet extend the roundworms’ life span?
Listen to part of a lecture in a biology class.
Professor: Aging of course is a natural part of life for all living things. Most theories of aging, of why aging occurs, fall into one of two categories. First, you have the biological clock category—theories in this category assert that all organisms have a sort of internal biological clock, that they are sort of genetically programmed to age in a certain age at a certain rate. The second category asserts that aging is the result of the gradual accumulation of damage to cells and tissues. In the second category, we have a theory called "the free radical theory," which we’ll now examine.
    According to this theory, an organism’s cells in their regular metabolic processes—by metabolic processes I mean... things like cellular respiration and reproduction... in their regular metabolic processes, cells generate oxygen molecules, some of which have an extra electron. That extra or free electron makes the whole molecule unstable. The unstable molecule is known as a free radical, okay?
    And according to this theory these free radicals cause damage to other molecules and structures within the cell, including, most significantly, to the cell’s DNA, and it’s this gradual degradation of DNA and tissues and larger structures that drives the aging process.
    You may be wondering how does this theory hold up? Well, some organisms do have some protection against free radical damage in the form of substances called antioxidants. Antioxidants through a series of chemical reactions, can help stabilized free radicals, so they can’t damage the cell. An important antioxidant is an enzyme called "superoxide dismutase" (or SOD). ln experiments with fruit flies, when researchers increased the level of SOD, there was less free radicals harm. The fruit flies had longer lifespans and stayed younger longer. So this seems to show that indeed, at least in some invertebrates, free radical damage is what causes aging.
    Ok, and there is another line of thought that at one point was used to support the free radical theory. It was discovered that when organisms, such as yeast—a fungus—were given a calorie restricted diet, they lived longer. If their diets included far fewer calories than normal but still included all the necessary nutrients, their lifespan increased. For a while researches explained this by arguing that the limited food intake leads the yeast cell’s metabolism to slow down. You’ll need to remember that according to the free radical theory, cells metabolic processes were what brought about those extra electrons that created free radicals, and, so the argument went at the time, if the cells metabolism slowed down, the rate of free radical production also slowed down, so there was less cell damage. And that’s why the yeast cells aged slower and lived longer.
    However, later research, this time on roundworms, suggested that while it was true that organisms on a calorie restricted diet live longer, it wasn’t because of a slower metabolism. In fact, the researchers in this roundworm study claimed that the special diet actually increased cell respiration, and what happened was this increased respiration activated a certain gene, a gene that has been linked to a longer lifespan. And these researchers claimed that it was actually the activation of this longevity gene, not the slowing of free radical damage, that lengthen the lifespan.

选项 A、By helping process nutrients more efficiently
B、By increasing the rate of cell repair
C、By activating a special gene
D、By slowing cell metabolism

答案 C

解析 细节题。在陈述反驳自由基理论时,教授提到对于蛔虫的研究:ln fact,the researchers in this roundworm study claimed that the special diet actually increased cell respiration,and what happened was this increased respiration activated a certain gene.特殊的饮食增加细胞的呼吸活动,正是活跃的呼吸活动刺激了长寿的基因,即C选项是正确答案。生物本身处理营养物质的方式未提到有变化,因此A选项不正确。该部分未提及提高细胞修复速率来延缓衰老,因此B选项不正确。教授提到蛔虫的实验结果:it wasn’t because of a slower metabolism.即控制饮食导致长寿的原因并不是降低了新陈代谢的速率,因此D选项不正确。
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