For some time scientists have believed that cholesterol (胆固醇) plays a major role in heart disease because people with familial hypercholesterolemia, a genetic defect, have six to eight times the normal level of cholesterol in their blood and they invariably develop heart disease. These people lack cell-surface receptors for low-density lipoproteins (LDL’s), which are the fundamental carriers of blood cholesterol to the body cells that use cholesterol. Without an adequate number of cell-surface receptors to remove LDL’s from the blood, the cholesterol-carrying LDL’s remain in the blood, increasing blood cholesterol levels. Scientists also noticed that people with familial hypercholesterolemia appear to produce more LDL’s than normal individuals. How, scientists wondered, could a genetic mutation that causes a slow-down in the removal of LDL’s from the blood also result in an increase in the synthesis of this cholesterol-carrying protein?
Since scientists could not experiment on human body tissue, their knowledge of familial hyper-cholesterolemia was severely limited. However, a breakthrough came in the laboratories of Yoshio Watanabe of Kobe University in Japan in 1980. Watanabe noticed that a male rabbit in his colony had ten times to the normal concentration of cholesterol in its blood. By appropriate breeding, Watanabe obtained a strain of rabbits that had very high cholesterol levels. These rabbits spontaneously developed heart disease. To his surprise, Watanabe further found that the rabbits, like humans with familial hypercholesterolemia, lacked LDL receptors. This, scientists could study these Watanabe rabbits to gain a better understanding of familial hypercholesterolemia in humans.
Prior to the breakthrough at Kobe University, it was known that LDL’s are secreted from the liver in the form of a precursor, called very low-density lipoproteins (VLDLs), which carry triglycerides as well as relatively small amounts of cholesterol. The triglycerides are removed from the VLDL’s by fatty and other tissues. What remains is a remnant particle that must be removed from the blood. What scientists learned by studying the Watanabe rabbits is that the removal of the VLDL remnant requires the LDL receptor. Normally, the majority of the VLDL remnants go to the liver where they bind to LDL receptors and are degraded. In the Watanabe rabbit, due to a lack of LDL receptors on liver cells, the VLDL remnants remain in the blood and are eventually converted to LDL’s. The LDL receptors thus have a dual effect in controlling LDL levels. They are necessary to prevent oversynthesis of LDL’s from VLDL remnants and they are necessary for the normal removal of LDL from the blood. With this knowledge, scientists are now well on the way toward developing drugs that dramatically lower cholesterol levels in people afflicted with certain forms of familial hypercholesterolemia.
54. In the passage, the author is primarily concerned with ______.
55. The passage supplies information to answer which of the following questions?
56. The passage implies that if the Watanabe rabbits had had as many LDL receptors on their livers as do normal rabbits, the Watanabe rabbits would have been ______.
57. The passage implies that Watanabe rabbits differ from normal rabbits in which of the following ways?
问题1选项
A.presenting a hypothesis and describing compelling evidence in support of it
B.raising a question and describing an important discovery that led to an answer
C.showing that a certain genetically caused disease can be treated effectively with drugs
D.explaining what causes the genetic mutation that leads to heart disease
问题2选项
A.Which body cells are the primary users of cholesterol?
B.How did scientists discover that LDL’s are secreted from the liver in the form of a precursor?
C.Where in the body are VLDL remnants degraded?
D.Why are LDL secreted from the liver in the form of a precursor?
问题3选项
A.less likely than normal rabbits to develop heart disease
B.less likely than normal rabbits to develop high concentrations of cholesterol in their blood
C.less useful than they actually were to scientists in the study of familial hypercholesterolemia in humans
D.unable to secrete VLDL from their livers
问题4选项
A.Watanabe rabbits have more LDL receptors than do normal rabbits.
B.The blood of Watanabe rabbits contains more VLDL remnants than docs the blood of normal rabbits.
C.Watanabe rabbits have fewer fatty issues than do normal rabbits.
D.Watanabe rabbits secrete lower levels of VLDL’s than do normal rabbits.
54.【试题答案】B
【试题解析】中心主旨题。题干意思是“在这篇文章中,作者主要关注……”。通读全文可知,文章第一段先是提出一个问题,即第一段最后一句How, scientists wondered, could a genetic mutation that causes a slow-down in the removal of LDL’s from the blood also result in an increase in the synthesis of this cholesterol-carrying protein?(科学家们想知道,一种导致血液中低密度脂蛋白清除速度减缓的基因突变,是如何导致这种携带胆固醇的蛋白质的合成增加的呢?),接着作者就第一段的这个问题展开了论述,并详细描述了能解决第一段提出的问题的重大发现,所以B项“提出一个问题,并描述一个导致答案的重要发现”正确。A项“提出一个假设,并描述有力的证据来支持它”中的hypothesis(假设)并不符合文意。C项“表明某种由基因引起的疾病可以用药物有效治疗”和D项“解释导致心脏病的基因突变的原因”都只是其中的一部分内容。因此,该题选择B项正确。
55.【试题答案】C
【试题解析】推理判断题。题干意思是“这篇文章提供信息来回答以下哪个问题?”。文章最后一段详细的描述了VLDL残留物是如何产生以及肝脏是如何清除VLDL残余物,第五句提到Normally, the majority of the VLDL remnants go to the liver where they bind to LDL receptors and are degraded.(正常情况下,大多数VLDL残体进入肝脏,与LDL受体结合并被降解),所以文章提供信息回答了“VLDL残留物在身体何处被降解”这个问题,故C项“VLDL残留物在身体的何处被降解?”正确。A项“哪些细胞是胆固醇的主要使用者?”、B项“科学家是如何发现低密度脂蛋白以一种前体的形式从肝脏分泌出来的?”和D项“为什么LDL以前体的形式从肝脏分泌?”在文中都没有相关信息解释其原因。因此,该题选择C项正确。
56.【试题答案】C
【试题解析】推理判断题。题干意思是“这篇文章暗示,如果渡边兔的肝脏中有和正常兔子一样多的低密度脂蛋白受体,渡边兔可能……”。最后一段中提到What scientists learned by studying the Watanabe rabbits is that the removal of the VLDL remnant requires the LDL receptor…In the Watanabe rabbit, due to a lack of LDL receptors on liver cells, the VLDL remnants remain in the blood and are eventually converted to LDL’s….With this knowledge, scientists are now well on the way toward developing drugs that dramatically lower cholesterol levels in people afflicted with certain forms of familial hypercholesterolemia.(通过研究渡边兔,科学家发现清除VLDL残余需要LDL受体……在渡边兔体内,由于肝细胞上缺乏低密度脂蛋白受体,VLDL残留在血液中并最终转化为低密度脂蛋白……有了这些知识,科学家们现在正在开发药物,以显著降低某些家族性高胆固醇血症患者的胆固醇水平。),可知,渡边兔因为缺乏LDL受体所以它血液中的低密度脂蛋白会过多从而引起高胆固醇,这对科学研究有很大帮助,从而反推,如果渡边兔的肝脏中有和正常兔子一样多的低密度脂蛋白受体,那么它对科学研究来说没什么意义,所以C项“在科学家对人类家族性高胆固醇血症的研究中,它们实际上没有那么有用”正确。A项“比正常兔子更不容易患心脏病”和B项“在它们的血液不太可能比正常的兔子发展高浓度的胆固醇”在文中没有相关信息可以推断。D项“无法从他们的肝脏分泌VLDL”,不管低密度脂蛋白受体的数量多少,肝脏都会分泌VLDL,所以D项错误。因此,该题选择C项正确。
57.【试题答案】B
【试题解析】题干意思是“这篇文章暗示渡边兔在以下哪方面不同于正常的兔子?”。最后一段中提到Normally, the majority of the VLDL remnants go to the liver where they bind to LDL receptors and are degraded. In the Watanabe rabbit, due to a lack of LDL receptors on liver cells, the VLDL remnants remain in the blood and are eventually converted to LDL’s.(正常情况下,大多数VLDL残体进入肝脏,与LDL受体结合并被降解。在渡边兔体内,由于肝细胞上缺乏低密度脂蛋白受体,VLDL残留在血液中并最终转化为低密度脂蛋白。),可知渡边兔由于缺乏低密度脂蛋白受体,所以不能降解VLDL,故VLDL残留物会比正常兔子的多,B项“渡边兔血液中VLDL残留量比正常兔子血液多”正确。A项“渡边兔比正常兔子有更多的LDL受体”,与文章表述相反,因此错误。C项“渡边兔的脂肪问题比正常兔子少”和D项“渡边兔分泌的VLDL水平比正常兔子低”在文中没有对它们的脂肪和分泌的VLDL水平进行比较。因此,该题选择B项正确。