Gre阅读真题200篇 brief contents
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Researchers are finding thatResearchers are finding that in many ways an individual bacterium is more analogous to a component cell of a multicellular organism than it is to a free-living, autonomous organism. Anabaena, a freshwater bacteria, is a case in point. Among photosynthetic bacteria, Anabaena is unusual: it is capable of both photosynthesis and nitrogen fixation. Within a single cell, these two biochemical processes are incompatible: oxygen produced during photosynthesis, inactivates the nitrogenase (nitrogenase: n.[生化]固氮酶) required for nitrogen fixation. In Anabaena communities, however, these processes can coexist. When fixed nitrogen compounds are abundant, Anabaena is strictly photosynthetic and its cells are all alike. When nitrogen levels are low, however, specialized cells called heterocysts (heterocyst: [生]异形细胞) are produced which lack chlorophyll (necessary for photosynthesis) but which can fix nitrogen by converting nitrogen gas into a usable form. Submicroscopic channels develop which connect the heterocyst cells with the photosynthetic ones and which are used for transferring cellular products between the two kinds of Anabaena cells. 24. According to the passage, which of the following statements is true of bacteria that engage in photosynthesis? (A) They eventually become two autonomous cells. (B) They cannot normally also engage in nitrogen fixation. (C) Oxygen normally inactivates them. (D) Cellular products are constantly transferred between such bacteria.（B） (E) They normally lack chlorophyll. 25. It can be inferred from the passage that cell differentiation within Anabaena is regulated by the (A) amount of oxygen Anabaena cells produce (B) season of the year (C) amount of fixed nitrogen compounds available (D) number of microscopic channels uniting Anabaena cells（C） (E) amount of chlorophyll in Anabaena cells 26. The passage supports which of the following inferences about heterocysts? (A) Heterocysts do not produce oxygen. (B) Nitrogen gas inactivates heterocysts. (C) Chlorophyll increases the productivity of heterocysts. (D) Heterocysts allow nitrogen fixation and photosynthesis to occur in the same cell.（A） (E) Heterocysts are more important for Anabaena’s functioning than are photosynthetic cells. 27. The author uses the example of Anabaena to illustrate the(A选项很具迷惑性，但A肯定不对。因为Among photosynthetic bacteria, Anabaena is unusual。Unusual是指Among photosynthetic bacteria。而非among unicellular organisms) (A) uniqueness of bacteria among unicellular organisms (B) inadequacy of an existing view of bacteria (C) ability of unicellular organisms to engage in photosynthesis (D) variability of a freshwater bacteria（B） (E) difficulty of investigating even the simplest unicellular organisms
The success of fluoride in combating dental decay is well established and, without a doubt (without a doubt: adv.无疑地), socially beneficial. However, fluoride’s toxic properties have been known for a century. In humans excessive intake (for adults, over 4 milligrams per day) over many years can lead to skeletal fluorosis, a well-defined skeletal disorder, and in some plant species, fluoride is more toxic than ozone, sulfur dioxide, or pesticides. Some important questions remain. For example, the precise lower limit at which the fluoride content of bone becomes toxic is still undetermined. And while fluoride intake from water and air can be evaluated relatively easily, it is much harder to estimate how much a given population ingests from foodstuffs because of the wide variations in individual eating habits and in fluoride concentrations in foodstuffs. These difficulties suggest that we should be wary of indiscriminately using fluoride, even in the form of fluoride-containing dental products. 24. In the passage, the author is primarily concerned with (A) analyzing and categorizing (C) synthesizing and predicting (D) describing and cautioning（D） (E) summarizing and reinterpreting 25. The passage suggests that it would be easier to calculate fluoride intake from food if (A) adequate diets were available for most people. (B) individual eating habits were more uniform (C) the fluoride content of food was more varied (D) more people were aware of the fluoride content of food（B） (E) methods for measuring the fluoride content of food were more generally agreed on 26. One function of the second paragraph of the passage is to (A) raise doubts about fluoride’s toxicity (B) introduce the issue of fluoride’s toxicity (C) differentiate a toxic from a nontoxic amount of fluoride (D) indicate that necessary knowledge of fluoride remains incomplete（D） (E) discuss the foodstuffs that are most likely to contain significant concentrations of fluoride 27. The passage suggests which of the following about the effect of fluoride on humans? (A) The effect is more easily measured than is the effect of exposure to pesticides. (B) The effect of fluoride intake from water and air is relatively difficult to monitor. (C) In general the effect is not likely to be as harmful as the effect of exposure to sulfur dioxide. (D) An intake of 4 milligrams over a long period of time usually leads to a skeletal disorder in humans.（E） (E) An intake of slightly more than 4 milligrams for only a few months is not likely to be life-threatening. One advantage of breeding African bees with other bee types (Africanization) may be resistance to the parasitic mite Varroa jacobsoni, a major threat to modern beekeeping. In parts of Europe, this mite is devastating honeybees and killing many colonies despite preventive measures by beekeepers. But in Brazil Varroa jacobsoni has been present in Africanized bees since 1972 without the loss of a single colony, even though beekeepers there undertook no preventive measures. The mites lay eggs within the brood cells of immature bees, and developing mites feed on the hemolymph (blood) of bee pupae. But fewer mites reproduce in Africanized bees than in European bees. Some researchers point out that this resistance may be related to the Africanized worker bee’s shorter development period, which prevents some mites from reaching maturity. Recently the mite has become a serious problem in colonies of European bees in North America. Africanization of these bees may be the best safeguard against this parasite. 24. The passage suggests that which of the following was true of the honeybee colonies described in line 4-6? (A) Their life expectancy, when free of disease, was shorter than that of European bee colonies in North America. (B) They were not Africanized. (C) Their life cycle did not accommodate the feeding habits of Varroa jacobsoni. (D) They responded well to measures to control Varroa jacobsoni.（B） (E) They were managed using methods that were more modern than those employed in Brazil. 25. The author cites all of the following as evidence that Africanized bees’ resistance to Varroa jacobsoni is superior to that of European bees EXCEPT: (A) Fewer Varroa jacobsoni mites reproduce in Africanized bees. (B) Varroa jacobsoni is killing many bee colonies in Europe. (C) Beekeepers in Brazil have not used preventive measures to protect their colonies. (D) Brazilian bee colonies have endured Varroa jacobsoni since 1972.（E） (E) At least some European bee colonies have been saved by preventive measures. 26. According to the passage, research suggests that one possible reason the Africanized bees in Brazil have successfully resisted Varroa jacobsoni is that (A) the life cycle of the Africanized bee may limit the Varroa jacobsoni mite’s opportunity to reach full development (B) the Africanized bees may have had an opportunity to develop a chemical resistance to Varroa jacobsoni (C) the location of bee colonies in Brazil may provide a natural deterrent to Varroa jacobsoni (D) Varroa jacobsoni may be relatively new to Brazil and may not have had time to become widespread（A） (E) beekeepers may have developed effective control techniques for Varroa jacobsoni 27. The author’s argument regarding the resistance of Africanized bees to Varroa jacobsoni would be most weakened if which of the following were true? (A) The bees in Brazil were resistant before being Africanized. (B) The number of bee colonies in North American increased dramatically whereas the number in Brazil remained unchanged. (C) Mites found in European bees reproduce at a faster rate than mites of identical species found in the bees in Brazil. (D) Africanized bees retain many of the characteristics of European bees.（A） (E) Bee colonies in Europe continue to produce greater quantities of honey than do those in Brazil.
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