北美gre考试阅读真题解析(A)

　　As people age, their cells become less efficient and less able to replace damaged components. At the same time their tissues stiffen. For example, the lungs and the heart muscle expand less successfully, the blood vessels become increasingly rigid, and the ligaments and tendons tighten.

　　Few investigators would attribute such diverse effects to a single cause. Nevertheless, researchers have discovered that a process long known to discolor and toughen foods may also contribute to age-related impairment of both cells and tissues. That process is nonenzymatic glycosylation, whereby glucose becomes attached to proteins without the aid of enzymes. When enzymes attach glucose to proteins , they do so at a specific site on a specific protein molecule for a specific purpose. In contrast, the nonenzymatic process adds glucose haphazardly to any of several sites along any available peptide chain within a protein molecule.

　　This nonenzymatic glycosylation of certain proteins has been understood by food chemists for decades, although few biologists recognized until recently that the same steps could take place in the body. Nonenzymatic glycosylation begins when an aldehyde group of glucose and an amino group of a protein are attracted to each other. The molecules combine, forming what is called a Schiff base within the protein. This combination is unstable and quickly rearranges itself into a stabler, but still reversible, substance known as an Amadori product.

　　If a given protein persists in the body for months or years, some of its Amadori products slowly dehydrate and rearrange themselves yet again, into new glucose-derived structures. These can combine with various kinds of molecules to form irreversible structures named advanced glycosylation end products . Most AGEs are yellowish brown and fluorescent and have specific spectrographic properties. More important for the body, many are also able to cross-link adjacent proteins, particularly ones that give structure to tissues and organs. Although no one has yet satisfactorily described the origin of all such bridges between proteins, many investigators agree that extensive cross-linking of proteins probably contributes to the stiffening and loss of elasticity characteristic of aging tissues.

　　In an attempt to link this process with the development of cataracts , researchers studied the effect of glucose on solutions of purified crystallin, the major protein in the lens of the eye. Glucose-free solutions remained clear, but solutions with glucose caused the proteins to form clusters, suggesting that the molecules had become cross-linked. The clusters diffracted light, making the solution opaque. The researchers also discovered that the pigmented cross-links in human cataracts have the brownish color and fluorescence characteristic of AGEs. These data suggest that nonenzymatic glycosylation of lens crystallins may contribute to cataract formation.

　　17. With which of the following statements concerning the stiffening of aging tissues would the author most likely agree?

　　 It is caused to a large degree by an increased rate of cell multiplication.

　　 It paradoxically both helps and hinders the longevity of proteins in the human body.

　　 It can be counteracted in part by increased ingestion of glucose-free foods.

　　 It is exacerbated by increased enzymatic glycosylation.

　　 It probably involves the nonenzymatic glycosylation of proteins.

　　18. According to the passage, which of the following statements is true of the process that discolors and toughens foods?

　　 It takes place more slowly than glycosylation in the human body.

　　 It requires a higher ratio of glucose to protein than glycosylation requires in the human body.

　　 It does not require the aid of enzymes to attach glucose to protein.

　　 It proceeds more quickly when the food proteins have a molecular structure similar to that of crystallin proteins.

　　 Its effectiveness depends heavily on the amount of environmental moisture.

　　19. According to the passage, which of the following is characteristic of enzymatic glycosylation of proteins?

　　 AGEs are formed after a period of months or years.

　　 Proteins affected by the process are made unstable.

　　 Glucose attachment impairs and stiffens tissues.

　　 Glucose is attached to proteins for specific purposes.

　　 Amino groups combine with aldehyde groups to form Schiff bases.

　　20. According to the passage, which of the following statements is true of Amadori products in proteins?

　　 They are more plentiful in a dehydrated environment.

　　 They are created through enzymatic glycosylation.

　　 They are composed entirely of glucose molecules.

　　 They are derived from Schiff bases.

　　 They are derived from AGEs.

　　21. Which of the following best describes the function of the third paragraph of the passage ?

　　 It offers evidence that contradicts the findings described in the first two paragraphs.

　　 It presents a specific example of the process discussed in the first two paragraphs.

　　 It explains a problem that the researchers mentioned in the second paragraph have yet to solve.

　　 It evaluates the research discoveries described in the previous paragraph.

　　 It begins a detailed description of the process introduced in the previous two paragraphs.

　　22. The passage suggests that which of the following would be LEAST important in determining whether nonenzymatic glycosylation is likely to have taken place in the proteins of a particular tissue?

　　 The likelihood that the tissue has been exposed to free glucose

　　 The color and spectrographic properties of structures within the tissue

　　 The amount of time that the proteins in the tissue have persisted in the body

　　 The number of amino groups within the proteins in the tissue

　　 The degree of elasticity that the tissue exhibits

　　23. If the hypothesis stated in lines 56-58 is true, it can be inferred that the crystallin proteins in the lenses of people with cataracts

　　 have increased elasticity

　　 do not respond to enzymatic glycosylation

　　 are more susceptible to stiffening than are other proteins

　　 are at least several months old

　　 respond more acutely than other proteins to changes in moisture levels

　　Writing of the Iroquois nation, Smith has argued that through the chiefs council, tribal chiefs traditionally maintained complete control over the political affairs of both the Iroquois tribal league and the individual tribes belonging to the league, whereas the sole jurisdiction over religious affairs resided with the shamans. According to Smith, this division was maintained until the late nineteenth century, when the dissolution of the chiefs council and the consequent diminishment of the chiefs political power fostered their increasing involvement in religious affairs.

　　However, Smith fails to recognize that this division of power between the tribal chiefs and shamans was not actually rooted in Iroquois tradition; rather, it resulted from the Iroquois resettlement on reservations early in the nineteenth century. Prior to resettlement, the chiefs council controlled only the broad policy of the tribal league; individual tribes had institutionsmost important, the longhouse )to govern their own affairs. In the longhouse, the tribes chief influenced both political and religious affairs.

　　24. The primary purpose of the passage is to

　　 question the published conclusions of a scholar concerning the history of the Iroquois nation

　　 establish the relationship between an earlier scholars work and new anthropological research

　　 summarize scholarly controversy concerning an incident from Iroquois history

　　 trace two generations of scholarly opinion concerning Iroquois social institutions

　　 differentiate between Iroquois political practices and Iroquois religious practices

　　25. It can be inferred that the author of the passage regards Smiths argument as

　　 provocative and potentially useful, but flawed by poor organization

　　 eloquently presented, but needlessly inflammatory

　　 accurate in some of its particulars, but inaccurate with regard to an important point

　　 historically sound, but overly detailed and redundant

　　 persuasive in its time, but now largely outdated

　　26. The author of the passage implies that which of the following occurred after the Iroquois were resettled on reservations early in the nineteenth century?

　　 Chiefs became more involved in their tribes religious affairs.

　　 The authority of the chiefs council over the affairs of individual tribes increased.

　　 The political influence of the Iroquois shamans was diminished.

　　 Individual tribes coalesced into the Iroquois tribal league.

　　 The longhouse because a political rather than a religious institution.

　　27. Which of the following best expresses an opinion presented by the author of the passage?

　　 Smith has overstated the importance of the political role played by Iroquois tribal chiefs in the nineteenth century.

　　 Smith has overlooked the fact that the Iroquois rarely allowed their shamans to exercise political authority.

　　 Smith has failed to explain why the chiefs council was dissolved late in the nineteenth century.

　　 Smith has failed to acknowledge the role prior to the nineteenth century of the Iroquois tribal chiefs in religious affairs.

　　 Smith has failed to recognize that the very structure of Iroquois social institutions reflects religious beliefs.

　　答案：17-27：ECDDEDDACBD