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  托福阅读真题文档免费下载【答案+解析+译文】passage1

  Symbiotic Relationships

  A symbiotic relationship is an interaction between two or more species in which one species lives in or on another species. There are three main types of symbiotic relationships: parasitism, commensalism, and mutualism. The first and the third can be key factors in the structure of a biological community; that is, all the populations of organisms living together and potentially interacting in a particular area.

  Parasitism is a kind of predator-prey relationship in which one organism, the parasite, derives its food at the expense of its symbiotic associate, the host. Parasites are usually smaller than their hosts. An example of a parasite is a tapeworm that lives inside the intestines of a larger animal and absorbs nutrients from its host. Natural selection favors the parasites that are best able to find and feed on hosts. At the same time, defensive abilities of hosts are also selected for. As an example, plants make chemicals toxic to fungal and bacterial parasites, along with ones toxic to predatory animals (sometimes they are the same chemicals). In vertebrates, the immune system provides a multiple defense against internal parasites.

  At times, it is actually possible to watch the effects of natural selection in host-parasite relationships. For example, Australia during the 1940 s was overrun by hundreds of millions of European rabbits. The rabbits destroyed huge expanses of Australia and threatened the sheep and cattle industries. In 1950, myxoma virus, a parasite that affects rabbits, was deliberately introduced into Australia to control the rabbit population. Spread rapidly by mosquitoes, the virus devastated the rabbit population. The virus was less deadly to the offspring of surviving rabbits, however, and it caused less and less harm over the years. Apparently, genotypes (the genetic make-up of an organism) in the rabbit population were selected that were better able to resist the parasite. Meanwhile, the deadliest strains of the virus perished with their hosts as natural selection favored strains that could infect hosts but not kill them. Thus, natural selection stabilized this host-parasite relationship......

  问题:

  Paragraph 1: A symbiotic relationship is an interaction between two or more species in which one species lives in or on another species. There are three main types of symbiotic relationships: parasitism, commensalism, and mutualism. The first and the third can be key factors in the structure of a biological community; that is, all the populations of organisms living together and potentially interacting in a particular area.

  1. Which of the following statements about commensalism can be inferred from paragraph 1? (3)

  ○It excludes interactions between more than two species.

  ○It makes it less likely for species within a community to survive.

  ○Its significance to the organization of biological communities is small.

  ○Its role in the structure of biological populations is a disruptive one.

  Paragraph 2: Parasitism is a kind of predator-prey relationship in which one organism, the parasite, derives its food at the expense of its symbiotic associate, the host. Parasites are usually smaller than their hosts. An example of a parasite is a tapeworm that lives inside the intestines of a larger animal and absorbs nutrients from its host. Natural selection favors the parasites that are best able to find and feed on hosts. At the same time, defensive abilities of hosts are also selected for. As an example, plants make chemicals toxic to fungal and bacterial parasites, along with ones toxic to predatory animals (sometimes they are the same chemicals). In vertebrates, the immune system provides a multiple defense against internal parasites.

  2. The word derives in the passage is closest in meaning to (2)

  ○Digests

  ○Obtains

  ○Controls

  ○Discovers

  3. According to paragraph 2. which of the following is true of the action of natural selection on hosts and parasites?(2)

  ○Hosts benefit more from natural selection than parasites do.

  ○Both aggression in predators and defensive capacities in hosts are favored for species survival.

  ○The ability to make toxic chemicals enables a parasite to find and isolate its host.

  ○Larger size equips a parasite to prey on smaller host organisms.

  ......

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  译文:

  参考译文:共生关系

  共生关系是两种或更多物种之间的一种交互作用,其中一个物种要么在另一个物种中生存要么依赖另外一个物种生存。共生关系共有三种类型:寄生、共栖和互利共生。其中第一种和第三种是一个生物群落结构的关键要素。所谓生物群落,指的是在某个特定区域内,所有生物体共同生存并且潜在地相互影响。

  寄生现象是一种捕食式的关系,其中,寄生物通过削弱其寄主而获得自身所需食物。寄生物的形体往往小于寄主。绦虫是寄生的例子之一,它生存在较大型动物的肠道中,并吸收寄主体内的营养。自然选择青睐那些寻找寄主并且寄生能力强的寄生虫,同时,防御能力强的寄主也被选择出来。比如说,有些植物会产生对真菌和细菌寄生物有毒的化学物质,也会产生那些对捕食动物有毒的化学物质(有时这些化学物质是一样的)。而对于脊椎动物来说,其身体的免疫系统可以对体内的寄生物进行多层防御。

  有时候,寄生关系的自然选择效应也可能在现实中被观察到。比如说,二十世纪四十年代,亿万只欧洲兔在澳大利亚泛滥成灾。兔子肆虐了广袤的土地,并给牛羊业带来了极大的威胁。1950年,为了控制兔灾,澳大利亚特意引进了一种名为粘液瘤病毒的可影响兔子的寄生虫。通过蚊子的快速传播,兔子数量急剧减少。然而,这种寄生虫对于生存下来的兔群的后代就没有那么致命了,而且这种伤害逐年减小。显然,该兔群的遗传性状(生物体的基因结构)经过了自然选择,已经具备了更好地抵抗粘液瘤病毒的能力。同时,由于自然选择更倾向于那些能够感染寄主但不致其死的品系,这种病毒最致命的品系也逐渐地衰败了。这样,自然选择便使得寄主-寄生虫的关系趋于稳定......

  以上内容是我们分别截取的托福阅读真题文档里面的三段内容,其中更包含答案、解析和译文,完整版需要学生点击面免费索取或者向在线咨询的老师们免费索取。最后祝愿大家在接下来的备考过程中都能够取得优异的成绩。



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