下面是美国留学：2012年10月13日雅思阅读考试机经回忆的内容，大家可以参考一下2012年10月13日雅思阅读考试的内容，更好的备考下一次的雅思阅读考试。

考试日期：2012年10月13日

Title：biological clock

Question types:heading; 多选;判断

文章内容回顾：可以参照20100115的旧题，但是版本众多，莫衷一是。可参阅《精六六五阅写补充练习》P294页，”how does the biological clock tick”, 这篇文章的题型和难度与此次考试的文章的题型和难度相近。

英文原文阅读：Biological Clocks

Survival and successful reproduction usually require the activities of animals to be coordinated with predictable events around them. Consequently, the timing and rhythms of biological functions must closely match periodic events like the solar day, the tides, the lunar cycle, and the seasons. The relations between animal activity and these periods, particularly for the daily rhythms, have been of such interest and importance that a huge amount of work has been done on them and the special research field of chronobiology has emerged. Normally, the constantly changing levels of an animal's activity—sleeping, feeding, moving, reproducing, metabolizing, and producing enzymes and hormones, for example—are well coordinated with environmental rhythms, but the key question is whether the animal's schedule is driven by external cues, such as sunrise or sunset, or is instead dependent somehow on internal timers that themselves generate the observed biological rhythms. Almost universally, biologists accept the idea that all eukaryotes (a category that includes most organisms except bacteria and certain algae) have internal clocks. By isolating organisms completely from external periodic cues, biologists learned that organisms have internal clocks. For instance, apparently normal daily periods of biological activity were maintained for about a week by the fungus Neurospora when it was intentionally isolated from all geophysical timing cues while orbiting in a space shuttle. The continuation of biological rhythms in an organism without external cues attests to its having an internal clock.

When crayfish are kept continuously in the dark, even for four to five months, their compound eyes continue to adjust on a daily schedule for daytime and nighttime vision. Horseshoe crabs kept in the dark continuously for a year were found to maintain a persistent rhythm of brain activity that similarly adapts their eyes on a daily schedule for bright or for weak light. Like almost all daily cycles of animals deprived of environmental cues, those measured for the horseshoe crabs in these conditions were not exactly 24 hours. Such a rhythm whose period is approximately—but not exactly—a day is called circadian. For different individual horseshoe crabs, the circadian period ranged from 22.2 to 25.5 hours. A particular animal typically maintains its own characteristic cycle duration with great precision for many days. Indeed, stability of the biological clock's period is one of its major features, even when the organism's environment is subjected to considerable changes in factors, such as temperature, that would be expected to affect biological activity strongly. Further evidence for persistent internal rhythms appears when the usual external cycles are shifted—either experimentally or by rapid east-west travel over great distances. Typically, the animal's daily internally generated cycle of activity continues without change. As a result, its activities are shifted relative to the external cycle of the new environment. The disorienting effects of this mismatch between external time cues and internal schedules may persist, like our jet lag, for several days or weeks until certain cues such as the daylight/darkness cycle reset the organism's clock to synchronize with the daily rhythm of the new environment.

Animals need natural periodic signals like sunrise to maintain a cycle whose period is precisely 24 hours. Such an external cue not only coordinates an animal's daily rhythms with particular features of the local solar day but also—because it normally does so day after day—seems to keep the internal clock's period close to that of Earth's rotation. Yet despite this synchronization of the period of the internal cycle, the animal's timer itself continues to have its own genetically built-in period close to, but different from, 24 hours. Without the external cue, the difference accumulates and so the internally regulated activities of the biological day drift continuously, like the tides, in relation to the solar day. This drift has been studied extensively in many animals and in biological activities ranging from the hatching of fruit fly eggs to wheel running by squirrels. Light has a predominating influence in setting the clock. Even a fifteen-minute burst of light in otherwise sustained darkness can reset an animal's circadian rhythm. Normally, internal rhythms are kept in step by regular environmental cycles. For instance, if a homing pigeon is to navigate with its Sun compass, its clock must be properly set by cues provided by the daylight/darkness cycle.

题型难度分析：heading题难度较大

题型技巧分析：heading题考查学生对段落大致含义的把握能力，虽然出题人可能希望我们好好的读懂全段，但是考生未必有这样的能力。我们可以根据一些特定的词语在段落中寻找主题句的出现位置：

1. 在段落开头有举例结构的地方往往说明该段的主题句在举例结构之前。

2. 在段落中间有比较明显的转折结构，那么转折后的内容有可能是段落的重点内容。

3. 在段末有表示结果的词语，那么该句句子有可能是段落的主题句。

常见的段落结构有：总分结构;对比结构;并列结构;分总结构。总分结构还是占大多数的。

剑桥雅思推荐：原文练习可参阅《精六六五阅写补充练习》P294页，”how does the biological clock tick”

以上就是关于美国留学：2012年10月13日雅思阅读考试机经回忆的全部内容。大家可以在备考雅思阅读考试的过程中根据自己的实际情况选择一些话题进行背景知识的准备。预祝大家考试成功。

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