Official 13 Passage 2


Biological Clocks


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  • 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.

  • 通常动物的繁衍生息需要动物的活动与周围可预测活动同步。因此,生物功能的时间与节律也就理所应当必须与昼夜交替、潮涨潮落、月圆月缺和四季更迭这样的周期性事件保持大体一致。动物的活动与这些周期之间的关系,特别是与昼夜交替之间的关系,引起人们浓厚的兴趣,而且因为大量的工作都是在其基础之上完成的而意义重大,从而也延伸出了一个特别的研究领域:生物钟学。通常意义上讲,动物活动的经常性转变——例如,睡觉、喂食、活动、繁殖、新陈代谢以及产生酶和荷尔蒙,都与环境的节律同步。但是关键问题在于,动物的作息时间是否受制于外界环境,比如日出日落,又或者是依赖于他们自身独立的生物节律。生物学家普遍认为,所有真核生物 (包括除病毒和某些藻类之外的所有生物)都有内部的生物钟。通过将生物与外界的周期性现象完全隔离,生物学家们发现生物的确有生物钟。例如,一种叫脉孢菌 的细菌在航天飞机中与一切地球时间线索隔离的情况下,所有生物日常活动周期可以持续一个礼拜左右。在没有外界信号的时候生物也能延续生物节律,这说明生物是具有生物钟的。




    脉孢菌属(NeurosPora) 因子囊孢子表面有纵形花纹,犹如叶脉而得名,又称链孢霉。
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