This preview shows page 1. Sign up to view the full content.
Unformatted text preview: ecial 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.
1. The word Consequently in the passage is closest in meaning to
2. In paragraph 1, the experiment on the fungus Neurospora is mentioned to illustrate
○the existence of weekly periods of activity as well as daily ones
○the finding of evidence that organisms have internal clocks
○the effect of space on the internal clocks of organisms
○the isolation of one part of an organism's cycle for study
3. According to paragraph 1, all the following are generally assumed to be true EXCEPT:
○It is important for animals' daily activities to be coordinated with recurring events in their environment.
○Eukaryotes have internal clocks.
○The relationship between biological function and environmental cycles is a topic of intense research.
○Animals' daily rhythms are more dependent on external cues than on internal clocks.
Paragraph 2 在 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...
View Full Document
- Fall '13