Circadian rhythms are maintained by a biological clock

It is clear that organisms have some way of measuring time, and that they are well adapted to the 24-hour day-night cycle of our planet. A biological clock resides within the cells of all eukaryotes and some prokaryotes. The major outward manifestations of this clock are known as circadian rhythms (from the Latin circa, "about," and dies, "day").

We can characterize circadian rhythms, as well as other regular biological cycles, in two ways: The period is the length of one cycle, and the amplitude is the magnitude of the change over the course of a cycle (Figure 39.13).

The circadian rhythms of cyanobacteria, protists, animals, fungi, and plants have been found to share some important characteristics:

► The period is remarkably insensitive to temperature, although lowering the temperature may drastically reduce the amplitude of the rhythmic effect.

► Circadian rhythms are highly persistent; they may continue for days even in an environment in which there are no environmental cues, such as light-dark periods.

► Circadian rhythms can be entrained, within limits, by light-dark cycles that differ from 24 hours. That is, the period an organism expresses can be made to coincide with that of the light-dark cycle to which it is exposed.

► A brief exposure to light can shift the peak of the cycle— it can cause a phase shift.

Plants provide innumerable examples of circadian rhythms. The leaflets of plants such as clover normally hang down and fold at night and rise and unfold during the day. The flowers of many plants show similar "sleep movements," closing at night and opening during the day. They continue to open and close on an approximately 24-hour cycle even when the light and dark periods are experimentally modified.

The period of circadian rhythms in nature is approximately 24 hours. If a clover plant, for example, were to be

Circadian rhythms are characterized on the basis of time, measured in periods of about 24 hours.

Effect

Circadian rhythms are characterized on the basis of time, measured in periods of about 24 hours.

Effect

.and on the basis of the magnitude of the rhythmic effect, measured by the cycle's amplitude.

.and on the basis of the magnitude of the rhythmic effect, measured by the cycle's amplitude.

39.13 Features of Circadian Rhythms Circadian rhythms, like all biological rhythms,can be characterized in two ways: by period and by amplitude.

placed in light on a day-night cycle totaling exactly 24 hours, it would express a rhythm with a period of exactly 24 hours. However, if an experimenter used a day-night cycle of, say, 22 hours, then over time the rhythm would change—it would be entrained to a 22-hour period.

If an organism is maintained under constant darkness, it will express a circadian rhythm with an approximately 24hour period. However, a brief exposure to light under these circumstances can cause a phase shift—that is, it can make the next peak of activity appear either later or earlier than expected, depending on when the exposure is given. Moreover, the organism does not then return to its old schedule if it remains in darkness. If the first peak is delayed by 6 hours, the subsequent peaks are all 6 hours late. Such phase shifts are per-manent—until the organism receives more exposures to light.

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