Correct option is B
Circadian Rhythms: These are biological rhythms that operate on a roughly 24-hour cycle and are driven by an internal biological clock located in the suprachiasmatic nucleus (SCN) of the brain in mammals. In other organisms, such as plants and fungi, circadian rhythms are controlled by similar molecular mechanisms.
The correct answer is option (b): A process that is rhythmic only in the presence of a 24-hour light and dark cycle. This statement is incorrect because circadian rhythms do not require a light and dark cycle to exist; they are endogenous rhythms generated by an internal biological clock. While light and dark cycles can synchronize circadian rhythms (a process known as entrainment), the rhythms themselves continue in a free-running state in constant conditions, such as in constant darkness, with a period close to 24 hours.
Now, let’s review why the other options are correct:
Option (a): Circadian rhythms are found in a wide variety of organisms, including bacteria, plants, fungi, and animals. This is a characteristic feature of circadian systems, as these rhythms are widespread across all forms of life.
Option (c): Circadian rhythms can indeed be synchronized by environmental cycles, particularly light-dark cycles. The external cues, known as zeitgebers (time-givers), such as light, temperature, and feeding times, help align an organism's internal clock with the external environment.
Option (d): Prolonged exposure to constant darkness can disrupt circadian rhythms because the absence of environmental cues (like light-dark cycles) can cause desynchronization of the biological clock, making it more difficult for the organism to maintain a stable rhythm.
Thus, option (b) is the incorrect statement because circadian rhythms are not dependent on external light-dark cycles to be rhythmic.
