Advanced Placement (AP) Biology Practice Exam

What characterizes circadian rhythms in organisms?

1. They are annual cycles

2. They are irregular and unpredictable

3. They follow a 24-hour biological cycle

4. They are solely based on lunar phases

The correct answer is: They follow a 24-hour biological cycle

Circadian rhythms are characterized by their consistent and predictable pattern that aligns with the 24-hour cycle of day and night. These rhythms are biological processes that occur in various organisms, including plants, animals, and even bacteria, allowing them to adapt their behavior and physiological processes to the regular changes in their environment, such as light and darkness. This 24-hour cycle influences numerous functions, including sleep-wake cycles, hormone release, eating habits, and other bodily functions that peak at specific times of the day. The mechanisms behind circadian rhythms involve clocks regulated by external cues like sunlight, which help organisms maintain synchronization with the Earth's rotation. Annual cycles, irregularity, and solely lunar influences do not accurately reflect the defining characteristics of circadian rhythms. Instead, the precise and rhythmic nature of the 24-hour biological cycle is what underscores the importance and function of these rhythms in living organisms.