CIRCADIAN RHYTHM:THE BIOLOGICAL CLOCK WITHIN

CIRCADIAN RHYTHMS IN PLANTS

• Daily  organism are  subjected to cycles of light and darkness  and in association with these changes of light and darkness both plants and animals often exhibit rhythmic behaviours. 

EXAMPLE- Leaf petal movement (day & night position )
                      Stomatal opening and closing
                      Sporulation pattern in fungi (e.g Pilobolus and Neurospora)
                     Time of day of pupal emergence (the fruit fly Drosophila)

• When organism are transferred from daily light dark cycles to continuous darkness,many of these rhythms continued to be expressed,atleast for several days.
• Under such uniform conditions the period of rhythm is then close to 24 hours ,then we often apply the term CIRCADIAN RHYTHM.

• Just because of the fact that they continue in a constant light or dark environment,these circadian rhythms cannot be direct responses to the presence or absence of light but must be based on an internal pacemaker,often called as ENDOGENOUS OSCILLATOR.

• This oscillator is coupled to a variety of physiological processes,such as leaf movements or photosynthesis and it maintains the  rhythm. For this reason endogenous oscillator can be considered as CLOCK MECHANISM and the physiological functions that are being regulated ,such as leaf movements or photosynthesis are sometimes referred to as HANDS OF THE CLOCK.

CIRCADIAN RHYTHMS EXHIBIT CHARACTERISTICS FEATURES

• Circadian rhythms arise from cyclic phenomenon that are defined by three parameters-

1. PERIOD- It is the  time between comparable points in the repeating cycle. Typically  a period is measured as the time between consecutive maxima( peaks) or minima (troughs).
2. PHASE- It is any point in the cycle that is  recognizable by its relationship to the rest of the cycle. Mostly the  phase points are peak and trough positions.
3. AMPLITUDE- It is  the distance between  peak and trough. The amplitude  can often vary while the period remains unchanged in a biological rhythms.

• In constant light or darkness the rhythm depart from an exact 24 hours period. 
• These  rhythms are then drifted  in relation to solar time ,either gaining or losing time dependence on whether the period is shorter or longer than 24 hours.
• Under  natural conditions,the endogenous oscillator is ENTRAINED (Synchronised) to a true 24 hour period by environmental signals,the most important of which are the light to dark transition at dusk and the dark to light transition at dawn. 

• Such environmental signals are termed as ZEITGEBERS (German for "time givers").
• When such signals are removed,  for example-transfer to continuous darkness-the rhythm is said to be FREE RUNNING  and that is characteristic of the particular organism. 

• Although the rhythms are generated internally,they normally require an environmental signals such as exposure to light or a change in temperature to be restarted.

• We should note that the clock itself doesn't damp out ,only the coupling between the molecular clock (Endogenous Oscillator ) and the physiological function is affected.

• The Circadian Clock will have  no value to the organism if it could not keep accurate time  under the fluctuating temperatures experienced by organism in natural conditions.
• Hence temperature has little or no effect on the period of the free running rhythm.
• The features that enables the clock to keep time at different temperatures is called TEMPERATURE CONDENSATION.
• Although all of the biochemical step in Pathway are temperature sensitive,their temperature responses will probably cancel out each other.
• EXAMPLE- Changes in the rate of synthesis of intermediates could be compensated for by parallel changes in their rate of degradation. In this way, the steady state levels of clock regulators would remain constant at different temperatures.