The below mentioned article provides notes on circadian rhythm with its suitable diagram.
Organisms often exhibits rhythmic behaviour in association with daily alterations of light and darkness. Interestingly, many of the rhythmic responses to day and night continue even when organisms are kept under artificially made constant light (LL) or continuous dark (DD) environment.
While in some organisms these rhythmic behaviours persist under constant conditions at least for a period of time, in others they persist indefinitely. Circadian rhythm report in bacteria like Escherichia coli (t = 24 hr.) with the growth oscillation. Similar movement was also reported in cyanobacterial species. Of course in eukaryotes, such rhythm is quite evident.
All circadian rhythms oscillate with a period length close to, but seldom equal to 24 hrs. when organisms are kept under constant conditions of light, temperature and other possible geophysical factors. The pattern of phase response curves may vary from organism to organism.
The persistence of rhythm varies with quality of light and also duration of exposure. In many animals, circadian rhythms in locomotory activity persist in continuous light (LL) as well as in constant darkness (DD) but with their ‘t’ altered.
The temperature is another important time eve of circadian rhythms. In nature, organisms experience temperature cycles with peaks coinciding with the early part of the afternoon and troughs in the late dark phase (close to dawn). The amplitude of temperature cycle as low as 1°C may synchronize circadian rhythms in a number of plant species and ectothermic vertebrates, whereas high amplitude thermal cycles.
Circadian rhythms are endogenous. They are both an organismal and a cellular phenomenon. Light/dark cycle works as one of the strongest entrainers of these rhythms, although other periodic cycles can also entrain circadian rhythms. They free-run in the absence of a light/dark cycle.
This suggests that there is a pacemaker(s) which generates circadian rhythms in various physiological, biochemical and behavioural variables. Thus a basic circadian system has necessarily three important components such as photoreceptors, pacemaker(s), and observable rhythmic outputs.
The entraining pathways transduce information between the photo receptive elements and the pacemaker(s). The coupling pathways are also necessary as a link (the efferent pathways) between the pacemaker(s) and the multiple effector systems. The effector outputs are the overt rhythms that allow us to study the properties of the central clockwork (pacemaker) (Figs. 11.1 and 11.2).
There may be multiple photoreceptors, multiple clocks and many overt rhythms. There components may interact with each over in a variety of ways. A possible operation of feedback of the clock into the photoreceptive pathways has been strongly suspected.
In recent years, molecular mechanisms of circadian rhythms with respect to gene control processes in Drosophila, Neurospora, and Gonyaulax were reported. Cytochrome a light absorbing protein was also discovered from Arabidopsis thaliana, which are connected with circadian rhythms.
Nobody knows when or how these clocks evolved through evolution? Photoperiodic behaviour in higher plants showed distinct circadian rhythm as regulated by phytochrome pigment system associated with hormonal regulation.
By and large circadian rhythm as noted in all categories of prokaryotes and eukaryotes, perhaps first evolved in cyanobacteria/bacteria and then evolved through evolutionary pathways. Circadian rhythms in breeding cycle or reproductive cycle of vertebrates including man are of paramount importance, which is regulated by pituitary, thyroid, gonadal or adrenal glandular secretion too.
The ecological significance of circadian rhythms are not understood clearly. However chronobiology has progressed tremendously in the past four decades. The molecular basis of cyanobacterial and eukaryotic circadian rhythm have been partly understood with respect to their period length, sustainability, and relationship with light.