William
Schwartz -
Professor
of Neurology at the University of Massachusetts Medical School.
What is a biological clock?
There have been many different
definition of what a biological clock is:
A biological clock is “an
innate physiological system capable of measuring the passage of time in a
living organism” (Coleman 1986).
A biological clock is “a
widespread cellular mechanism that underlies diverse rhythmic functions in
organisms” (Young and Kay, 2001).
Biological clocks are a way for
organisms to track the passage of time so that they can anticipate and adapt
to changing environmental conditions e.g. plants anticipating the day so
that they can prepare their photosynthetic apparatus in time.
Properties of a Biological Clock
Biological clocks have certain
properties that define them, irrespective of where they are found:
They continue to run under
constant conditions: Biological clocks continue to run in the
absence of environmental cues, producing rhythmic patterns. This is a
phenomenon called freerunning.
They are temperature
compensated: Biological clocks are one of the few biological systems
that are not sensitive to temperature. Normally the activity of many systems
alters according to temperature e.g. enzymes catalyse reactions at a faster
rate at a warm temperature than a cold one. Temperature compensation is
important property for the clock, as they have to remain accurate
irrespective of environmental temperature changes – otherwise image the
problems that would occur on a cold day! Temperature compensation is one of
the key defining properties of a biological clock.
They are altered by light:
Biological clocks are altered (entrained) by light levels and changing
lengths of photoperiod. This allows an organism to anticipate seasonal
changes and adjust to changing day lengths.
They are under Genetic Control:
Alterations in certain clock genes affect the period length of the clock so
that it runs shorter or longer than 24 hours. The genetics of biological
clocks is covered in the “cogs” section.
They are adjustable: A
biological clock can be reset by external environmental cues (Zeitgebers).
These include photoperiod, temperature and social cues. It allows the clock
to run with a rhythm most suitable for the organism’s surroundings.
The
Clock
This section covers the "visible" actions of biological clocks
such as overt rhythms, entrainment, resetting, and effects such as jet lag.
The
Clockwork
This section covers the "nuts and bolts" of the clock - the molecular and
cellular mechanisms that make the clock work.
Inputs
Outputs
The
Time of our Lives