26 Biological Clocks

Key Idea: A biological clock is the endogenous timing system an organism uses to synchronise its activities with the external environment. A biological clock is an endogenous (internal) timing system that helps to control the physiological responses and activities of an organism. Rhythms established by the biological clock

Where is the biological clock located?

The location of the biological clock varies between organisms. In birds, reptiles, and amphibians it is located in the pineal gland (in the brain). In insects each cell has its own biological clock. In mammals the biological clock is located in the hypothalamus.

For most humans, the biological clock runs at about a 25½ hour day. To keep it synchronised with the 24 hour-day cycle it needs to be reset each day, reacting to outside stimuli such as light and dark and meal times. The clock is made up of a collection of cells in the hypothalamus, called the suprachiasmatic nucleus (SCN), just behind the eyes. Light from the eyes stimulates the nerve pathways to the SCN and regulates its activity.

Eye

(endogenous rhythms) will continue even in the absence of environmental cues, although the period (duration) of the rhythm may be slightly different to the environmental rhythm. Biological clocks have an adaptive function, such as helping anticipate environmental changes and preparing the body for the activities that will predictably follow.

The pineal gland secretes the sleep-inducing hormone melatonin in the dark. Melatonin production is suppressed by bright light.

Once exposed to light, the suprachiasmatic nucleus (SCN) communicates with the hypothalamus and pineal gland to promote wakefulness (e.g. by raising body temperature, releasing stimulating hormones, and suppressing melatonin production.

Melatonin

Functions of the biological clock

The biological clock helps to control internal rhythms such as heart activity, hormone secretion, blood pressure, oxygen consumption, and metabolic rate. When the rhythms controlled by the biological clock become out-of-sync with the environment various short or long term disorders can occur, e.g. jet lag. Some functions of the biological clock are described below:

Prediction of and preparation for events in the environment (e.g. storing food reserves as fat for periods of torpor or hibernation).

Gannets

Synchronisation of migration, reproduction, or social activities. Animals congregate at breeding grounds at the same time of year. Synchronising circadian and annual rhythms, e.g. basking in tuatara and other reptiles, with changes in the environment.

Honeybee

Time compensation in navigation and sun compass orientation using a continuously consulted clock (e.g. honeybee food collection).

1. (a) Where is the biological clock located in mammals?

(b) What is the main stimulus that helps synchronise the biological clock with the environment?

2. Describe two functions of the biological clock and their adaptive value:

(a)

(b)