Plants and animals have to be able to orientate themselves in time, as well as in space. Humans do this, we know it as our body clock. It is the mechanim that makes you hungry at lunch time and tired at night.
Although we can establish time from external queues, it is better to be able to use an internal clock. This is because it will continue to run even if conditions are changed, and does not need constant updating. If you didn't have an internal clock, you would fall asleep whenever you walked into a dark place, and would stay awake all night if you left a light on.
This is often tested by removing an item from its natural habitat. People have lived in totally isolated caves for periods of weeks or months, and have established that the human body clock runs on a 25 hour day. This allows for adaption to the environment, as a 24 hour day would lock the body clock up too much. However this extra hour means that the body needs constant re-adjustment. This is why you need to get outside every day to keep happy and healthy.
|
An amoeba does not have an endogenous clock. When external stimulus is removed the amoeba will: |
A zeitgeber is an external stimulus used by an organism to adjust its endogenous clock. This allows it to adjust to the environment and stops the clock from slipping. Basically it serves the same function as setting a clock for daylight saving, and keeping it in time by checking it daily. This is a continuous process keeping everything in order.
|
A crab is put in a tank where the tide no longer goes up and down. It will |
If an internal clock is free running clock it lacks a zeitgeber and continues to run. This allows an animal to live in it's enviroment without any external input/output from the environment.
|
Without an external queue, an endogenous clock is: |
|
If an animal has no endogenous clock, and no external inputs it is: |
Entrainment is the process of an animal re-ajusting to it's surroundings based on the zeitgeber available. This is not an instatnaneous process, as it is based around complex biochemistry that takes time to ajust.
|
A treatment for jet lag is to go out into the sun. This is because it provides a: |
|
If an animal has no endogenous clock, and no external inputs it is: |