Life Support Systems
By Tas - tas@oustudent.me.uk
Usable Energy and Food Chains
Energy provides the fuel for life to grow, move, reproduce and, simply put, exist. The Sun provides the vast majority of all the energy on Earth. As a result of the fusion reactions taking place at the heart of the Sun, enormous quantities of energy bombard the planet every second of every day, in the form of radiation (e.g. sunlight etc.,).
This is not what most mammals would consider a usable form of energy, however it is taken up by the green plants that provide the basis for all food chains. These autotrophs (i.e. an organism capable of synthesising its own organic substances from inorganic compounds) have the unique ability to use this energy to power the reactions in which carbohydrates (C6H12O6) are formed from naturally occurring carbon, hydrogen and oxygen in the atmosphere (Westcott, R., p 21).
For all other life on Earth, this form of energy isn’t usable and energy must be obtained in other ways, for example, from the carbohydrates provided by the autotroph. The herbivores (plant eaters) that consume the base level autotrophs are known as primary consumers, the ones that feed off the primary consumers are secondary consumers and so on. This is known as a food chain – where each organism feeds off organisms lower down the chain. By doing this, the energy that arrived in the biosphere from the Sun is passed on in a usable form.
This form of energy conversion is far from perfect and at each stage energy is lost into the environment, as heat etc., and can play no further role in supporting life (Westcott, R., p 22). The effect of this is that as each organism moves further away from the base level there is less overall energy available to support life. As a result, most heterotrophs feed from a variety of sources (food chains) and form what is called a food web. This combination of food chains helps ensure there is sufficient energy at each level to support life.
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Reference
Westcott, R., ed. (2004) T210 Block One: The Environment, Risk and Public Health, Milton Keynes, The Open University.
