The term ‘energy’ is quite familiar to us and we use it in a very general sense. Often this usage does not correspond to the precise scientific definition. It is, therefore, important to first define it precisely.
Energy is defined as the capacity to do work. We use energy to cook food. We need energy to raise our feet. A truck climbs up a hill when energy is supplied to it from combustion of diesel oil and a light bulb glows when electric energy is supplied to it. The developing countries of the Third World face perpetual energy shortage, and in the present day world, energy and prosperity go hand in hand.
As such, sun is the ultimate source of all our energy, which caters to the need of our ecosystem. In the interior of the sun, a thermonuclear reaction is continuously going on at a temperature of about 108 K wherein hydrogen is converted into helium. This is accompanied by a release of huge amount of energy, which manifests itself as heat and light.
Observations made from artificial satellites indicate that nearly 30% of the total solar radiation entering our atmosphere is reflected by the earth-atmosphere system. The remaining 70% of the radiation is absorbed by the earth’s atmosphere. Of this 19% is absorbed directly by the atmosphere and the rest by the earth’s surface. The blue and red component (400-500 nm and 600-700 nm band respectively) of solar radiation are strongly absorbed by chlorophyll, the green pigment, present in vegetation and are converted into chemical energy. That is how energy for the ecosystem is trapped.
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The energy captured by the autotrophs will never revert back to the sun. Similarly, the energy, which passes to the herbivore, does not revert back to autotrophs and so on. Thus, the flow of solar energy is unidirectional. Its immediate implication is that an ecosystem would collapse if the sun stops giving out energy.
The second important fact is that at each tropic level energy content decreases progressively. This factor is easily explained by noting that the trapped solar energy is used up in metabolic activity and measured as respiration.
In an ecosystem, energy is transferred in an orderly sequence. We have explained above that energy is always unidirectional. In a chain of events, some useful energy may be lost as heat. Two descriptive physical laws apply to such situations. These are the first and the second laws of thermodynamics.
The first laws of thermodynamics deals with the conservation of matter and energy and states that energy cannot be created or destroyed but can only change from one form to another. For example, the energy of visible light is absorbed by green plants through photosynthesis; it is changed into chemical energy stored in the glucose molecules. Almost all living organisms including plants consume glucose in respiration and use the stored chemical energy for their metabolic activity. Some of the energy is dissipated as heat, another form of energy.
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The second law of thermodynamics states that some useful energy is converted into unusable waste heat during every energy transformation. This heat energy escapes into the surrounding environment. Another way of saying the same thing would be that in energy transformation, some energy is always lost in the form of heat that is thereafter unavailable to do further useful work. For example, if we have to push an object on the floor some of the work, which we are putting in pushing the object, is used up as heat energy due to friction.
In the same way when energy stored in the body is used in doing some work, some of the useful energy is lost as body heat. In other words, energy transformation in the physical as well as biological worlds are less than one hundred per cent efficient, because energy’s natural and unavoidable tendency is to spread out, i.e., become disorganized or disordered. The degree of disorder that occurs in any given system can be measured and expressed mathematically as entropy. In fact, the universe as a whole is tending towards a state of maximum entropy. In order to continue to function, organisms must continue to receive new input of energy in the ecosystem.