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The steam engine uses the power generated from steam pressure to move. Coal is used in furnaces to heat water in the boiler to generate steam. Steam pressure makes the wheels of the engine move. As coal is burnt, smoke is emitted and ash is left behind in the furnace. Smoke is let off through the chimney and the ash is removed periodically.
In this functional system, smoke and ash are the waste products that need to be expelled out from the engine to ensure its effective functioning.
In living organisms also, energy is produced by metabolically burning food. But this is accompanied by the formation of a variety of by-products which are harmful to the organism. These wastes have to be eliminated.
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