Every organism needs a continuous source of energy for maintenance and growth of their body. Energy comes from the intake of food through absorption. After absorption, it goes into a chemical process which converts it to energy. We use the term cellular respiration for it.
Cellular respiration needs oxygen. We get oxygen through the lungs by the process called Respiration.
In a multi-cellular organism, the transportation of food to a different cell is necessary. So a circulatory system is present in them.
Conversion of energy also leads to by-product which is not only useless but also may be harmful. So the process of removal of these waste product through excretion becomes necessary.
So we can say absorption, respiration, transportation and excretion are essential for maintaining life.
Next: What are the differences between autotrophic nutrition and heterotrophic nutrition?
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