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What is meant by living? Give any four defining features of life forms.

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Ans: An organism is considered to be living if it is capable of evolving, self-replicating, responding to external stimuli, etc. Nutrition, response to stimuli, reproduction, growth are several aspects of living beings, but they are not the defining features of life forms. Growth is often mistaken to be a defining feature of life forms, but it is not the case as a few non-living things like snowball, cloud, etc. can also gain mass, volume, and space. So, growth is not a defining feature for living beings.

The four defining features of life forms are: -


  • Metabolism: The collection of carrying out several biological reactions in the living being comprises of the metabolism. Metabolism includes the breakdown and the formation of various molecules to carry out the biological reactions in the living organisms. Since metabolism requires a system comprised of cells to be carried out, it is quite evident that this cannot be done in non-living things. Though a few in-vitro reactions have been tried in the laboratories to imitate metabolism, that is not real metabolism. Thus, metabolism is one of the defining features of life.


  • Reproduction: Reproductions is carried out by the living organisms to produce off-springs like them. The reproduction could be either sexual reproduction or asexual reproduction, but the act of producing off-springs of your own kind is a feature of the living organisms alone. Though there are a few living organisms like the worker bee, that does not reproduce, that does not take reproduction down as a defining feature of the life forms because reproduction cannot be carried out in the non-living things. Thus, reproduction is a defining feature of life forms.



  • Response to stimuli: The stimuli can be felt by the organisms wither externally in the form of temperature, touch, light, etc. or internally via the hormones like dopamine, estrogen, etc. The response to the stimuli is a defining feature for the living beings because they are able to sense a change in their environment whether externally or internally and respond to it in the most suitable way according to their own convenience. This sense of consciousness is absent in the non-living beings, and thus, it is a defining feature of the living forms.

Structural organization: The structural organization, whether on a cellular level or at an upper level, is found in the living beings. Various molecules come together and arrange themselves in a particular arrangement to form cell organelles, and these cell organelles further arrange themselves in a specific fashion to form cells. These cells come together to form various tissues, and then these tissues further arrange in a way to forms various organs present in living beings. These organs are what the body of the living being function on. 


This arrangement from the cellular level up to the organs level is an extremely advanced and complicated manner of structural organization which cannot be made in the non-living beings. The arrangement of each cell organelle matter as the different arrangements form different cells; each cell arrangement further makes different tissues, and each arrangement of tissues makes different organs. This level of the hierarchy is not possible in the non-lining things, and thus the structural organization is a defining feature of the living beings.


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