What are defining characteristics of living organism? | Why some features of living organisms are their characteristics, but not their defining characteristic? | NEET Biology | NTA NEET

We have been learning the differences between living organisms and non-living from the very early classes of our school lives.

We can list a lot of features which distinguishes a living from the non-living like growth, reproduction and ability to sense our surroundings. 

But do all these features which we think are only the characters of living organism make them the defining features of life forms?

To watch video lectures on NEET Biology: Click here.

Let us explore one by one:

1) Growth: When we talk about growth we think of it as increase in mass, or maybe increase in numbers Which indeed are twin characters of growth. So when increase in mass and increase in numbers are twin characters of growth. And when we can observe this growth in living organisms when they undergo cell division (in multicellular this results in both increase in overall body mass, while in unicellular organism this results in increase in numbers). Why can’t we say that growth is a defining characteristic of living organism? Because if we considering increase in body mass as a feature of growth because this grown due to increase in body mass also occurs in non-living objects like mountains, boulders and sand mounds by accumulation of material over them. While in living organisms this growth occurs from inside, so when we simply say growth, it doesn’t qualify to become a characteristic defining feature, growth from `inside’ has to be mentioned along with it. 

2) Reproduction: this is also a characteristic of living organisms, but does it qualify to become a characteristic feature? Again the answer is No! Firstly because if we talk about unicellular organisms like bacteria, unicellular algae or Amoeba a simple mitotic devision in them will result into the new organism so cell division itself is a mode of reproduction, also we have learnt that increase in cell no is a characteristic of growth, so in unicellular organism growth is synonymous with reproduction. Secondly because there are some living organisms which do not reproduce! Yes, like we can take an example of mules or sterile worker bees. So reproduction also cannot become an all inclusive defining characteristic.

To watch video lectures on NEET Biology Click here.

3) Cellular organisation: every living organism is made up of a lot of chemicals and these chemicals are continuously being made and break from one form to another like from complex to simpler molecules or from simples to complex molecules. These chemical reactions inside the cells of all living organisms occur without any exception which makes metabolism the defining feature and the cellular organisations of the body of the living organism the defining characteristic! No non living exhibits metabolism.

4) Consciousness: this is one characteristic characteristic of the living organisms which undoubtedly is the defining characteristic of the living organisms. It is the ability of the living organism to sense their surrounding and environment. From the smallest of bacteria to the most complex animal, every living organism can sense their surrounding and mount a suitable response. Human beings on the other hand are the organisms who are of themselves, humans are self-conscious.

So while Growth, and Reproduction being a feature of living organism are not characteristic feature, but Metabolism, cellular organisation and consciousness are the defining characteristics of the living organism.

To watch lectures on NEET Biology Click here.

Author: Shivansh Gaur (M.sc Biotechnology - CSJMU) 

Reference: NCERT Biology Class 11.