Diversity In The Living World

How fascinating is the living world ! The spacious range of living types is amazing. Where ever we dart our eyes we see living habitat there, be it cold mountains, deciduous forests, oceans, freshwater or marine water. So the question is what indeed life is? This question itself has two implicit questions within it. The first one is a technical one that defines what makes a living organism all together different from non-living and the second is a philosophical one, and seeks to answer what the purpose of life is? As science students, we shall not attempt answering the second question. Let's try to reflect on - what is living?

What is 'Living'?

Let me try to define 'living'. We mostly think living organisms have some distinctive characteristics such as growth, reproduction, ability to sense the environment and mount a suitable response. But let me prove you some of them are wrong.

Why is growth not the defining property of living organisms?

Before we get to the main point, let's know what growth means:

Increase in mass and increase in number of individuals are twin characteristics of growth

Multicellular organisms grow by cell division. In plants, growth is ever lasting or we can say continuous throughout their life span. In unicellular organisms growth occurs by cell division, which often leads to reproduction. Let's recall the above mentioned definition. It has two parts, first one-Increase in mass and second one-Increase in number of individuals. Non living things also grow. If we take increase in body mass as a criterion for growth. Mountains, boulders and sand mounds to grow. However this kind of growth exhibited by non-living objects is due to the accumulation of mass over them. In living organisms, growth is from inside. Growth, therefore, cannot be taken as a defining property of living organisms

Why is reproduction not the defining property of living organisms?

Fungi multiply and spread easily due to the millions of asexual spores they produce. Lower organisms viz yeast and hydra, budding is observed. In Planaria (flatworms), true regeneration is observed. [I myself have seen the regeneration of Planaria under a microscope. You believe or not I was surprised to see such a fast regeneration in them. From a little bit of mass a new organism develops. wow ! ] The fungi, the filamentous algae, the mosses, all easily multiply by fragmentation. Humans reproduce by sexual reproduction. Then why s reproduction is the defining property of living organisms? The answer is simple: There are many organisms which do not reproduce (mules, sterile worker bees, infertile human couples, etc ).

What is the defining property of living organisms?

1) Cellular organization or metabolism

2) Consciousness

Metabolism the defining property of living organisms

The sum total of all the chemical reactions occurring in our body is metabolism

All living organisms are made up of chemicals and these chemicals are continuously made and break in our body. These chemicals are called biomolecules. No non-living object exhibits metabolism. Metabolic reactions can be demonstrated outside the body in cell-free systems also. But that isolated metabolic reaction performed outside the cell is neither living or non-living. Hence, cellular organization or metabolism is the defining property of living organisms.

Consciousness the defining property of living organisms

It is the most obvious and technically complicated feature of all living organisms i.e. ability to sense their surroundings. We sense our environment through our sense organs. Photoperiod affects reproduction in seasonal breeders, both plants and animals. All organisms have the ability to define their friend and foe. This is all due to consciousness. Only human beings are the organisms who are aware of themselves, i.e., has self-consciousness.

We can say that living organisms are self-replicating, evolving and self-regulating interactive system capable of responding to external stimuli

References:

1. NCERT Biology page 1,2,3 and summary

2. Wikipedia for image

3. Notes by Dr. Shahnawaz

4. Concept maps by Dr. Pakeezah Mushtaq