Friday, July 3, 2020

The Living World - Notes (Class 11) - Part 1: Properties of Living Organisms


Life is a unique, complex organization of molecules, expressing through chemical reactions which lead to growth, development, responsiveness, adaptation & reproduction.

A living organism is self-replicating, evolving and self-regulating interactive system capable of responding to external stimuli.


1. Growth

·    It is the increase in number & mass of cells by cell division.

·   In plants, growth continues throughout their lifespan.

·   In animals, growth is only up to a certain age. However, cell division occurs to replace lost cells.

·   Basically, growth is the increase in mass & size. Thus non-living objects also grow (surface accumulation of material). So growth is not a defining property of living organisms.

·   In living organisms, growth is from inside.

2. Reproduction

·   It is the production of progeny having features similar to those of parents.

·   Organisms reproduce asexually and sexually.

·    In unicellular organisms, growth & reproduction are same because they reproduce by cell division.

·   Many organisms do not reproduce (e.g. mules, worker bees, infertile human couples, etc). Hence, reproduction is not a perfect defining property of living organisms.

3. Metabolism

·   It is the sum total of all biochemical reactions taking place inside a living system.

·   It is the defining feature of living organisms.

·   Metabolic reactions can be demonstrated outside the body in cell-free systems. Isolated metabolic reactions in vitro are not living things but are living reactions.

4. Cellular organization

·   Organisms are made up of one or more cells.

·   It is the defining feature of living organisms.

5. Consciousness

·   It is the ability of organisms to sense their environment and respond to environmental stimuli (like light, water, temperature, other organisms, chemicals, pollutants, etc).

·   All organisms are ‘aware’ of their surroundings. So, it is the defining property of living organisms.

·   Human is the only organism having self-consciousness.

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