1. THE LIVING WORLD
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.
PROPERTIES OF LIVING ORGANISMS
1. Growth
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
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 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
It is the defining feature of living organisms.
5. Consciousness
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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