1. THE LIVING WORLD
Life is a unique, complex organization of molecules, expressed 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
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.
