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 a 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 the 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.
- Humans are the only organisms having self-consciousness.
