What is 'Living'? — The Defining Features of Life
Before we can classify the living world, we must answer a deceptively simple question: what does it mean to be living? NCERT identifies a set of characteristics shared by all living organisms. Some are only descriptive (non-living things can show them too), while a few are truly defining.
The features usually listed are: growth, reproduction, metabolism, cellular organisation, consciousness, and the ability to sense the surroundings and respond to stimuli.
1. Growth
Growth is an increase in mass and an increase in the number of individuals/cells. These are its twin characteristics.
- In living organisms, growth is intrinsic — it occurs from inside the body (new cells/material are added internally).
- In non-living objects (a mountain, a boulder, a sand mound, a crystal), apparent "growth" is extrinsic — material simply accumulates on the surface.
- In unicellular organisms, growth (an increase in the number of cells) happens by cell division.
Key point: Growth is NOT a defining property of life. A dead organism does not grow, and many non-living things do grow (by accumulation). So growth alone cannot tell the living from the non-living.
Reproduction, Metabolism, Cellular Organisation & Consciousness
2. Reproduction
Reproduction is the production of progeny that resemble the parents. It occurs by many methods — asexual (binary fission, budding, fragmentation, spore formation) and sexual.
Examples (NCERT): budding — Hydra and yeast; fragmentation — the filamentous algae, the fungi and the protonema of mosses; true regeneration — Planaria (a body fragment regrows the whole organism).
- In unicellular organisms, reproduction is synonymous with growth — both simply mean an increase in cell number by division.
- Reproduction is also NOT a defining property. Many living organisms do not reproduce: mules, sterile worker bees, and infertile human couples are all very much alive yet do not reproduce.
3. Metabolism
Metabolism is the sum total of all the chemical reactions occurring in the body of an organism (anabolism + catabolism together).
- All living organisms — and only living organisms — show metabolism. No non-living object exhibits metabolism.
- Metabolic reactions can be carried out outside the body, in a cell-free (in vitro) system. Such isolated reactions are living reactions, but the isolated system itself is neither living nor non-living.
- Because every living thing shows it and no non-living thing does, metabolism is a defining feature of all living organisms.
4. Cellular Organisation
All living organisms are made of one or more cells. This cellular organisation of the body is the defining feature of life forms — there is no exception.
5. Consciousness — the Defining Property
Consciousness is the ability to sense the environment/surroundings and respond to external stimuli (physical, chemical or biological — light, temperature, water, other organisms, pollutants, etc.).
- All organisms, from the simplest prokaryote to the most complex eukaryote, can sense and respond to environmental cues (photoperiod affects flowering; seasonal breeders respond to day length).
- Humans are the only organisms aware of themselves — i.e., they have self-consciousness.
- For this reason consciousness is regarded as the defining property of living organisms.
Emergent Properties
As we move up levels of organisation (molecules to cells to tissues to organisms), new properties emerge that were not present at the lower level — for example, the properties of a tissue are not present in its isolated constituent cells. These are called emergent properties.
At the deepest level, living organisms are best described as self-replicating, evolving, self-regulating interactive systems capable of responding to external stimuli.
Quick summary — defining vs non-defining
| Feature | Defining property? |
|---|---|
| Growth | No (non-living things also grow) |
| Reproduction | No (mules, worker bees don't reproduce) |
| Metabolism | Yes |
| Cellular organisation | Yes |
| Consciousness | Yes — the defining property |
How Living Processes Are Studied - Reductionist Biology
Living organisms can be studied at many levels. The reductionist approach studies life by breaking living processes down to their underlying physico-chemical reactions - that is, understanding organisms through physics and chemistry at the molecular level.
NEET note: 'Reductionist biology' = the physico-chemical approach to study and understand living organisms (asked in NEET 2025). It contrasts with looking at the organism only as a whole.