Defining Life: What Makes an Organism Living?

Defining life is deceptively difficult because no single property is both universal to all living organisms and absent from all non-living entities. NEET consistently exploits this ambiguity.

Growth refers to the irreversible increase in mass and number of cells. However, non-living crystals also grow by accumulation of material from outside, and dead organisms obviously do not grow. Therefore, growth alone cannot define life.

Reproduction is the capacity to produce offspring (sexual or asexual). Yet mules — sterile hybrids of a horse and a donkey — cannot reproduce and are unquestionably alive. Worker bees (sterile females) and infertile humans likewise reproduce nothing yet are living. Reproduction fails as a universal defining criterion.

Response to stimuli is broad — plants undergo phototropism (bending toward light), animals exhibit withdrawal reflexes. But a thermostat also responds to temperature. Response alone is insufficient.

Metabolism — the sum total of all anabolic $\frac{biosynthesis}{constructive}$ and catabolic $\frac{breakdown}{destructive}$ chemical reactions occurring in an organism — is accepted as the TRUE defining feature of life. Crucially, no non-living entity exhibits metabolism. Even an isolated enzyme reaction in a test tube is metabolism, confirming that metabolism is intrinsic to life.

Consciousness — the ability to sense the environment and respond to it — is proposed as a broader defining property, encompassing even single-celled organisms and plants that lack a nervous system.