What is Living? Characteristics of Living Organisms

Chapter 1 opens the whole NCERT syllabus and is a favourite for one-mark board and NEET questions: "Which is NOT a defining property of life?" is asked almost every year. Get the defining-vs-non-defining distinction right and these are free marks.

The characteristics of living organisms

All living things share: growth, reproduction, metabolism, cellular organisation, consciousness, and the ability to sense and respond to their surroundings.

Growth — not a defining property

Living organisms grow from inside by cell division, so growth is intrinsic. Non-living things (a mountain, a boulder, a sand mound) also grow — but by accumulation of material on the surface (extrinsic). Because non-living objects grow too, and a dead organism does not grow, growth is not a defining feature. In unicellular organisms, growth and reproduction are the same event (cell division increases both cell number and mass).

Reproduction — not a defining property either

Organisms reproduce asexually (fission, budding, fragmentation) or sexually. But mules, sterile worker bees and infertile human couples do not reproduce, yet are living. So reproduction is not an all-inclusive defining feature.

Metabolism and cellular organisation — defining

Metabolism is the sum total of all chemical reactions in the body. Every living organism is a chemical factory, so metabolism is a defining feature without exception. Isolated metabolic reactions in a test tube are "living reactions" but are not, by themselves, alive — hence cellular organisation is the true defining property.

Consciousness — the defining property

The capacity to sense the environment and respond to stimuli (light, temperature, other organisms) is consciousness — the defining property of all living organisms. Humans alone show self-consciousness. Living organisms are self-replicating, evolving, self-regulating interactive systems.

Exam Tricks & Tips

• 🎯 Defining trio to memorise: metabolism, cellular organisation, consciousness. Growth and reproduction are the two "trap" non-defining properties.
• 🎯 Growth in living = intrinsic (from inside); growth in non-living = extrinsic (surface deposition). This exact contrast is a stock MCQ.
• 🎯 In unicellular organisms, growth = reproduction (both are cell division) — a classic assertion-reason question.
• 🎯 Consciousness is the defining property of living organisms; self-consciousness is unique to humans.
• 🎯 "In vitro isolated metabolic reactions are living reactions but not living" — remember this exact NCERT line.
• ❌ Common mistake: ticking "growth and reproduction" as defining features. They are NOT, because non-living things grow and many living things never reproduce.

Quick recap

Life = metabolism + cellular organisation + consciousness (defining) plus growth and reproduction (present, but not defining). Growth from inside marks the living; sensing and responding to the environment is the ultimate hallmark of life.

Cover the answer, recall, then check. 12 cards on the defining features of life.

Q1. Name the three DEFINING properties of living organisms.
A1. Metabolism, cellular organisation, and consciousness.

Q2. Why is growth NOT a defining property of life?
A2. Non-living objects (mountains, boulders) also grow by surface accumulation, and dead organisms do not grow.

Q3. How does growth differ in living vs non-living things?
A3. Living: intrinsic, from inside by cell division. Non-living: extrinsic, by deposition of material on the surface.

Q4. Why is reproduction not an all-inclusive defining feature?
A4. Mules, sterile worker bees and infertile couples are living but do not reproduce.

Q5. Define metabolism.
A5. The sum total of all the chemical reactions occurring in the body.

Q6. Are isolated metabolic reactions in a test tube "living"?
A6. No — they are living reactions but are not alive; only cellular organisation of these reactions makes a living thing.

Q7. What is consciousness in a biological sense?
A7. The ability to sense the surroundings/environment and respond to stimuli (physical or chemical).

Q8. Which property is unique to human beings?
A8. Self-consciousness.

Q9. In unicellular organisms, growth and reproduction are the same event — why?
A9. Both occur by cell division: one division increases cell number and cell mass simultaneously.

Q10. Which is called the defining property of all living organisms without exception?
A10. Metabolism (with cellular organisation as its structural basis).

Q11. List the general characteristics shown by living organisms.
A11. Growth, reproduction, metabolism, cellular organisation, consciousness, and response to stimuli.

Q12. Living organisms are described as what kind of systems?
A12. Self-replicating, evolving, self-regulating interactive systems capable of responding to external stimuli.

A crystal grows, a river flows, a flame consumes fuel — yet none is alive. So what separates a bacterium from a salt crystal? Biology answers not with a single magic property but with a checklist of defining features.

Definition / core idea

A living organism is a self-organising system that shows a combination of defining traits: growth, reproduction, metabolism, cellular organisation, consciousness (responsiveness), self-replication, self-regulation and the ability to sense and respond to the environment. No single feature is exclusive to life — it is their combination that defines it. The most defining, non-negotiable properties are metabolism and cellular organisation; the most fool-proof, all-inclusive defining feature is the capacity for self-consciousness/responsiveness to stimuli.

Deep explanation

Beginner — the twin defining features

• Growth: an increase in mass and number of cells. In living things growth is intrinsic (from inside, by cell division). A mountain or a crystal grows only by extrinsic deposition of material on the surface — so growth alone cannot define life.
• Reproduction: production of offspring resembling the parent (asexual or sexual). But mules, sterile worker bees and many organisms cannot reproduce, so this too is not universal.

Intermediate — why growth and reproduction fail as sole criteria

Unicellular organisms like Amoeba and bacteria show growth and reproduction as the same event — the cell divides. In multicellular organisms, growth (more cells) and reproduction (offspring) are separate processes. Because sterile individuals live without reproducing, reproduction is not an all-inclusive defining property. This is why NCERT stresses: properties of tissues are not present in their constituent cells — living organisation shows emergent properties at higher levels.

Advanced — metabolism, consciousness and self-organisation

• Metabolism: the sum of all chemical reactions in the body. Every living organism, without exception, shows metabolism — this makes it a defining feature. Reactions carried out in a test tube are metabolic reactions but are neither living nor non-living; isolated metabolism is not "living".
• Cellular organisation of the body is the defining feature of all living forms.
• Consciousness / responsiveness: the ability to sense the surroundings and respond to physical, chemical and biological stimuli (light, temperature, other organisms). NCERT calls consciousness the defining property of living organisms, and considers self-consciousness the most fool-proof marker.
  Life is therefore a self-replicating, evolving, self-regulating interactive system capable of responding to external stimuli.

Worked example

Q. A student argues that a growing sand dune and a growing child are both "living" because both grow. Where is the flaw?
A. Growth is not a defining property by itself, and the type of growth differs. The child grows by intrinsic growth — cell division from within, adding mass and cell number internally. The sand dune grows by extrinsic accretion — new sand deposited on its outer surface. Only intrinsic growth accompanies the full living checklist (metabolism, cellular organisation, responsiveness), so the dune is non-living.

Real-world application

Defining life precisely matters for virology (viruses sit on the boundary — no independent metabolism), astrobiology (NASA screens for metabolism and self-replication when hunting extraterrestrial life), and medicine (declaring biological death uses cessation of integrated function, not just stopped growth).

Exam tricks & shortcuts

• Two features are truly defining (universal): metabolism and cellular organisation. Growth and reproduction are NOT defining (crystals grow; mules don't reproduce).
• Growth in living = intrinsic; in non-living = extrinsic (surface deposition).
• Mnemonic "GRIMCC R" for characteristics of life: Growth, Reproduction, Irritability (responsiveness/consciousness), Metabolism, Cellular organisation, Consciousness, Regulation/self-replication.

Worked Example

Problem: A student argues that a car is "living" because it moves, consumes fuel (like eating), and gives out exhaust (like breathing). Using the defining characteristics of living organisms, evaluate this claim.

Solution:
Step 1 — List the defining, distinctive properties of life: metabolism, growth (from inside/intrinsic), reproduction, self-replication, cellular organisation, response to stimuli, and consciousness.

Step 2 — Examine "movement". A car moves only when externally driven; living organisms show self-generated (autonomous) movement and locomotion. So external movement alone is not a criterion.

Step 3 — Examine "metabolism". Fuel combustion in an engine is a single chemical reaction, not the organised network of thousands of enzyme-controlled reactions (metabolism) that characterises a cell. Only living systems carry out metabolism, and no non-living object performs it.

Step 4 — Examine growth and reproduction. Living organisms grow intrinsically (by cell division from within) and reproduce to make copies of themselves. A car neither grows from within nor produces offspring cars.

Step 5 — Apply the strongest single criterion. The defining features unique to life are cellular organisation and self-consciousness; a car has neither.

Answer: The car is non-living: it lacks metabolism, intrinsic growth, reproduction, cellular organisation and consciousness — external movement and fuel use do not make it alive.