What is Living and Characteristics of Life

NCERT lists key features of life. GROWTH: increase in mass and number of cells. In living organisms it is INTRINSIC (from inside), while in non-living (mountains, crystals) it is EXTRINSIC (by accumulation on surface). Growth and reproduction are NOT defining/foolproof properties (mules, sterile worker bees, infertile humans cannot reproduce; non-dividing cells do not grow). The ONLY DEFINING property is METABOLISM (sum of all chemical reactions). All living things have metabolism without exception. Memory aid: 'CRiMP' - Cellular organisation, Reproduction, Metabolism, self-consciousness (Property of). The most defining property at higher levels is SELF-CONSCIOUSNESS / self-replication, and cellular organisation is the defining feature of all living forms.

If someone asked you to write a single sentence that separates a living tiger from a stuffed one in a museum, what would you choose — movement? breathing? growth? The NCERT answer is sharper than any of these: metabolism, paired with cellular organisation, is what truly defines life. This lesson unpacks why metabolism is the most carefully phrased "defining feature" in Class 11 Biology, and why the NEET examiners love to test the fine print here.

Definition: Metabolism is the sum total of all chemical reactions occurring inside a living organism's body.

Definition: A defining feature is one that is present in every living being and absent in every non-living object — so it can be used as a yes/no test for life.

Metabolism — the universal feature

Every cell in every living organism — from a bacterium in a Ganga water sample to a human neuron — is running thousands of chemical reactions every second. These reactions break down food (catabolism), build new molecules (anabolism), generate energy (respiration), and recycle waste. Together, they form the organism's metabolism.

Two crucial facts to memorise:

First, no non-living object ever shows metabolism. A rock does not metabolise. A car does not metabolise (combustion is not metabolism — there is no enzymatic regulation, no homeostatic feedback, no integrated network). Even a virus outside a host cell shows no metabolic activity. Metabolism is therefore a property unique to life.

Second, metabolism is the defining feature precisely because of that universality. NCERT is careful with the word "defining": growth and reproduction are characteristic of living things but not defining, because exceptions exist (a sterile worker bee does not reproduce; an adult human stops growing in height). Metabolism, by contrast, never takes a holiday. The moment metabolism stops, the organism is dead.

The cell-free system puzzle

Here is the subtle bit NEET adores. If you grind up a yeast cell and extract its enzymes, those enzymes will still perform metabolic reactions in a test tube. This is called an in vitro (literally "in glass") cell-free system. Sugar can still be broken down; ATP can still be produced. The reactions are unmistakably metabolic.

But is that test tube alive? No.

NCERT phrases this beautifully: "Metabolic reactions can be demonstrated outside the body in cell-free systems. An isolated metabolic reaction(s) outside the body of an organism, performed in a test tube is neither living nor non-living." Such reactions are called "living reactions" — they are biochemistry happening just as it happens inside cells — but the test tube as a whole is not a "living thing."

This raises an obvious question: if metabolism is the defining feature of life, how can metabolism happen outside life? The resolution is the second defining feature.

Cellular organisation — the other defining feature

Definition: Cellular organisation is the arrangement of a living being's body into one or more cells, each with the structural and functional machinery needed for metabolism.

A cell-free extract has the chemistry but not the structure — no membrane to maintain a separate internal environment, no genetic material to direct repair and replacement, no integrated network of organelles. The reactions run for a short while and then stop, because nothing maintains them. A living cell, on the other hand, organises its metabolism inside compartments, feeds in raw materials continuously, removes waste, and repairs itself.

So the NCERT answer is precise: a living organism is one that shows metabolism AND is organised at the cellular level. Together, these two are the unbreakable defining features. Other properties — growth, reproduction, response to stimuli, consciousness, homeostasis — are characteristic but not strictly defining, because exceptions exist or because non-living objects can mimic them superficially.

Why growth and reproduction are NOT defining

Growth is one of the trickiest traps in this chapter. Living organisms grow by adding new cells from inside (intrinsic growth). Non-living objects can also grow — a mountain grows, a sand dune grows, a crystal grows. But they grow by accretion (adding from outside). So growth is characteristic but not defining, because:

• Non-living things also show some form of growth.
• Some living things (sterile mules, post-mitotic neurons in adults) stop growing.

Reproduction is also non-defining:

• Sterile worker bees, mules, and post-menopausal women cannot reproduce, yet are alive.
• Some non-living objects (crystals, fires) show forms of self-propagation.

Hence NCERT places metabolism and cellular organisation at the top of the hierarchy of defining features, and demotes growth and reproduction.

Worked example

Question: A student isolates the enzyme amylase from his own saliva, puts it in a test tube with starch solution at 37 °C, and observes the starch breaking down into maltose. He claims the test tube is now "alive." Evaluate this claim.

Solution:
Step 1: Identify what is happening. A metabolic reaction (enzymatic hydrolysis of starch) is occurring in vitro.
Step 2: Apply the NCERT definition. Living = metabolism + cellular organisation. The test tube has metabolism but lacks cellular organisation.
Step 3: Therefore, this is a "living reaction" but not a "living thing." It is neither living nor non-living in the classical sense — it is just isolated biochemistry.
Conclusion: The student is wrong. The reaction is a property of life, but the test tube is not alive. Cellular organisation is missing.

Why it matters: This lesson is the conceptual foundation for NEET's "What is Living?" chapter, which contributes roughly 1–2 questions every year, often as assertion-reason or "which is the defining feature" MCQs. Marking metabolism as the universal defining feature, while keeping cellular organisation as its essential partner, is a frequent trap.

Real-world example: When ISRO scientists searched for signs of life on Mars (and earlier, when Carl Sagan designed the Viking lander experiments), they did not look for movement or shape. They looked for metabolism — specifically, evidence that something was breathing in atmospheric gases and breathing out altered gases. Why? Because metabolism is the only universal defining feature. If a Martian organism existed, it might look like nothing on Earth, but it would have to metabolise.

Common misconception: Many students confuse "alive" with "complex." They imagine that a complicated machine (say, a self-driving car) might be considered alive. NCERT rules this out cleanly: machines have no metabolism — they consume energy through combustion or batteries, but they do not run integrated enzymatic networks regulated by their own genes. They lack cellular organisation. Complexity is not life.

A mountain or boulder grows by accumulation of material on its OUTER surface (extrinsic growth). A living organism (e.g. a plant or animal) grows from WITHIN by cell division (intrinsic growth). In plants, growth continues throughout life (open form), whereas in most animals growth occurs only up to a certain age. Unicellular organisms (e.g. bacteria) show growth as increase in cell number, which overlaps with reproduction - here growth = reproduction (synonymous). Hence growth cannot be taken as a defining property of living organisms.

Cover the answer, recall, then check. 12 cards on the NCERT must-know facts for NEET.

Q1. What are the twin characteristics of growth?
A1. Increase in mass and increase in number of individuals (cells). Both are twin characteristics of growth.

Q2. Why is growth NOT a defining property of living organisms?
A2. Non-living objects (mountains, boulders, sand mounds) also grow — but by accumulation of material on the surface. In living organisms growth is from inside. A dead organism does not grow, so growth cannot define life.

Q3. How does growth differ between plants and most animals?
A3. Plants grow continuously throughout life (indefinite growth); most animals grow only up to a certain age. Cell division continues in some tissues even in adults.

Q4. Why is reproduction not a defining property of life?
A4. Many organisms do not reproduce — mules, sterile worker bees, infertile human couples. They are still living. Hence reproduction cannot be an all-inclusive defining feature.

Q5. Give NCERT examples of asexual reproduction modes.
A5. Budding (yeast, Hydra), fragmentation (filamentous algae, protonema of mosses), true regeneration (Planaria), spore formation (fungi/algae).

Q6. Define metabolism and state its status as a life property.
A6. Metabolism = sum total of all chemical reactions occurring in the body. All living organisms — unicellular and multicellular — exhibit metabolism. It is a defining feature of all living organisms without exception.

Q7. Are isolated metabolic reactions in a test tube 'living'?
A7. No. Isolated metabolic reactions in vitro are neither living nor non-living — but they are surely 'living reactions'. Hence cellular organisation of the body is the defining feature of life forms.

Q8. What is the ultimate defining property of living organisms per NCERT?
A8. Consciousness — the ability to sense the surroundings/environment and respond to stimuli (physical, chemical, biological). It is the most obvious yet technically complicated defining feature.

Q9. Which organism is the only one aware of itself?
A9. The human being — the only organism who is self-conscious (aware of itself).

Q10. What are 'emergent' properties in the hierarchy of organisation?
A10. Properties present at a higher level of organisation that are absent at lower levels — e.g. tissue properties absent in constituent cells; organelle properties absent in constituent molecules. Living organisms are self-replicating, evolving, self-regulating interactive systems.

Q11. How is growth in a unicellular organism observed?
A11. By increase in number (cell division). In unicellular organisms, growth is measured as increase in cell number, seen in vitro in cultures.

Q12. State the key NCERT one-liner distinguishing living from non-living.
A12. All living organisms grow, reproduce, respond to stimuli and metabolise; but the truly all-inclusive defining properties are cellular organisation and consciousness.