What is life? The signs we look for
There is no one-line test for life. So biologists use a checklist. A thing is living if it shows all of these signs:
- Cells: it is made of one or more cells. A cell is the smallest unit that can stay alive.
- Metabolism: it uses chemical reactions to get energy and build its body. Taking in food (nutrition), releasing energy (respiration) and removing waste (excretion) are parts of metabolism.
- Response: it senses a change (a stimulus) and reacts. A plant bends to light. You pull your hand from a hot pan.
- Homeostasis: it keeps its inside steady. Your body stays near 37 °C and sweats when hot.
- Growth and development: it gets bigger and changes form, like seed → plant.
- Reproduction: it makes new living things like itself.
- Adaptation: over many generations, a population changes to suit its surroundings (evolution).
Many living things also move. Plants move slowly, for example their leaves turn to the sun.
Living, dead and never alive
Fire, crystals and cars show some signs. Fire uses fuel and spreads. A crystal grows. A car uses energy. But none is made of cells, and none keeps its inside steady or passes on genes. So they are non-living.
A dead thing (a fallen leaf, a wooden table) was alive and is made of cells, but has stopped metabolism. A resting thing, like a seed or a frozen bacterium spore, is alive: its metabolism is just very slow.
Two big ideas tie the signs together: living things maintain themselves (repair, steady inside, use energy) and they reproduce with change, so life can adapt.
Are viruses alive?
A virus is a tiny packet of genes (DNA or RNA) inside a protein coat. It is much smaller than a cell.
- Like life: it has genes, it makes copies of itself, and it changes (evolves). That is why new flu and COVID-19 variants appear.
- Not like life: it has no cell, no metabolism of its own, and it cannot grow. Outside a host cell it is like a lifeless particle. It can copy itself only inside a living cell, using that cell's machinery.
So most biologists say viruses are on the border between living and non-living.
Life science and how biologists work
Life science (biology) studies living things at every level: molecule → cell → tissue → organ → organism → population → ecosystem. It links with chemistry, physics and computing, and it is used in medicine, farming and protecting nature.
Biologists work by inquiry:
- Observe and ask a question.
- Make a hypothesis (a testable guess).
- Do a fair test with a control group, changing only one thing.
- Collect data and draw a conclusion.
- Share results so others can check them.
Some knowledge comes from induction (many observations → a general rule, e.g. "every organism we checked has cells"). Some comes from deduction (rule → prediction to test).
Good scientists also explain their work simply to non-experts. Use everyday examples, avoid jargon and say what is still unknown.
Try it: the seed test
Take 10 dry seeds (moong, rajma or chana). Put 5 on wet cotton in a cup and 5 on dry cotton. Check every day for 4 days. Write which signs of life you see: growth, response (roots grow down, shoot grows up), use of water. Then predict: if you boil 5 seeds first, will they sprout? Try it. Boiled seeds are dead and will not grow. In the 3D, pick "Seed" in free play and compare.
Key formulas and definitions
- Cell: smallest unit that can carry out life
- Metabolism: all chemical reactions in a living thing (builds up + breaks down)
- Stimulus → response: a change the organism detects, and how it reacts
- Homeostasis: keeping conditions inside the body steady (temperature, water, sugar)
- Growth: permanent increase in size or mass
- Reproduction: making new organisms (sexual or asexual)
- Adaptation: a feature that helps a population survive in its home; built over generations
- Virus: genes in a protein coat; copies only inside a host cell
Worked examples
1. Is a mobile phone alive? Use the checklist.
It uses energy (battery) and responds to touch. But it is not made of cells, does not grow, cannot reproduce and does not adapt over generations. Not alive.
2. A seed has been in a jar for a year and shows no change. Is it alive?
Probably yes. It is made of cells and its metabolism is very slow (dormant). Give it water and warmth: if it sprouts and grows, it was alive.
3. Why do we shiver in the cold? Which sign of life is this?
Shivering makes muscles release heat, which keeps the body near 37 °C. It is homeostasis, and also a response to a stimulus (cold).
Common mistakes
- Thinking "moves = alive". Cars and wind move; plants are alive but barely move.
- Calling a dead leaf "non-living". It is dead: it was alive once and is still made of cells.
- Saying growth alone proves life. Crystals and fires also get bigger, but from outside, not by making new cells.
- Saying viruses are just small bacteria. Bacteria are cells; viruses are not.