How do we know something is alive?
A dog, a mango tree and a mushroom are alive. A stone, a chair and a bicycle are not. A cloud moves and a crystal of salt can "grow" in a solution, yet neither is alive. So how do we decide?
Scientists look for a set of characteristics that all living things share. No single feature is enough; a living thing shows all of them during its life.
The characteristics of living things
Characteristic | What it means | Examples |
|---|---|---|
Need food | food gives energy and materials for growth | plants make their own food; animals eat plants or other animals |
Growth | living things increase in size from within | a seedling becomes a tree; a puppy becomes a dog |
Respiration | releasing energy from food, usually using oxygen | humans breathe through lungs, fish use gills, leaves use tiny pores |
Response to stimuli | reacting to changes around them | you pull your hand from a hot pan; touch-me-not leaves fold |
Excretion | getting rid of wastes made in the body | sweating and urine in humans; some plants shed wastes through leaves |
Reproduction | producing more of their own kind | birds lay eggs; plants form seeds |
Movement | moving the body or its parts | animals walk, swim, fly; flowers open and close |
Death | every living thing has a limited life span | all organisms eventually die |
Respiration
Respiration is the process that releases energy from food. Breathing (taking in air and giving it out) is only the first part of it.
Humans and most animals breathe through lungs.
Fish take in oxygen dissolved in water through their gills.
An earthworm breathes through its moist skin.
Plants exchange gases through tiny pores called stomata, found mostly on the leaves.
Response to stimuli
A change in the surroundings that causes a living thing to react is a stimulus (plural: stimuli).
Many flowers, such as the sunflower, turn towards the sun.
The leaves of the touch-me-not plant (Mimosa) fold up when touched.
Cockroaches run to hide when a light is switched on in a dark kitchen.
Your mouth waters when you see or smell your favourite food.
Movement in plants
Plants are fixed in one place, but parts of them do move: roots grow down into the soil, shoots grow towards light, and tendrils of a pea plant coil around a support. Animals can usually move from place to place.
Seeds: alive or not?
A seed looks lifeless. It doesn't seem to eat, breathe or grow. But give it water, air and warmth and it germinates into a new plant. Seeds are alive; they are simply dormant (resting) until conditions are right.
Is a car alive?
A car moves, uses fuel for energy and gives out smoke as waste. But it cannot grow by itself, reproduce or respond to stimuli on its own. That is why it is non-living. A living thing must show all the characteristics, not just one or two.
Summary
Living things need food, grow, respire, respond to stimuli, excrete, reproduce, move and die.
Breathing is part of respiration; plants exchange gases through stomata.
A seed is alive but dormant; a moving car is still non-living because it lacks most features of life.