The big idea
A fire grows, uses oxygen and gives off waste gases. A robot vacuum moves and senses walls. Neither is alive. Biologists decide whether something is alive by checking for seven characteristics (features shared by every living thing). An organism has to show all seven at some point in its life.
Explained simply
A handy way to remember them is the name MRS GREN:
- Movement — an action by an organism, or part of it, that changes its position or place. A cheetah runs; a sunflower slowly turns its head; even cells move things around inside them.
- Respiration — the chemical reactions in cells that break down nutrients (such as glucose) to release energy. This is not the same as breathing. Breathing just moves air in and out of the lungs.
- Sensitivity — the ability to detect changes in the surroundings (called stimuli) and respond to them. You pull your hand from a hot pan; a plant shoot bends towards light.
- Growth — a permanent increase in size and dry mass. It happens by cells getting bigger, cells dividing, or both. A balloon getting bigger does not count; there is no new material.
- Reproduction — the processes that make more of the same kind of organism.
- Excretion — getting rid of waste made by the body's own chemical reactions (metabolism), toxic materials and substances in excess. Carbon dioxide from respiration and urea in urine are excreted.
- Nutrition — taking in materials for energy, growth and development. Plants make their own food using light, carbon dioxide and water; animals eat other organisms.
Each characteristic on its own is not enough. Crystals "grow" and rivers "move", but they do neither of the other six things.
Worked example
Exam-style question: A student says a car is alive because it moves, takes in fuel and gives off waste gases. Explain why the student is wrong. [3 marks]
Step 1 — match each feature honestly. The car moves (looks like movement). It takes in fuel (looks a bit like nutrition). It releases exhaust gases (looks a bit like excretion).
Step 2 — look for the missing processes. A car cannot reproduce — it does not make new cars. It does not grow; it stays the same size all its life. It does not show true sensitivity; it only reacts because a human or a program controls it.
Step 3 — check respiration. Burning fuel in an engine is not respiration, because respiration happens inside living cells and is controlled by enzymes.