What Makes Something Alive?
Have you ever wondered what separates a rock from a rabbit, or a flame from a fern? Scientists have developed a clear set of criteria , a checklist of conditions , to decide whether something is living or non-living.
If an object meets all of the criteria, it is considered a living thing. If it fails even one, scientists classify it as non-living.
Scientists prefer using a checklist rather than a simple definition because life comes in many surprising forms , from tiny bacteria to giant whales , and a checklist covers all of them.
The Seven Criteria for Life
Scientists use the following seven criteria to determine if something is living. A handy way to remember them is the acronym MRSGREN:
- Movement , Living things can move. Even plants move slowly towards light.
- Respiration , Living things release energy from food through a chemical process called respiration. This involves taking in oxygen and releasing carbon dioxide (in most organisms).
- Sensitivity , Living things detect and react to changes in their environment (e.g. a plant turning towards sunlight, you pulling your hand away from something hot).
- Growth , Living things grow and develop over their lifetime. A seed grows into a tree; a baby grows into an adult.
- Reproduction , Living things are able to produce more individuals like themselves.
- Excretion , All living things produce and remove waste products as a result of their internal chemical processes.
- Nutrition , Living things need an energy source. Animals eat food; plants make their own through photosynthesis. All living things also need water to survive.
Use the acronym MRSGREN to remember the seven criteria: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, Nutrition. Every letter matches a criterion , learn this and you'll always have the full list!
You may also see 'Cells' added to some versions of this list , all living things are made of one or more cells, the basic building blocks of life. We'll explore cells in more detail below.
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