Characteristics of living things

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.

This subtopic sits within the MYP Sciences global context of Scientific and Technical Innovation — the tools and frameworks scientists use to understand life around us are themselves products of centuries of careful inquiry and creative thinking.

Scientists use seven criteria to determine if something is living. A handy way to remember them is the acronym MRSGREN — each letter stands for one criterion:

Now let's explore what each letter means:

1. Movement — Living things can move. Even plants move slowly towards light, although you can't always see it happening.
2. Respiration — Living things release energy from food through a chemical process called respiration. Many organisms use oxygen and release carbon dioxide in a process called aerobic respiration. However, some organisms (like certain bacteria and yeast, and even human muscle cells during intense exercise) can release energy without oxygen — this is called anaerobic respiration. The key point is that all living things respire to release energy from food.
3. 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).
4. Growth — Living things grow and develop over their lifetime through cell division and development. A seed grows into a tree; a baby grows into an adult.
5. Reproduction — Living things are able to produce more individuals like themselves.
6. Excretion — All living things produce and remove metabolic waste — harmful by-products generated by the chemical reactions happening inside their own cells. For example, animals excrete carbon dioxide through breathing and urea through urine.
7. Nutrition — Living things need an energy source. Animals eat food; plants make their own through photosynthesis. All living things also need water to survive.