Coaching Centre
Is it alive?
The eight characteristics of living things (MRN GREWW), cells, autotrophs and heterotrophs, and how to tell living, dead and non-living things apart, including tricky cases like fire, seeds and viruses.
A bushfire moves across the landscape by itself. It "eats" fuel, takes in oxygen, gives off carbon dioxide and smoke, grows bigger and even spreads by throwing out sparks that start new fires. So is a fire alive? Most people say no straight away, but explaining why takes a bit more thought.
Eight characteristics of living things
A living thing is called an organism. After a lot of observation and argument, biologists agree that all organisms, from bacteria to blue whales, share the same set of characteristics. A quick way to remember them is MRN GREWW.
| Characteristic | What it looks like | |
|---|---|---|
| M | Movement | Moves by itself. Animals are obvious, but plants move too: a sunflower turns to follow the Sun and a Venus flytrap snaps shut. Movement can be slow, or happen inside the cells. |
| R | Reproduction | Makes new individuals like itself. Elephants give birth, plants make seeds, bacteria split in two. |
| N | Nutrition | Takes in nutrients for energy and building material. Autotrophs (plants, algae) make their own food by photosynthesis; heterotrophs (animals, fungi) get it from other organisms. |
| G | Growth | Grows and develops: a spore becomes a mushroom, a tadpole becomes a frog, a baby becomes an adult. |
| R | Responds to stimuli | Reacts to changes. You pull your hand off a hot iron; a kangaroo bolts at a sudden noise; roots grow towards water. |
| E | Exchanges gases | Takes in and gives out gases, through lungs, gills, skin or leaves. Some bacteria are killed by oxygen but still exchange other gases. |
| W | Wastes | Gets rid of wastes from chemical reactions in its cells: breathing out carbon dioxide, sweating, urinating. Plants lose wastes through their leaves. |
| W | Water | Needs water. It transports substances and is where most of the cell's chemical reactions happen. You are about 60% water. |
There's a ninth idea that sits underneath all of these: every organism is made of one or more cells, and every cell contains genetic material (DNA) that carries instructions for building a copy. You'll study cells in Chapter 3.
Living, dead or non-living?
Non-living things can show some of the characteristics, but never all of them by themselves. A steam train moves and needs fuel, but it doesn't grow, reproduce or respond to its surroundings. A crystal grows, but it doesn't take in nutrients or exchange gases.
That gives three categories, and the difference between the last two matters:
- Living: currently carrying out all the characteristics (a gum tree, your dog, yeast in bread dough).
- Dead: was once alive but has stopped (a leather belt, a dinosaur skeleton, a dry autumn leaf, paper, a shark's tooth).
- Non-living: was never alive (a rock, a phone, plastic, water, a silver chain).
Watch out for plastic: most plastic is made from crude oil, which formed from organisms that died millions of years ago. But the plastic itself was made in a factory and was never a living thing, so we call it non-living. The fair test is whether the object itself was ever an organism.
- Pick an object. Tick each characteristic you think it shows by itself, then decide: living, dead or non-living.
- Do the fire. Which characteristics does it seem to have? Which one does it fail on?
- Try the seed and the virus. These are the cases scientists actually argue about.
The tricky cases
Fire. Fire seems to move, grow, "feed" and give off gases, but none of this is controlled from inside: it's just a chemical reaction spreading wherever there's fuel and oxygen. A fire has no cells, no DNA and doesn't respond to stimuli in any purposeful way. Its "offspring" are not copies built from instructions. Non-living.
Seeds. A dry seed in a packet barely exchanges gases and doesn't grow, but it is alive: it is dormant, with its cells working extremely slowly. Add water and warmth and it springs into action. Some seeds stay alive for centuries; a date palm seed about 2000 years old from Masada in Israel was germinated in 2005.
Viruses. A virus is a tiny package of genetic material in a protein coat. It can't feed, grow or reproduce by itself; it has to get inside a living cell and hijack it to make copies. Outside a cell it does nothing at all. Most biologists say viruses are not living organisms, and they aren't placed in any kingdom, but some scientists disagree. That debate is real science in progress.
Animals that pause. Some Australian animals push "alive" to the limit. The water-holding frog of inland Australia buries itself, wraps itself in a cocoon of shed skin and can wait underground through years of drought, barely breathing, until heavy rain comes.
Test yourself
Which of these is dead (rather than non-living)?
Wood came from a tree, which was once alive.
Plants are autotrophs. This means they
"Auto" means self; autotrophs make their own food by photosynthesis.
A fire is not alive mainly because
Fire seems to move, feed and give off gases, but it has no cells or genetic instructions; it is just a chemical reaction.
A dry bean seed sits in a packet for two years, then grows when planted. While it was in the packet it was
Its cells were working very slowly. Something dead can't start growing again.
When you sweat, which characteristic of living things are you showing?
Sweat removes water, salts and some wastes (it also cools you, which is a response).
Sort each item.
Name each characteristic of living things.
A living thing is called an . Plants are because they make their own food, while animals and fungi are . All living things are made of . Something that was once alive, like a leather belt, is described as , while something that was never alive is .
A robot vacuum cleaner moves around a room by itself, avoids obstacles and returns to its dock to recharge. Using the characteristics of living things, explain why it is not alive.
The robot shows movement, responds to obstacles in its way and takes in energy when it recharges, so it seems to show some characteristics of life. However, it cannot reproduce or grow, it does not exchange gases or need water, and it is not made of cells. Because it does not show all the characteristics of living things, it is non-living.
Explain the difference between something that is dead and something that is non-living. Use an example of each.
A dead thing was once a living organism but has stopped carrying out life processes, for example a fallen leaf or a leather belt. A non-living thing was never alive, for example a rock or a plastic bottle.