What Is Life? Characteristics of Living Things
In 2019, CSIRO researchers found up to one billion living microorganisms in a single teaspoon of Australian soil — every one of them carrying out all seven characteristics of life, day and night.
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Q1 · Write down three things you think are alive and three things you are sure are NOT alive. What clue did you use to decide?
Q2 · A car can move, use fuel, give off waste gases and even get warmer. Why isn't a car alive?
â—Ź Know
- The seven characteristics of living things (MRS GREN)
- That all living things are made of cells
- The difference between living, dead, and non-living
â—Ź Understand
- Why a single feature (like movement) is not enough to call something alive
- Why scientists need agreed rules to decide what is alive
- That viruses sit in a "grey area" of the rules
â—Ź Can do
- Sort objects into living, dead, or non-living using MRS GREN
- Explain why something is or isn't alive using evidence
- Give a clear example of each of the seven characteristics
- Organism
- Cell
- Respiration
- Excretion
- Stimulus
- Smallest building block of life
- A change the body can detect
- An individual living thing
- Getting rid of waste chemicals
- Releasing energy from food inside cells
Pick up a teaspoon of soil from a garden and you're holding up to a billion living organisms — each one feeding, growing, and reproducing right now. Scientists use a checklist of seven characteristics that all living things share to decide what counts as alive. A handy way to remember them is the made-up name MRS GREN.
| Letter | Stands for | What it means |
|---|---|---|
| M | Movement | The organism can move part or all of itself. Plants bend toward light; animals walk, swim or fly. |
| R | Respiration | Cells release energy from food (usually by reacting it with oxygen). |
| S | Sensitivity | The organism detects and reacts to a stimulus (light, touch, heat, chemicals). |
| G | Growth | Gets bigger or more complex over time by making more of its own cells. |
| R | Reproduction | Can make new copies of itself (offspring) of the same species. |
| E | Excretion | Removes waste chemicals made by the body. |
| N | Nutrition | Takes in food or makes its own (plants do this with sunlight). |
If a thing shows all seven, scientists agree it's alive. Missing even one is enough to disqualify it.
MRS GREN stands for Movement, , Sensitivity, , Reproduction, , and Nutrition. A living thing must show seven.
Wrong: "Fire is alive — it moves, grows, and 'eats' fuel." Fire ticks a few boxes but is missing most of MRS GREN — it does not respond to a stimulus, it cannot reproduce (one fire doesn't make a baby fire) and it has no cells.
Right: Fire is a chemical reaction, not a living thing. It fails MRS GREN — no cells, no reproduction, no real sensitivity.
Wrong: "Plants aren't really alive — they don't move." Plants do move — they bend stems toward light, open flowers in the morning, close leaves at night. The movement is just slower than animals.
Right: Plants do all seven MRS GREN characteristics. Their movement and response to stimulus is slower and harder to see, but it is real.
Wrong: "Respiration is the same as breathing." Breathing is moving air in and out of your lungs. Respiration is a chemical reaction inside every single one of your cells that releases energy.
Right: Breathing brings oxygen to your cells. Respiration is the chemical reaction inside the cells that uses that oxygen to release energy from food.
Many students mix up these three categories. Use this rule:
- Living — shows all seven MRS GREN characteristics right now. Examples: you, a magpie, a gum tree, a yeast cell.
- Dead — was once alive but is not now. Examples: a fallen leaf, a fossil, a piece of timber, a sausage.
- Non-living (sometimes called "never alive") — was never a living thing. Examples: a rock, water, the air, a glass marble.
The trick: ask "was this ever alive?". If yes and it's still alive → living. If yes but not anymore → dead. If no → non-living.
Every living thing on Earth is made of one or more cells. A cell is the smallest unit that can still be called "alive" on its own.
- A bacterium = one cell.
- An ant = about a million cells.
- A human = about 37 trillion cells.
- A blue whale = roughly 100 quadrillion cells.
You can't see a cell with your eye — most are about 0.01 mm across. That's why microscopes were such a huge discovery for biology (you'll meet the scientists who built them in Lesson 6).
So when we say "all living things are made of cells", we mean every plant, animal, fungus and microbe — without exception.
This is one of the most argued-about questions in biology. A virus (like the flu virus or SARS-CoV-2) is much smaller than a cell. It is not made of cells at all — it's just a tiny piece of genetic material wrapped in a protein coat.
Run a virus through MRS GREN:
| Characteristic | Does a virus do it? |
|---|---|
| Movement | No — it just drifts. |
| Respiration | No — no chemical reactions of its own. |
| Sensitivity | No. |
| Growth | No — does not grow. |
| Reproduction | Sort of — only inside a host cell, using the host's machinery. |
| Excretion | No. |
| Nutrition | No. |
One out of seven. That's why most biologists say viruses are not alive — they sit at the edge of life. But they need a living host to spread, which is why they cause infectious diseases.
A dry seed sitting in a packet for ten years shows almost none of the seven MRS GREN characteristics — no movement, no respiration you can measure, no growth. Predict: is a dry seed alive, dead, or non-living? Explain in one sentence, then reveal.
How close was your prediction?
Nice — you spotted that "no activity" can hide life that is just paused.
Good — being surprised is the point. Dormant life is a real and important category.
At the start of the lesson, you thought about whether a virus counts as alive — even though it can copy itself.
Now that you've worked through MRS GREN and the virus table, write a fuller answer. Which of the seven characteristics does a virus pass, and which does it fail? Why do most biologists say it sits outside life rather than at the edge of it?
Q1. Write what each letter of MRS GREN stands for and give a one-line example of each from a living thing of your choice. (3 marks)
Q2. Sort the following into "living", "dead" or "non-living" and explain your choice for each: a fossil seashell, a sprouting bean, a piece of granite, a fallen autumn leaf. (4 marks)
Q3. A student claims: "If something can reproduce, it must be alive." Evaluate this claim. Use the MRS GREN checklist and at least one specific example (e.g. virus, fire, crystal). (4 marks)
Answers
â–ľMCQ 1
B — In MRS GREN, the first R stands for Respiration (the chemical reaction in cells that releases energy from food). The second R stands for Reproduction.
MCQ 2
D — A wooden chopstick is dead (it came from a tree, which was alive). Marbles, water and pebbles are non-living because they were never alive.
MCQ 3
C — Non-living things can show a single feature alone. Fire moves; rivers flow; crystals "grow". Using just one feature would wrongly include these. The seven-feature rule excludes them.
MCQ 4
A — Every living thing is made of at least one cell. B is wrong (animals and microbes are cells too). C is wrong (most cells need a microscope). D is wrong (a bacterium = one cell, a human has trillions).
MCQ 5
B — A virus is not made of cells and on its own does almost none of MRS GREN. (D is wrong: viruses can reproduce, but only inside a host cell.)
Short Answer 1
Model answer: M = Movement (a magpie flies); R = Respiration (cells release energy from glucose using oxygen); S = Sensitivity (a magpie turns its head toward a sound); G = Growth (chicks grow into adults by making more cells); R = Reproduction (a pair of magpies lay eggs); E = Excretion (waste like CO₂ is breathed out, urea passed); N = Nutrition (eats insects, fruit and seeds). Examples can come from any organism — accept any correct one.
Short Answer 2
Model answer: Fossil seashell = dead (came from a once-living mollusc, even though it has been mineralised). Sprouting bean = living (now doing all seven MRS GREN — actively growing, respiring, responding to water and warmth). Piece of granite = non-living (never alive — it is a rock formed by cooling magma). Fallen autumn leaf = dead (was part of a living tree, no longer respiring or growing).
Short Answer 3
Model answer: The claim is partly correct because reproduction is one of the seven MRS GREN characteristics — every truly living thing can produce offspring. However, reproduction alone is not enough. A virus can reproduce (only inside a host cell) but is not made of cells and fails the other six characteristics, so most biologists say it is not alive. On the other hand, fire and crystals can "make more of themselves" in a loose sense, but they fail most of MRS GREN — they have no cells, no respiration and no excretion. A scientist needs all seven characteristics, not just one, to call something alive.