What Is Life? Characteristics of Living Things
You'll be able to sort objects into living, dead, or non-living using MRS GREN.
A teaspoon of healthy soil can contain more than one billion living organisms. Many are microscopic, yet their cells carry out the processes that keep them alive.
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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
A teaspoon of healthy soil can contain more than a billion living organisms. Their activity varies with conditions, but their cells carry out the processes needed for life. Soil bacteria help break down plant material and recycle nutrients. Some fungi supply plant roots with water and nutrients in exchange for sugars. These are examples of living organisms obtaining the resources they need. 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 | The species can produce offspring; an individual organism does not need to reproduce during its own lifetime. |
| E | Excretion | Removes waste chemicals made by the body. |
| N | Nutrition | Takes in food or makes its own (plants do this with sunlight). |
MRS GREN is a school-level checklist of the life processes and capacities found across an organism's life. A living organism is capable of these processes, although it may not show every one at every moment. Reproduction is a capacity of the species: an individual does not need to produce offspring to be alive.
Why seven, and not one good test? Because every single characteristic on that list, taken by itself, is shared by something that is definitely not alive. A river moves and a river grows. A bushfire takes in fuel and oxygen, releases energy and produces waste, which looks a great deal like nutrition and respiration. A salt crystal in a jar grows and gets more complex. A car responds to a stimulus every time you turn the key. Pick any one characteristic as your test and you will let one of those in. The checklist helps distinguish living organisms by considering this set of life processes and capacities together over an organism's life.
Text description
- Centre: a living thing, joined to all seven characteristics.
- MRS (left column): M, Movement. R, Respiration. S, Sensitivity.
- GREN (right column): G, Growth. R, Reproduction. E, Excretion. N, Nutrition.
- Takeaway: this school-level checklist describes life processes and capacities across an organism's life; dormancy can slow visible activity and reproduction is a species-level capacity.
MRS GREN stands for Movement, , Sensitivity, , Reproduction, , and Nutrition. Together they describe the life processes and capacities of an organism.
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 is carrying out or capable of the MRS GREN life processes across its life, even when some are slowed or not currently visible. Examples: you, a magpie, a gum tree, a dormant viable seed.
- 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.
How many cells an organism has says nothing about whether it is alive, only about how big it is. A bacterium is one cell and is completely alive. An ant runs on about a million, a human on about 37 trillion, and a blue whale on roughly 100 quadrillion.
So why is a whale a hundred quadrillion small cells instead of one enormous one? Because of how a cell feeds. Everything a cell takes in, food, oxygen and water, has to cross its outer surface, and everything it takes in has to supply the whole of its inside. Make a cell twice as wide and its inside grows about eight times bigger while its surface grows only about four times, so there is suddenly twice as much inside to supply through each patch of surface. Past a certain size the middle of a cell simply cannot be reached in time. Staying small keeps every part of a cell close to its own surface, and the only way to build something whale-sized out of parts that all have to stay small is to use an enormous number of them.
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 COâ‚‚ 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.