What does it mean to be alive? The seven life processes
By the end of this lesson you will be able to
- List the seven life processes shared by all living things: movement, reproduction, sensitivity, growth, respiration, excretion (elimination) and nutrition.
- Distinguish living, dead and non-living, and apply the seven processes to decide which is which.
- Explain the difference between respiration and breathing, and write the word equation for aerobic respiration with the correct reactants and products.
- Describe how each life process looks in an animal and in a plant.
- Design a fair test of one condition a seed needs to germinate.
The puzzle
A car moves, uses fuel, gives off waste gases, gets warm, responds when you press the brake and can even “reproduce” — a factory makes more of it. A dry bean seed in a packet does none of those things; it just sits there for years. Yet every biologist will tell you the seed is alive and the car is not. What test are they using?
Write your own list of things every living thing must do. Then test it: does a fire pass your list? Does a virus? Does a seed?
Compare your answer
Most people list moving, eating, breathing, growing and having babies. A fire “eats” fuel, “breathes” oxygen, grows, spreads and produces waste — and is not alive, because it does not sense and respond, does not excrete waste it made from its own processes in a controlled way, and does not reproduce with inherited instructions. A seed passes every test, but slowly: it respires a tiny amount, and given water it will grow, move its root downwards, sense light and eventually reproduce. Biologists use all seven processes together, and they insist on all seven — which is why viruses, which do not respire or grow, sit on the border.
The science — structured notes
1.1 The seven life processes — MRS GREN
All living things — organisms — carry out seven processes. Anything that does all seven is alive; anything that does none is non-living; something that once did them and has stopped is dead. The initials spell MRS GREN:
| Process | What it means | In an animal (a meerkat) | In a plant (a mopane tree) |
|---|---|---|---|
| Movement | Changing position of the whole body or parts of it | Runs, digs, turns its head | Leaves turn to the light; flowers open; roots grow towards water |
| Reproduction | Producing offspring | Gives birth to pups | Flowers, seeds and seedlings |
| Sensitivity | Detecting changes (stimuli) and responding | Sees a hawk and dives for the burrow | Shoots grow towards light; leaves fold in drought |
| Growth | Getting permanently bigger by making more cells | Pup grows to adult | Seedling to tree; new leaves each spring |
| Respiration | Releasing energy from food inside every cell | Every cell, all the time | Every cell, all the time — day and night |
| Excretion (elimination) | Getting rid of waste made by the body’s own chemistry | Urine, carbon dioxide breathed out, sweat | Oxygen (by day), carbon dioxide (at night), stored wastes dropped with leaves |
| Nutrition | Taking in or making food | Eats insects, scorpions | Makes sugar by photosynthesis; takes in minerals from soil |
A stone, a cut flower in a vase, a robot vacuum cleaner, a mushroom.
Stone: none of the seven — non-living.
Cut flower: its cells are still respiring, it still takes up water and its petals still open (movement, sensitivity) — alive, for a few days; when the cells stop respiring it is dead.
Robot: moves, senses, uses energy — but does not grow, reproduce, respire (it uses electricity, not food broken down in cells) or excrete metabolic waste — non-living.
Mushroom: grows, reproduces by spores, respires, excretes, feeds on dead material, responds to moisture, moves very slowly as it grows — alive. It is neither plant nor animal (Lesson 4.3).
For each, decide living, dead or non-living, naming the life processes that decide it.
(a) A candle flame.
(b) A dry seed in a packet.
(c) A piece of firewood.
(d) The yeast in bread dough.
Reveal answer
(a) Non-living: it moves, uses fuel and makes waste, but never grows by making cells, never reproduces with inherited instructions, does not sense and respond, and does not respire.
(b) Living, but dormant: it respires very slowly and, given water and warmth, will grow, move its root, sense gravity and light, and eventually reproduce.
(c) Dead: it was once part of a living tree that did all seven; its cells no longer respire or grow.
(d) Living: yeast cells respire (making the carbon dioxide that raises the dough), grow, reproduce by budding, feed on the sugar and excrete alcohol and carbon dioxide.
1.2 Respiration is not breathing
The most-marked-down confusion in this unit. Breathing is moving air in and out of the lungs — a movement that only some animals do. Respiration is a chemical reaction inside every cell of every organism, plants included, that releases the energy stored in food:
glucose + oxygen → carbon dioxide + water (+ energy released)
The reactants (left side) are glucose and oxygen. The products are carbon dioxide and water. Energy is released — it is not a substance, so it is not a reactant and is written in brackets or as a note, never as “+ energy” on the left.
“Plants do not respire; they photosynthesise instead.”
Reveal answer
False. Plants respire in every cell, day and night, exactly as animals do — that is how their cells get energy to grow, move and repair. Photosynthesis is an additional process, in green cells in the light, that makes the glucose which respiration later uses. A plant in the dark still respires; a plant that stopped respiring would be dead.
1.3 Excretion is not egestion
Excretion means removing waste products made by the organism’s own chemical reactions: carbon dioxide from respiration, urea (from breaking down excess protein) in urine, excess water and salts in sweat. Egestion — passing out undigested food as faeces — is not excretion, because the food was never part of the body’s chemistry. The IB uses the word “elimination” for the life process; in an exam, describe excretion as the removal of metabolic waste and name an example.
Carbon dioxide dissolves in blood to form an acid; if it built up, enzymes would stop working within minutes. Urea is poisonous in high concentration. Excretion is not tidiness; it is survival. A kidney failing is fatal within days without a machine to do its job.
1.4 Sensitivity, growth and movement in plants
Students readily accept that animals sense and move; plants seem to do neither. They do both, slowly. A shoot bends towards the light within hours (the cells on the shaded side grow longer); a root grows downwards however the seed is planted; a mimosa leaf folds when touched; a sunflower tracks the Sun across the sky; leaves drop before drought. These are responses to stimuli — light, gravity, touch, water — and they are the plant’s sensitivity and movement. Plant growth is also different: a tree keeps growing for its whole life, and mostly at the tips of its shoots and roots.
“A bean seedling on a windowsill bends towards the window. This is sensitivity: the shoot detects the direction of light (the stimulus) and responds by growing more on the shaded side, so the tip turns towards the light. It is also movement of part of the plant. The response helps the plant survive because its leaves receive more light for photosynthesis — its nutrition.”
Three life processes in one observation, each named and linked to survival. That linkage — process → what it does → why it helps — is what the top levels of Criterion A ask for.
A student says: “Seeds are not alive because they don’t do anything.” Using the seven processes, explain what a seed is doing, what it needs to start doing the rest, and what would show it was dead.
Reveal answer
A dry seed respires very slowly, releasing just enough energy to keep its cells alive; every other process is paused. Given water (to activate its enzymes), oxygen (for respiration) and warmth (for the reactions to run), it grows — the root emerges and moves downwards (sensitivity to gravity), the shoot grows up towards light, it begins to make its own food and, eventually, reproduces. A seed that is given all three conditions and never germinates, whose cells no longer respire, is dead — which is how seed banks test their stocks.
1.5 Investigating a life process: what a seed needs
Germination is the easiest life process to investigate in a school, and it appears in the eAssessment in exactly the form the examiner reports warn about. A plan that scores:
Try it — germination
Twenty bean seeds on damp paper. Change the temperature five times (three trials each) and count how many have germinated after five days. Plot the count against temperature: is it a line or a hill? Then set the paper to dry and repeat one temperature — what happens? Finally, put the seeds under water: they have water, so why do so few germinate?
Suppose your means were: 5 °C → 0, 15 °C → 12, 25 °C → 19, 35 °C → 15, 45 °C → 3 (out of 20), and flooded seeds at 25 °C → 6.
(a) Describe the trend with numbers and name its shape.
(b) Explain the fall above 25 °C using the idea of enzymes.
(c) Explain why flooded seeds germinate poorly even though they have plenty of water, using the respiration equation.
Reveal answer
(a) As temperature rises from 5 °C to 25 °C the number germinating rises from 0 to 19 of 20; above 25 °C it falls again, to 3 at 45 °C. The graph is a curve with a peak (an optimum) near 25 °C — not a straight line and not proportional.
(b) Germination depends on enzymes (proteins that speed up the seed’s reactions, including respiration). They work faster as it gets warmer, up to a point; above the optimum their shape is damaged (denatured) and they stop working, so the reactions — and germination — fail.
(c) Respiration needs oxygen: glucose + oxygen → carbon dioxide + water. Under water the seed cannot get oxygen fast enough (it dissolves poorly in water), so it cannot release the energy needed to grow — it drowns.
Common mistakes to avoid
- Respiration = breathing — respiration is a chemical reaction in every cell; breathing is moving air.
- Energy or water as a reactant — glucose + oxygen on the left; carbon dioxide + water on the right; energy released.
- “Plants don’t respire” — they respire all the time, and photosynthesise as well in the light.
- Excretion = going to the toilet — excretion is removing metabolic waste (CO₂, urea); faeces are egestion.
- “Plants don’t move or sense” — shoots bend to light, roots grow down, leaves fold: slow, but real.
- Deciding “alive” on one process — go through all seven and name the ones that fail.
How we found out — the flask with the swan’s neck
For two thousand years people believed that life could arise on its own from non-living matter: maggots from meat, mice from grain, frogs from mud. It matched what everyone saw. In 1668 Francesco Redi covered jars of meat with gauze and showed that maggots appeared only where flies could lay eggs — but for microscopic life, which appeared in every broth left standing, the idea of spontaneous generation survived, and in the 1850s it was still respectable science.
Louis Pasteur settled it in 1859 with an experiment of beautiful simplicity. He boiled meat broth in flasks whose necks he had drawn out into long S-shaped curves — swan necks. Air could reach the broth freely, but dust and microbes settled in the bend of the neck and never reached it. The broth stayed clear for months. Tip the flask so the broth touched the dust in the neck, and it turned cloudy with life within a day. Life came only from life; the “generation” everyone had seen was contamination. Some of Pasteur’s original flasks are still sterile today, on display in Paris. The result gave us sterilisation, pasteurised milk and the whole science of microbiology — and it settled what “alive” means: something that comes from a parent, and reproduces.
Why was the swan neck a better design than simply sealing the flask?
Reveal answer
A sealed flask kept out air as well as microbes, so supporters of spontaneous generation could say the broth had been “starved of the air life needs”. The swan neck let air in freely and kept only dust and microbes out — so the one variable that differed was the microbes. It is a control-variable argument: change one thing, and the conclusion cannot be dodged. That is the same logic as the dry-paper dish in the germination lab.
Where it is used now
- Seed banks. Botswana’s national genebank and the global vault at Svalbard store seeds cold and dry so that they respire as slowly as possible; samples are germinated every few years to check they are still alive.
- Medicine. “Signs of life” — breathing, pulse, response to stimulus — are the first three processes a paramedic checks. Brain death is defined by the permanent loss of sensitivity and response.
- Food. Bread rises, beer brews and yoghurt sets because living yeast and bacteria respire and excrete; refrigeration slows their life processes, and cooking stops them.
- Farming. Germination tests decide whether a bag of seed is worth planting; a farmer counts 100 seeds on damp paper before sowing a field.
- Space. The Mars rovers test soil for signs of respiration and metabolism — the seven processes, applied to another planet.
Science now — and the scientist you need to be
What is happening in 2026
The border of “alive” is being tested from both sides. Biologists have built synthetic cells with the smallest genome that can still carry out all seven processes — under 500 genes — to find out what the minimum for life actually is. Viruses, which do not respire or grow, are now known to outnumber every other kind of organism on Earth; giant viruses discovered in the last twenty years carry genes for processes once thought to belong only to cells, and the argument over whether they are alive is a live scientific one. At the other border, engineers build robots that move, sense, respond, take in energy and repair themselves — and machine-learning systems are now used to search telescope data and Mars-rover samples for the chemical signatures of respiration and metabolism, because the seven processes are the checklist for finding life anywhere.
The MRS GREN list is not a fact to memorise; it is a definition scientists still argue about, and the arguments are settled by the same method as Pasteur’s: change one thing, control the rest, and see what the evidence forces you to conclude.
The mindshift
What you need to be able to do
Every branch of biology, medicine, agriculture and food science starts with the seven processes, and every exam question about an unfamiliar organism can be answered by asking which processes are visible and how. The habit is to describe an organism by what it does, and to explain each process by what it is for: respiration releases energy, excretion removes poisons, sensitivity keeps it alive. Description plus purpose is the shape of a full-mark biology answer.
See an example
Basil plant on the windowsill. Movement: leaves turned towards the window within two days of turning the pot. Reproduction: flower buds forming. Sensitivity: the same leaf-turning (light) and drooping when dry. Growth: two new leaf pairs in a week, measured with a ruler. Respiration: not visible — but the plant would die in the dark in weeks, having used its stored sugar. Excretion: not visible; water droplets on leaves in the morning are water the plant has moved out. Nutrition: greener leaves after feeding; sugar made in the light. The AI wrote “the plant breathes in carbon dioxide” — I corrected it to “respires using oxygen, and photosynthesises using carbon dioxide”.
Key terms
| Organism | Any individual living thing — a bacterium, a mushroom, a tree, an animal. |
| Life processes (MRS GREN) | Movement, reproduction, sensitivity, growth, respiration, excretion (elimination), nutrition — the seven processes carried out by every living thing. |
| Respiration | The chemical reaction in every cell that releases energy from glucose: glucose + oxygen → carbon dioxide + water (energy released). |
| Breathing | Moving air into and out of the lungs to supply oxygen and remove carbon dioxide; not the same as respiration. |
| Excretion | The removal of waste products made by the organism’s own chemical reactions, such as carbon dioxide and urea. |
| Stimulus | A change in the surroundings that an organism detects and responds to — light, touch, temperature, chemicals. |
| Germination | The growth of a seed into a seedling, needing water, oxygen and a suitable temperature. |
| Enzyme | A protein that speeds up a chemical reaction in a cell; works best at an optimum temperature and is destroyed by heat. |
Think like a scientist
Criterion A A visitor to the Okavango sees a termite mound. She says: “The mound is alive — it grows taller every year, it stays warm inside, air moves in and out of it, and new mounds appear near old ones.”
(a) Using the seven life processes, explain why the mound itself is not alive.
(b) Explain which organisms are responsible for each of the four observations she made.
(c) Write the word equation for the respiration of the termites and explain why the mound needs air moving through it.
(d) Suggest one observation that would show sensitivity in the termites.
Model answer
(a) The mound is built of soil and saliva; it does not respire, grow by making cells, reproduce, sense or respond, feed, or excrete. It gets taller because termites add to it, and it stays warm because millions of respiring termites and their fungus gardens release energy inside it. It is a product of living things, like a nest or a house, not an organism.
(b) Growth of the mound — termites (movement and nutrition: they carry and cement soil). Warmth — respiration of the termites and the fungus they farm. Air moving — the termites build and open or close tunnels so that warm air rises out and fresh air is drawn in (their movement and sensitivity to temperature). New mounds — reproduction: winged termites fly from the old colony to found new ones.
(c) glucose + oxygen → carbon dioxide + water (energy released). Millions of termites use oxygen and produce carbon dioxide continuously; without ventilation the oxygen would run out and the carbon dioxide would build up to poisonous levels, so the colony would die.
(d) Termites move away from light when a mound is opened, repair a breach within minutes (responding to the change in humidity and air flow), or retreat deeper when the surface heats up — each is detection of a stimulus followed by a response.
Probe deeper — exam-style questions
Attempt every question before opening the answer. Marks in brackets show what a full answer is worth.
State the seven life processes.
Reveal answer
Movement, reproduction, sensitivity, growth, respiration, excretion (elimination), nutrition [2: all seven for 2; five or six for 1].
Distinguish between respiration and breathing, and write the word equation for aerobic respiration.
Reveal answer
Respiration is a chemical reaction in every cell that releases energy from glucose [1]; breathing is the movement of air into and out of the lungs to exchange gases [1]; glucose + oxygen → carbon dioxide + water (energy released) [1 — energy not written as a reactant].
A student writes: “energy + glucose → carbon dioxide + water + oxygen”. Identify two errors.
Reveal answer
Energy is not a reactant — it is released by the reaction [1]; oxygen is a reactant (left side), not a product, and water is a product [1].
A robot lawnmower moves around a garden, senses obstacles, returns to its charger when its battery is low and “grows” when its owner adds a bigger blade. Explain why it is not alive.
Reveal answer
It moves and senses, but it does not respire — its energy comes from electricity, not from food broken down in cells [1]; it does not grow by making new cells or reproduce with inherited instructions — additions are made by a person [1]; it does not feed or excrete metabolic waste; a living thing must carry out all seven processes [1].
True or false, with a reason: “A plant excretes oxygen.”
Reveal answer
True in daylight [1]: oxygen is a waste product of photosynthesis that the plant releases; at night the plant excretes carbon dioxide from respiration instead [1].
Seeds are stored in a seed bank at −18 °C and 5 % moisture. Explain how each condition keeps the seeds alive for decades, and describe how the bank checks that they are still alive.
Reveal answer
Cold slows the enzymes so respiration uses the seed’s stored food extremely slowly [1]; dryness keeps the enzymes inactive and prevents germination and decay [1]; every few years a sample is placed on damp paper at a suitable temperature and the number germinating is counted — if it falls, fresh seed is grown [1].
Summary
- Living things carry out all seven life processes — MRS GREN. Non-living things do none; dead things have stopped.
- Respiration is a reaction in every cell: glucose + oxygen → carbon dioxide + water, releasing energy. Breathing is gas exchange; plants respire too.
- Excretion removes the body’s own chemical waste (CO₂, urea); egestion is undigested food.
- Plants move, sense and respond — slowly, by growth.
- A life process is investigated like any other variable: measurable dependent variable, qualified controls, five values, three trials, a control dish.
Next time → Adaptations: how a gemsbok survives the Kalahari and a cactus survives the desert — structure, behaviour and the meaning of “fit”.