Characteristics of living organisms
A sunflower, a mosquito, a mushroom and you look nothing alike.
Yet biologists group all of them as living, and a candle flame or a robot vacuum cleaner as non-living.
You need to understand how living organisms share the following characteristics:
- they require nutrition
- they respire
- they excrete their waste
- they respond to their surroundings
- they move
- they control their internal conditions
- they reproduce
- they grow and develop.
- Each marking point is worth one mark, shown as (1) in the answers in these notes.
- A "/" separates alternatives: any one of them earns the point. "eq" means an answer with the same meaning also scores.
- "An answer that makes reference to three of the following" means any three points from a longer list. Write one more point than the marks if you can.
- A correct final answer to a calculation scores full marks, even with no working. Working still matters: a slip in the last step then loses only that mark.
- There are no levels and no marks for "quality of writing": every mark is a separate point.
The idea: life is a set of jobs, done by cells
A living organism is built from one or more cells, and inside those cells thousands of chemical reactions run all the time.
Those reactions need raw materials and energy, they make waste, and they only work if conditions inside stay right.
Every characteristic in the list above follows from that.
- An organism needs nutrition because cells need raw materials to build new parts and a fuel (food) that can be broken down for energy.
- It must respire because respiration is how cells release the energy stored in food, for example in glucose.
- It must excrete because the reactions in cells make waste, for example carbon dioxide from respiration and urea, which would become toxic if they built up.
- It must respond and move because food, light, water and danger change from moment to moment, and an organism that cannot react to them dies.
- It must control its internal conditions because enzymes inside cells work only within a narrow range of temperature, pH and water content.
- It must reproduce because every individual eventually dies; only reproduction keeps the species going.
- It must grow and develop because a new individual starts small and simple, often as a single cell.
What each characteristic looks like in an animal and in a plant
Plants seem to do much less than animals.
They do every one of the eight; they just do some of them slowly or in a less obvious way.
Characteristic
What it means
In a human
In a bean plant
they require nutrition
taking in or making the substances needed for energy, growth and repair
eats food and digests it
makes glucose by photosynthesis; absorbs water and mineral ions through the roots
they respire
releasing energy from food inside cells
every cell respires glucose, using oxygen
every cell respires, day and night, including root cells
they excrete their waste
removing the waste products of the chemical reactions in cells
lungs remove carbon dioxide; kidneys remove urea in urine
releases oxygen made by photosynthesis and carbon dioxide made by respiration, through the stomata of its leaves
they respond to their surroundings
detecting a change (a stimulus) and reacting to it
the pupil narrows in bright light
the shoot grows towards light; the root grows downwards
they move
changing the position of the whole organism or of a part of it
walking; the heart pumping blood
leaves turn to face the light; flowers open and close
they control their internal conditions
keeping conditions inside the body within narrow limits
body temperature stays near 37 °C
its cells keep a steady water content, so they stay firm (turgid)
they reproduce
producing new individuals
sexual reproduction
seeds from flowers (sexual); some plants also grow new plants from runners (asexual)
they grow and develop
a permanent increase in size, and becoming more complex
a baby grows into an adult and develops new features at puberty
a seed grows into a seedling, then a plant with flowers
The diagram below shows a bean plant and a rabbit carrying out some of the characteristics.
Key to the diagram: A the shoot tip bending towards the light from the window (they respond to their surroundings); B roots taking in water and mineral ions (they require nutrition); C a bean pod holding seeds (they reproduce); D grass being eaten (they require nutrition); E the back legs pushing off in a hop (they move); F thick fur that cuts heat loss and keeps the body temperature steady (they control their internal conditions); G a young rabbit that will grow into an adult (they reproduce; they grow and develop).
Respiration and excretion take place inside cells, so a drawing of the whole organism cannot show them, but both organisms do them all the time.
Three of these deserve a closer look, because they are the ones students most often get wrong.
Respiration is not breathing. Breathing moves air in and out of the lungs.
Respiration is a chemical process in every living cell that releases energy from food.
A tree respires, a bacterium respires, a seed respires, and none of them breathes.
Write "respiration releases energy"; never write that it "produces" or "makes" energy, because energy cannot be made.
Excretion is not the same as getting rid of faeces. Excretion removes waste made by the chemical reactions inside cells, such as carbon dioxide from respiration and urea from the breakdown of excess amino acids.
Faeces are mainly food that was never absorbed into the body's cells, so passing faeces (egestion) is not excretion.
Growth is permanent. Drinking a litre of water makes you heavier for an hour, but that is not growth.
Growth is a permanent increase in size, normally made by cells dividing (by mitosis) and by new cells getting larger.
Development means the organism becomes more complex: cells become specialised, and new tissues and organs form.
Why one or two characteristics are not enough
Many non-living things show some of the characteristics.
- A candle flame uses oxygen, releases carbon dioxide and energy, gets bigger and can "reproduce" when a spark lights another candle.
- A car takes in fuel, releases waste gases and moves.
- A salt crystal in a saturated solution grows.
None of them shows all eight, and none of them is built from cells.
A flame cannot control its own internal conditions; a car does not grow or reproduce; a crystal does not respire.
That is why a biologist uses the whole list.
An object counts as a living organism only if it shows all eight characteristics, at least at some stage of its life.
A dry seed in a packet looks lifeless, yet it is alive.
Its cells still respire, very slowly, and when it is given water and warmth it grows, develops and in time reproduces.
Being alive does not mean showing every characteristic every second.
Viruses sit on the boundary.
They reproduce, but only inside living cells, and on their own they do none of the other things.
You will meet them in the section on pathogens, where the reason they are not living organisms is explained.
Connecting the idea
Each characteristic leads to a later part of the course:
- nutrition: photosynthesis and human digestion (unit 2e)
- respiration: aerobic and anaerobic respiration (unit 2f-2g)
- excretion and control of internal conditions: the organs of excretion and homeostasis (unit 2i-2j)
- response: the nervous system, hormones and plant tropisms (unit 2i-2j)
- reproduction, growth and development: reproduction, mitosis and inheritance (units 3a and 3b)
So this list is a map of most of the biology course.
When you meet a new topic, ask which characteristic it explains.
- Respiration: a chemical process in living cells that releases energy from food (such as glucose).
- Excretion: the removal of the waste products of chemical reactions in cells (metabolism), for example carbon dioxide and urea.
- Homeostasis: the maintenance of a constant internal environment.
- Growth: a permanent increase in size.
A Venus flytrap is a plant.
Its leaves close in less than a second when an insect touches small hairs on them.
The plant then releases enzymes that digest the insect, and it absorbs the products.
Give two characteristics of living organisms shown by the Venus flytrap in this description, and the evidence for each.
(2)
Full-mark answer:
1. it responds to its surroundings: the leaves close when an insect touches the hairs (1)
2. it requires nutrition: it digests the insect and absorbs the products / it moves: the leaves close (1)
A weaker answer: "It is alive and it eats."
This scores 0.
"It is alive" is not a characteristic, and "it eats" gives no evidence from the description.
Each mark needs the characteristic and the matching evidence from the stem.
A student says: "A fire is alive, because it uses oxygen, gives out carbon dioxide and grows."
Explain why a fire is not a living organism.
(3)
Full-mark answer, any three from:
1. a living organism must show all eight characteristics (1)
2. a fire is not made of cells (1)
3. a fire does not respond to its surroundings / does not detect stimuli (1)
4. a fire cannot control its internal conditions (1)
5. a fire does not reproduce by making new individuals like itself / does not develop (1)
A weaker answer: "A fire is not alive because it is not an animal or a plant."
This scores 0.
Fungi, protoctists and bacteria are alive too.
The answer must use the characteristics: which ones the fire lacks.
Explain why a tree is said to respire, even though it does not breathe.
(2)
Full-mark answer:
1. respiration is a chemical process in (every) living cell that releases energy from food / glucose (1)
2. breathing is only the movement of air in and out of lungs, which a tree does not have, but its cells still need energy, so they still respire (1)
A weaker answer: "The tree respires because it takes in carbon dioxide."
This scores 0.
Taking in carbon dioxide is part of photosynthesis, not respiration, and the answer never says what respiration is.
When a question names an organism and asks for characteristics, quote the list's own words ("they respond to their surroundings", "they control their internal conditions") and attach each one to evidence from the stem.
A characteristic with no evidence, or evidence with no characteristic, earns nothing.
- "Respiration is breathing." Breathing is a movement of air; respiration is a chemical process in every cell that releases energy. Plants and bacteria respire but do not breathe.
- "Respiration produces energy." Energy is never made. Respiration releases energy that is stored in food.
- "Getting rid of faeces is excretion." Faeces are undigested food that never entered the cells. Excretion removes waste made by reactions in cells.
- "Plants do not move or respond." They do, slowly: shoots bend towards light, leaves turn, flowers open, stomata open and close.
Check your understanding
1. Which of these lists contains only characteristics shared by all living organisms? (1)
- A breathing, growing, reproducing
- B excreting waste, responding to surroundings, moving
- C eating food, respiring, controlling internal conditions
- D photosynthesising, reproducing, growing and developing
Total [1]
2. A sample of soil is put into a sealed flask. The air in the flask is tested each day. Over five days the carbon dioxide concentration rises. When an identical sample is heated to 120 °C before it is sealed in a flask, the carbon dioxide concentration does not change.
Explain what these results suggest about the first soil sample.
(3)
Total [3]
3. A baker mixes yeast with sugar solution in a warm flask. Over one day the number of yeast cells rises, bubbles of gas form in the liquid and the concentration of sugar in the solution falls.
Give three characteristics of living organisms shown by the yeast.
For each one, give the evidence from the description.
(3)
Total [3]
4. Euglena is a single-celled organism with chloroplasts that lives in ponds. When a lamp is placed at one side of a dish of pond water, the Euglena cells swim towards the lamp.
Explain how this shows two characteristics of living organisms, and how this behaviour helps Euglena to survive.
(3)
Total [3]
5. Read the passage.
The animal that pauses its life
Line
Passage
Water bears are animals less than 1 mm long that live in damp moss.
They walk slowly on eight stubby legs and pierce moss cells to suck
out the contents. When the moss dries out, a water bear pulls in its
legs and loses almost all of its body water. In this dry state no
5
growth or movement can be seen, and the rate of respiration falls to
almost zero. The animal can stay like this for years, and survives
being frozen. When water returns, it takes in water within an hour,
starts to move, feeds again and later lays eggs.
(a) In the dry state, the water bear shows almost no characteristics of a living organism. Suggest why biologists still describe it as alive. (Lines 4 to 8) (2)
(b) State two characteristics of living organisms that the water bear shows when water returns. (Lines 7 and 8) (2)
Total [4]
Answers
1. B (1). A is not the answer as breathing is not shared by all organisms; plants and bacteria respire without breathing. C is not the answer as plants and many bacteria do not eat food; the characteristic is that they require nutrition. D is not the answer as animals and fungi do not photosynthesise.
2. Any three from (1 each):
- the first sample contains living organisms / microorganisms
- they respire, releasing carbon dioxide
- heating to 120 °C kills the organisms / denatures their enzymes
- so no respiration takes place in the heated sample / the heated flask is a control Do not credit "the soil breathes".
3. Any three from (1 each); each needs the characteristic and its evidence:
- they reproduce: the number of yeast cells rises
- they require nutrition: the sugar is used up / taken in
- they respire: gas / carbon dioxide is released
- they excrete their waste: carbon dioxide (the gas) is released
- they grow and develop: the population of cells increases / new cells form Do not credit the same evidence for two characteristics unless the characteristics differ (gas for respire and excrete is allowed).
4. Any three from (1 each):
- it responds to its surroundings: it detects light (a stimulus) and reacts to it
- it moves: it swims (changes its position) towards the light
- its chloroplasts absorb more light, so more photosynthesis takes place
- so it makes more food / glucose (for respiration and growth) Ignore "it likes the light".
5. (a) Any two from (1 each):
- it still respires, at a very low rate (lines 5 and 6)
- it is made of cells that are not dead
- when water returns it shows the other characteristics (moves, feeds, reproduces)
- an organism need not show every characteristic at every moment Ignore "it is an animal" alone.
(b) Any two from (1 each):
- they move / it starts to move
- they require nutrition / it feeds
- they reproduce / it lays eggs Do not credit "it drinks water" alone.