Characteristics of living organisms
How can you tell whether something is alive?
A car moves, takes in fuel and gives out waste gases, but nobody calls it living.
Biologists use a list of seven characteristics of living organisms.
Every living organism carries out all seven at some stage of its life, and non-living things never show the full set.
A handy way to remember them is MRS GREN: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion and Nutrition.
The mnemonic only reminds you of the names.
In the exam you need the precise meaning of each one, because each definition depends on its key words.
The seven characteristics
Characteristic
What it means
In a human
In a green plant
Movement
an action by an organism or part of an organism causing a change of position or place
walking; the heart pumping
a flower opening in the morning and closing at night
Respiration
the chemical reactions in cells that break down nutrient molecules and release energy for metabolism
glucose broken down in muscle cells
glucose broken down in root and leaf cells
Sensitivity
the ability to detect and respond to changes in the internal or external environment
pulling a hand away from a hot pan
the leaves of some plants folding up when they are touched
Growth
a permanent increase in size and dry mass
a child getting taller
a seedling gaining leaves and roots
Reproduction
the processes that make more of the same kind of organism
producing a baby
making seeds
Excretion
the removal of waste products of metabolism and substances in excess of requirements
carbon dioxide made in respiration, breathed out
oxygen from photosynthesis given out through the leaves
Nutrition
the taking in of materials for energy, growth and development
eating food
taking in carbon dioxide, water and mineral ions and making food by photosynthesis
Three ideas that need care
Respiration is not breathing. Breathing moves air in and out of the lungs.
Respiration is a set of chemical reactions that happens inside every living cell, including plant cells, all the time.
Growth needs the words "permanent" and "dry mass". A plant that takes up water swells, then shrinks again when it loses that water.
That change is not growth.
Dry mass is the mass left when all the water has been removed, so an increase in dry mass shows that new material has really been built.
Excretion is not the same as egestion. Excretion removes substances that were made inside cells by metabolism, meaning all the chemical reactions in an organism.
Faeces are mostly undigested food that never entered the body's cells, so passing faeces is called egestion, not excretion.
Learn the definitions of respiration, growth and excretion word for word: Cambridge mark schemes credit a definition only in its exact wording.
The idea behind it
The seven characteristics are not a random list.
They are one connected system, and each depends on the others.
Nutrition brings in materials.
Respiration releases energy from some of those materials.
That energy powers movement, growth, reproduction and the work of detecting and responding to changes.
All these chemical reactions together are the organism's metabolism, and they make waste products, which excretion removes before they build up and do harm.
Take away any one link and the others soon stop.
A good way to picture a living thing is as a fountain, not a statue.
A statue keeps its shape because nothing is happening to it.
A fountain keeps its shape only because water is flowing through it all the time.
You are like the fountain.
Materials are constantly taken into your body, used, rebuilt and replaced, and keeping that flow going needs a steady supply of energy from respiration.
When the flow stops for good, the organism has died, even though its body is still there for a while.
This picture explains why "dry mass" matters.
A lettuce is about 95% water, so its mass goes up and down with how much water it has taken in.
Dry mass counts only the material the organism has actually built, such as proteins, carbohydrates and fats.
There is a catch: you cannot dry out a living seedling and keep it alive.
So scientists grow many similar seedlings, dry a few of them each day and use their average dry mass to follow the growth of the rest.
Notice the words "at some stage of its life".
A dry seed can lie in a packet for years showing almost nothing: it does not grow or move, and its respiration is so slow it is hard to detect.
It is still alive, because given water and warmth it will carry out every characteristic.
In the same way, a mule cannot reproduce, but it is still a living organism: the list describes what living things as a kind can do.
Viruses, which you will meet in the unit on disease, sit right on the edge of this definition.
- Metabolism: all the chemical reactions that take place in an organism.
- Respiration: the chemical reactions in cells that break down nutrient molecules and release energy for metabolism.
- Growth: a permanent increase in size and dry mass.
- Excretion: the removal of waste products of metabolism and substances in excess of requirements.
- Dry mass: the mass of an organism once all its water has been removed.
State the characteristic of living organisms shown in each case: (a) a leaf gives out the oxygen made in photosynthesis; (b) a dog turns its head towards a sudden noise; (c) a bean seedling's dry mass increases over two weeks.
1. Oxygen is a waste product of photosynthesis, one of the plant's metabolic reactions, so giving it out is excretion.
2. The dog detects a change (a sound) and responds to it, so this is sensitivity. Turning the head is also a movement, but detecting a change and responding to it is the idea being tested.
3. A rise in dry mass is a permanent gain of new material, so this is growth.
Answer: (a) excretion; (b) sensitivity; (c) growth.
A crystal of copper sulfate hanging in a strong solution gets larger each day.
A student says this proves the crystal is alive.
Explain why the student is wrong.
1. Getting larger looks like growth, but one characteristic is not enough: a living organism shows all seven.
2. Check the others. The crystal does not respire, has no nutrition, does not excrete, cannot detect or respond to changes, and cannot reproduce.
Answer: the crystal only gets larger; it does not show the other characteristics of living organisms, such as respiration, sensitivity, excretion or reproduction, so it is not alive.
A definition depends on its key words, so learn each one as a set phrase.
For growth, a full definition includes both "permanent" and "dry mass"; for respiration, say that the reactions happen in cells and release energy.
"Getting bigger" or "making energy" will not score.
- Treating respiration as breathing. Respiration is chemical reactions in cells; breathing is moving air in and out of the lungs.
- Calling the removal of faeces excretion. Faeces are undigested food, so this is egestion.
- Saying energy is "made" in respiration. Energy is released from nutrient molecules, never made.
- Thinking plants cannot move or respond. Their movements are slower, but leaves and flowers do both.
Check your understanding
1. Define growth. [2]
2. State the characteristic of living organisms shown when a person's mouth waters at the smell of food. [1]
3. Explain why removing carbon dioxide from the lungs is excretion but removing faeces from the body is not. [2]
4. Describe how a green plant carries out nutrition. [2]
5. A car moves, takes in fuel and releases waste gases. Give two characteristics of living organisms that a car does not show. [2]
Answers
1. A permanent increase [1]; in size and dry mass [1].
2. Sensitivity (the person detects a change, the smell, and responds by producing saliva). [1]
3. Carbon dioxide is a waste product of metabolism, made by respiration in cells [1]; faeces are undigested food that was never part of the cells' metabolism, so their removal is egestion [1].
4. It takes in carbon dioxide and water (and mineral ions) [1]; and uses light energy to make its own food by photosynthesis [1].
5. Any two from: growth; reproduction; respiration (a car burns fuel, but not by chemical reactions inside cells). [1 each]