Characteristics of living thingsIB MYP Biology: Revision notes
Section 1
The seven life processes
All living things (organisms) carry out seven life processes. A useful way to remember them is MRS GREN:
- Movement
- Respiration
- Sensitivity
- Growth
- Reproduction
- Excretion
- Nutrition
An object must carry out all seven to be classed as living.
Section 2
Movement, respiration and sensitivity
- Movement: a change of position or place. Animals move their whole bodies. Plants move parts of themselves, such as leaves turning towards light.
- Respiration: the chemical reactions in cells that release energy from food such as glucose. It is not the same as breathing.
- Sensitivity: detecting changes in the surroundings (stimuli) and responding to them, for example a shoot growing towards light.
Respiration is not breathing. Breathing brings oxygen in, but respiration happens in cells and releases energy.
Section 3
Growth, reproduction, excretion and nutrition
- Growth: a permanent increase in size or mass.
- Reproduction: making new organisms of the same kind, for example seeds from plants or young from animals.
- Excretion: removing the waste products of the organism's own chemical reactions, such as carbon dioxide from respiration.
- Nutrition: taking in or making the nutrients needed for energy and growth. Animals take in food; plants make their own food by photosynthesis.
Excretion is not the removal of undigested food. That is egestion.
Section 4
Deciding whether something is living
To decide whether something is alive, look for evidence of each of the seven processes.
- A burning candle uses oxygen and releases carbon dioxide, but it does not reproduce, grow by making cells, or respond to stimuli. It is not living.
- A rusting nail changes by chemical reaction only. It is not living.
- A dry seed may show no signs of life because it is dormant: its life processes are very slow. It is living, because it can germinate and grow.
- A bubbling yeast culture respires, takes in sugar, grows and reproduces. It is living.
Do not decide from one process. A flame moves and uses oxygen, but it fails the other tests.
Must Know
- Seven life processes: MRS GREN
- Living things carry out all seven
- Respiration releases energy in cells
- Excretion removes metabolic waste
- Dormant seeds are living
- Test an object against each process
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Characteristics of living things
- A class in Wellington watches a candle burn. The flame gets bigger when more wax melts, it uses oxygen from the air, it releases carbon dioxide and heat, and it flickers and moves in the draught. A student suggests that the candle must be alive.Explain why the burning candle is not a living thing.2 marks
- A student in Vancouver grows bean seedlings on a windowsill. Over two weeks the shoots bend towards the window and the plants grow from 5 cm to 12 cm tall. The dry mass of the plants increases. The student also notes that the plants make their own food using light.Outline how a bean plant shows sensitivity and how it carries out reproduction.2 marks
- A scientist is given three samples from a field station: a dry seed that has been stored in a jar for 20 years and shows no obvious movement or growth, a flask of yeast and sugar solution that is bubbling, and an iron nail that is covered in orange rust.Describe three pieces of evidence that could show that the yeast is living, naming the life process for each.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).