Lesson focus: Living organisms carry out coordinated life processes through cells and systems.
Use MRS GREN as evidence for life while recognising the limits of a checklist.
You will be successful when you can…
- Name and define the seven MRS GREN life processes.
- Distinguish respiration from breathing.
- Distinguish excretion from egestion.
- Evaluate whether observations provide sufficient evidence for life.
Words that carry the science
- movement
- A change in position of part or all of an organism.
- respiration
- Chemical reactions in cells that release energy from nutrients.
- sensitivity
- Detecting and responding to changes in surroundings.
- excretion
- Removal of waste products made by metabolism.
- nutrition
- Obtaining or making materials needed for energy, growth and repair.
Use life processes as evidence, not a tick-box definition
01Seven connected processes
MRS GRENA memory aid for the seven life processes: movement, respiration, sensitivity, growth, reproduction, excretion and nutrition. stands for movementOne of the life processes: all living things can move all or part of themselves., respirationA chemical reaction in every living cell that releases energy from glucose; it is not the same as breathing., sensitivityOne of the life processes: being able to notice and respond to changes around you., growth, reproduction, excretionGetting rid of waste made by chemical reactions inside cells, such as carbon dioxide or urea. and nutritionGetting the food or materials needed for energy, growth and repair.. Organisms coordinate these through cells; they are not seven independent tricks.
respirationA chemical reaction in every living cell that releases energy from glucose; it is not the same as breathing. occurs in cells and releases energy. Breathing ventilates lungs and supports gas exchange; it is not respiration itself.
02Use precise boundaries
excretionGetting rid of waste made by chemical reactions inside cells, such as carbon dioxide or urea. removes metabolic wastes such as carbon dioxide or urea. Egestion removes undigested material that was never absorbed into cells.
A dormant seed may show little visible movement or growth but remains alive and can resume activity under suitable conditions.
03Cases at the boundary
Viruses contain genetic material and evolve, but lack cellular structure and depend on host cells to reproduce and carry out many processes. Scientists therefore discuss them as a boundary case rather than forcing a simple checklist answer.
Compare the evidence and avoid treating a typical example as a universal rule.
- Dormant seedCellular organism; metabolism is greatly reduced and growth can resume.
- BacteriumOne cell carries out all essential life processes.
- FlameMoves and spreads but has no cells or inherited cellular reproduction.
- RobotMay sense and move, but its materials do not carry out cellular metabolism.
Evaluate a flame
A student claims a flame is alive because it grows, moves and uses oxygen.
- Accept the accurate observations: the flame can spread and fuel reacts with oxygen.
- Identify what is missing: cells, controlled metabolism and genetic reproduction.
- Explain that combustion is a chemical reaction, not cellular respiration.
- Conclude that the evidence does not support classifying the flame as an organism.
How to make the reasoning visible: Separate observations from the biological criteria used to interpret them.
Life-process evidence table
Decide what each observation shows and what it cannot establish alone.
| Observation | Possible process | Limitation |
|---|---|---|
| A shoot bends towards light | Sensitivity and growth | One response does not define all life |
| A bacterium divides | Reproduction | Also needs cellular/metabolic evidence |
| A person breathes faster | Ventilation response | Breathing is not cellular respiration |
| A seed appears unchanged | No visible process | Dormancy can hide activity |
Why is breathing not itself respiration?
Why is a dormant seed still considered living?
Look at the table. Why is “a bacterium divides” evidence of reproduction?
Check your understanding
Answer from memory, then check against the suggested answer.
What does the R in MRS GREN mean?
What is excretion?
Why is one life-like feature insufficient?
What does MRS GREN stand for?
What is sensitivity?
A crystal grows when placed in solution. Does growth alone show it is alive?
Give evidence for and against calling a virus alive.
A car moves, uses fuel and gives out waste gases. Is it alive? Use MRS GREN.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A flame can move and grow. Answer the parts to explain why it is still not a living thing.
What does a flame do that makes it look alive?
(1)What are all living things made of? Is a flame made of this?
(1)How does a flame release energy? Is this the same as respiration?
(1)Finish the sentence: “So moving and growing are not enough …”
(1)Total for this question: 4 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., say how the flame seems alive, then give the reasons it is not.