Life processes are the seven characteristics that every living organism carries out, and KS3 biology teaches them through the mnemonic MRS GREN: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion and Nutrition. An organism must perform all seven, not just some, to be classed as truly alive.
What is MRS GREN?
MRS GREN is a memory aid that lists the seven life processes in a fixed order, making them easy to recall in an exam.
| Letter | Life process | What it means |
|---|---|---|
| M | Movement | An action that changes position, either of the whole organism or a part of it |
| R | Respiration | The chemical release of energy from food inside cells |
| S | Sensitivity | Detecting and responding to changes in the surroundings, called stimuli |
| G | Growth | A permanent increase in size or number of cells over time |
| R | Reproduction | Producing offspring, either sexually or asexually |
| E | Excretion | Removing waste products made by the body's own chemical reactions |
| N | Nutrition | Taking in and using nutrients for energy, growth and repair |
Some versions of the mnemonic add a further C for "control" or an S for "seven" at the front, but MRS GREN covers the seven processes required by the KS3 curriculum.
Why must an organism perform all seven life processes to be classed as alive?
Biologists use all seven characteristics together, not any single one, because several life processes overlap with non-living things. A crystal can grow, and a car can move, yet neither is alive. Only something carrying out every one of the seven processes, from taking in nutrients to reproducing, is classified as a living organism. This is why the full list matters more than any individual process on its own.
What is the difference between movement and locomotion?
Movement is any change in position, and it is the "M" in MRS GREN. Locomotion is a more specific type of movement: moving the whole organism from one place to another, such as an animal walking or a bird flying. Plants show movement, for example a sunflower turning to face the sun, but they do not show locomotion because the whole plant does not relocate. This distinction is a common source of KS3 exam confusion.
Why is respiration different from breathing?
Respiration is a chemical process that happens inside every living cell, releasing energy from food. Breathing is a physical process, the movement of air in and out of the lungs, that supplies oxygen for respiration and removes carbon dioxide. Respiration takes place in plants, animals and microorganisms, but only animals with lungs actually breathe. Confusing the two terms is one of the most common mistakes made in KS3 biology assessments.
How do you apply MRS GREN to decide if something is alive?
Worked example: Use MRS GREN to compare a garden snail and a lit candle flame.
A garden snail moves using its muscular foot, respires inside its cells to release energy, senses light and touch through its tentacles, grows a larger shell over time, reproduces by laying eggs, excretes waste through its body, and takes in nutrition by eating plant matter. It satisfies all seven processes, so a snail is classified as a living organism.
A candle flame appears to move as it flickers, and it "consumes" wax, which can look like nutrition and excretion of soot and gas. However, a flame does not respire in cells, does not sense and respond to a stimulus in a controlled way, does not grow by adding new cells, and cannot reproduce itself. Because it fails most of the seven tests, a flame is not alive — it is simply a chemical reaction (combustion) that shares a few surface similarities with life processes.
This method, checking each of the seven letters in turn against the object in question, is exactly how KS3 exam questions expect you to justify a classification decision.
Are viruses alive?
Viruses are a classic KS3 exam question because they fail the MRS GREN test. A virus cannot respire, grow, or reproduce by itself; it can only make copies of itself once it has hijacked the machinery inside a living host cell. Because a virus cannot independently carry out all seven life processes, most biologists classify viruses as non-living, even though they share some features, such as genetic material, with living organisms.
Frequently asked questions
What does MRS GREN stand for?
MRS GREN stands for Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion and Nutrition. It is a mnemonic used in KS3 biology to help students remember the seven life processes that together define a living organism. Each letter corresponds to one process that must be present for something to be classified as alive.
Do plants show all seven life processes?
Yes, plants carry out all seven life processes, but some look different to how they appear in animals. Plant "movement" includes growth towards light and closing leaves, plant "sensitivity" includes responding to gravity and touch, and plant "nutrition" is photosynthesis rather than eating food. Because plants perform every process, even in these different forms, they are classified as fully living organisms.
Why can't a single life process prove something is alive?
Some non-living things display individual features that look like a single life process without being alive at all. Fire appears to "grow" and "move", and a crystal genuinely grows by adding new layers, yet neither respires, excretes or reproduces. Only an organism that carries out every one of the seven MRS GREN processes together counts as truly alive.
How is excretion different from egestion?
Excretion is the removal of waste made by the body's own chemical reactions, such as carbon dioxide from respiration or urea from the breakdown of excess protein. Egestion is the removal of undigested food as faeces, which was never actually absorbed into the body's cells. Because egested material was never part of the body's chemistry, egestion does not count as one of the seven MRS GREN life processes.
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