Living and Non-Living Things
Understand the difference between living, non-living, and once-living things for PSLE Science. Learn the 7 life processes (MRS GREN) with Singapore examples, exam tips, and a free quiz.
What Makes Something Living?
Everything in the world can be sorted into three groups: living, non-living, and once-living. The tricky part is knowing exactly what separates them — because some non-living things (like fire and robots) seem to behave like living things, and some living things (like seeds) seem completely still and lifeless.
A living thing is anything that is currently carrying out all seven life processes. Scientists use the memory trick MRS GREN to remember all seven. A non-living thing never had life at all. A once-living thing was alive at some point in the past but is no longer carrying out any life processes.
Spotting Living, Non-Living, and Once-Living Things in Singapore
At a hawker centre in Singapore, you can find all three groups in one place. The grass growing in the planter boxes outside is living — it grows, absorbs nutrients, responds to rain, and reproduces. The plastic tray your food sits on is non-living — it was manufactured from oil, never had life, and carries out no life processes. The wooden chopsticks are once-living — they were made from a tree that once grew and photosynthesised, but no life processes occur in the wood any more.
The SingPost robot at some post offices can move, detect people, and respond to them — but it is still non-living. It does not grow, reproduce, excrete waste products from chemical reactions, or carry out respiration. Movement and sensitivity alone are not enough.
In the Botanic Gardens, a fallen tree trunk covered in moss shows both once-living (the dead wood) and living (the moss growing on it) in one object.
MRS GREN — All 7 Must Be Present
A living thing must carry out all seven of these processes. Missing even one means it does not qualify as living under the scientific definition.
- M — Movement: Living things move, but not always the whole body. Animals move their whole bodies; plants move parts — leaves turn towards light, roots grow towards water and gravity.
- R — Respiration: Converting food (glucose) into energy that can be used for all life processes. This happens inside every living cell. Respiration is NOT the same as breathing — breathing is just the mechanism some animals use to get oxygen for respiration.
- S — Sensitivity: Detecting and responding to changes in the environment (called stimuli). Your eye blinking when something flies towards it, a plant bending towards light — these are both sensitivity.
- G — Growth: A permanent increase in size, mass, and complexity. A crystal can grow, but not in this biological sense — crystal growth is just molecules stacking up, not cells multiplying and developing.
- R — Reproduction: Producing offspring of the same species. Without reproduction, a species becomes extinct. Note: not every individual must reproduce, but the species as a whole must be capable of it.
- E — Excretion: Removing waste products made by the body's own chemical reactions. This is different from egestion (releasing undigested food). Carbon dioxide (from respiration), urea (from protein breakdown), and sweat are all examples of excretion.
- N — Nutrition: Taking in food or making food to provide raw materials and energy. Plants do this through photosynthesis; animals do it by eating other organisms.
Why Do Scientists Need This Classification?
Understanding whether something is living or non-living helps us predict how it will behave and how it interacts with its environment. A rock in MacRitchie Reservoir does nothing — it does not consume oxygen, reproduce, or respond to pollution. A water hyacinth plant in the same reservoir actively takes up nutrients, reproduces rapidly, and can cover the water surface so densely that it blocks sunlight for fish and other plants below. Knowing it is living explains why it spreads and why it needs to be controlled.
The once-living category matters too. Coal, oil, and natural gas are all once-living — they are the compressed remains of ancient organisms. Understanding this explains why burning them releases carbon that was locked away millions of years ago, contributing to climate change.
Living vs Non-Living vs Once-Living
| Feature | 🌱 Living | 🪨 Non-living |
|---|---|---|
| All 7 MRS GREN? | Yes — all seven | No |
| Was it ever alive? | Yes (currently) | Never |
| Examples | Dog, tree, mushroom, bacteria, you | Rock, water, air, plastic, robot, fire |
| Once-living examples | Dried leaf, wooden furniture, coal, charcoal, leather, dead insect, cotton cloth | |
The Mistakes Students Make Every Year
Everything You Need to Know — at a Glance
- MRS GREN: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, Nutrition
- All 7 must be present for something to be classified as living
- Non-living: never alive. Once-living: was alive, no longer is
- Seed = living (dormant). Fire = non-living. Robot = non-living. Dead leaf = once-living
- Excretion = removal of metabolic waste (CO₂, urea, sweat) — not egestion of undigested food
- Respiration ≠ breathing. All living things respire; not all breathe
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