Living and Non-Living Things: The Complete P3 Guide

One of the first big questions in science is also one of the most fundamental: what makes something alive? This is not always as obvious as it sounds. Fire grows and moves — is it alive? A robot can walk and respond to its surroundings — is it alive? A dried seed looks completely lifeless — but is it? These are exactly the kinds of questions that appear in P3 Science exams, and getting them right requires a deep understanding of the seven characteristics of living things.

MRS GREN — The Seven Characteristics of Living Things

Scientists use a checklist of seven life processes to decide whether something is alive. Every single living thing — from the tiniest bacterium to a blue whale — must carry out all seven processes. The mnemonic MRS GREN helps you remember them:

• M — Movement: Living things move. Animals move their whole bodies to find food, escape predators, and find mates. Plants move more slowly but they do move — roots grow downward (towards gravity and water), shoots grow upward (towards light), and flowers turn to follow the Sun. Movement in living things is self-generated, unlike a rock rolling downhill.
• R — Respiration: This is NOT the same as breathing. Respiration is the chemical process inside cells that releases energy from glucose. Every cell in every living thing respires 24 hours a day — even when you are sleeping. The energy released powers all life processes. Breathing is just the mechanism that brings oxygen to the body for aerobic respiration.
• S — Sensitivity: Living things detect changes in their environment (called stimuli) and respond to them. You pull your hand away from a hot flame. A plant grows towards a light source (phototropism). A bacterium swims towards a food source. This ability to detect and respond is a hallmark of life.
• G — Growth: Living things grow by increasing in size and complexity — not just by swelling up with water or having material piled on top. Real biological growth involves cell division and the production of new, complex organic molecules. A crystal can grow bigger by adding material, but this is not biological growth.
• R — Reproduction: Living things produce offspring — new individuals of the same kind. This can be sexual (requiring two parents) or asexual (one parent). Without reproduction, a species would die out in one generation. Reproduction does not need to occur in every individual (a worker bee never reproduces) but the capacity must exist in the species.
• E — Excretion: Living things produce waste products from their chemical reactions and must remove these wastes before they poison the organism. Humans excrete carbon dioxide (lungs), urine and urea (kidneys), and sweat (skin). Plants excrete oxygen as a by-product of photosynthesis and carbon dioxide as a by-product of respiration.
• N — Nutrition: Living things need to take in or make food to provide energy and raw materials for growth. Plants make their own food by photosynthesis (they are autotrophs). Animals must eat other organisms to obtain food (they are heterotrophs).

Three Categories: Living, Non-Living, and Once-Living

In science, we divide everything in the world into three categories based on whether it is alive, has never been alive, or was once alive but has since died:

The once-living category is particularly important because many students forget it. A wooden table was once part of a living tree. Paper was once wood pulp from a living tree. Coal formed over millions of years from ancient plants and animals. These things are no longer alive, but they were once alive — and this matters for answering exam questions correctly.

Common Exam Traps You Must Know

These are the most frequently tested "trick" organisms that students get wrong:

• Fire — Fire can grow, move, and even "spread" to new areas. But fire does not have cells, does not reproduce biologically, does not excrete, and does not carry out nutrition. It is NOT living. The correct answer is always: fire is non-living.
• Seeds — A dried seed looks like a small stone, completely lifeless. But it is very much ALIVE — it is in a state of dormancy (temporary inactivity). Given the right conditions (water, warmth, oxygen), it will germinate. Never classify a seed as non-living.
• Robots — A robot can move, sense its surroundings, and even be programmed to "grow" by printing new parts. But it does not respire, excrete biological waste, or reproduce biologically. It is NOT living.
• Coral — Coral looks like a rock but is actually a living colony of tiny animals called coral polyps. It is LIVING.
• Viruses — Viruses are unusual: they can reproduce, but only inside a living cell, and they lack most other life processes. They sit in a grey zone — most scientists do not classify them as truly alive, but this is beyond PSLE scope.

Vertebrates and Invertebrates

Animals can be divided into two major groups based on whether they have a backbone (vertebral column):

Vertebrates are animals with a backbone. There are five groups of vertebrates, which every P3 student must know:

Key distinctions to remember: warm-blooded animals (birds and mammals) maintain a constant body temperature regardless of the environment. Cold-blooded animals (fish, amphibians, reptiles) have a body temperature that changes with the environment. Mammals feed their young with milk; no other group does this. Bats are the only flying mammals; whales and dolphins are mammals despite living in water.

Invertebrates are animals without a backbone. They make up about 97% of all animal species. Key groups include:

• Insects — 6 legs, 3 body parts (head, thorax, abdomen), usually have wings. Examples: ant, butterfly, bee, grasshopper, beetle.
• Arachnids — 8 legs, 2 body parts. Examples: spider, scorpion, mite, tick.
• Crustaceans — more than 8 legs, hard exoskeleton, mostly aquatic. Examples: crab, prawn, lobster, woodlouse.
• Myriapods — many legs. Centipedes (one pair per segment) and millipedes (two pairs per segment).
• Molluscs — soft body, often with a shell. Examples: snail, slug, clam, squid, octopus.
• Echinoderms — five-fold symmetry, live in the sea. Examples: starfish, sea urchin.
• Worms — soft, elongated body. Examples: earthworm, tapeworm.