What makes something alive — introducing MRS GREN

Look around you. Some things are clearly alive — the plant on the windowsill, the bird flying past, the people around you. Other things are clearly not alive — a stone, a chair, a phone. But what exactly makes the difference? Biologists answer this with seven characteristics that all living things share. WAEC tests this in every paper.

The seven characteristics — MRS GREN

This is the most important mnemonic in WAEC Biology. Memorise it. Each letter stands for one characteristic:

• M — Movement
• R — Respiration
• S — Sensitivity (irritability)
• G — Growth
• R — Reproduction
• E — Excretion
• N — Nutrition

If something shows ALL seven of these, it is alive. If it shows fewer, it is either dead or never was alive.

The next seven lessons cover one characteristic each in detail. This lesson introduces the idea and explains why scientists use this seven-point test.

Why seven characteristics?

For a long time, people argued about what "life" really means. Is a virus alive? Is a flame alive? Is a robot alive?

Biologists eventually agreed on a working definition: a living thing must show ALL seven of these characteristics. This is not a perfect definition (some edge cases like viruses still cause debate), but it works for almost everything.

This is also the definition WAEC uses. Memorise the seven, learn what each means, and you are ready.

A quick tour of each characteristic

Here is a one-sentence preview of each — the next lessons go deeper.

M — Movement. All living things move. Animals move from place to place. Plants move parts of themselves (leaves toward light, roots toward water).

R — Respiration. All living things release energy from food. This is respiration. Without respiration, no living thing can function.

S — Sensitivity. All living things respond to changes in their environment. They detect light, sound, heat, smell, touch, chemicals — and respond.

G — Growth. All living things grow. They get bigger or develop new parts. A seed becomes a tree. A baby becomes an adult.

R — Reproduction. All living things produce more of their own kind. Plants make seeds. Animals make offspring. Bacteria divide. Without reproduction, no species could last.

E — Excretion. All living things produce waste, and they must get rid of it. Waste from chemical reactions inside the body must be removed, or it builds up and harms the organism.

N — Nutrition. All living things need food. Plants make their own (photosynthesis). Animals eat plants or other animals. Without food, no living thing can grow or respire.

Examples — testing the seven characteristics

Let's apply MRS GREN to a few examples.

A mango tree

• Movement? Yes — leaves turn toward light, roots grow toward water.
• Respiration? Yes — happens in every cell, day and night.
• Sensitivity? Yes — responds to light (phototropism), gravity (geotropism), touch (some plants close leaves when touched).
• Growth? Yes — seedling becomes a tall tree.
• Reproduction? Yes — makes flowers, then mangos containing seeds.
• Excretion? Yes — releases oxygen (during photosynthesis) and waste from metabolism.
• Nutrition? Yes — makes food through photosynthesis.

Mango tree shows all seven. It is alive.

A goat

• Movement? Yes — walks, runs, jumps.
• Respiration? Yes — breathes in air, releases CO₂.
• Sensitivity? Yes — responds to sights, sounds, smells.
• Growth? Yes — kid becomes adult goat.
• Reproduction? Yes — gives birth to young.
• Excretion? Yes — produces urine and faeces.
• Nutrition? Yes — eats grass, leaves, and other plant material.

Goat shows all seven. Alive.

A rock

• Movement? No (does not move on its own).
• Respiration? No.
• Sensitivity? No.
• Growth? No (does not get bigger on its own).
• Reproduction? No.
• Excretion? No.
• Nutrition? No.

Rock shows none of the seven. Not alive. Never was.

A bonfire

This is a tricky example. A fire seems to do many of the seven things:

• It moves (flames flicker).
• It "grows" (gets bigger if there's more fuel).
• It "produces waste" (smoke and ash).
• It "consumes" (eats fuel).
• It "reproduces" in a way (sparks can start new fires).
• It "responds" to oxygen (burns brighter with more air).

But it does NOT respire, in the biological sense. It does not have cells. It is a chemical reaction, not a living organism. A fire FAILS the test — it is not alive, even though it superficially mimics some characteristics.

This is why biologists need a strict definition. "Looking alive" is not enough.

A virus (the edge case)

Viruses are tricky:

• They do not respire on their own.
• They cannot reproduce on their own — they need to invade a living cell.
• They have no excretion, no nutrition, no growth in the usual sense.

Some biologists say viruses are NOT alive — they only show life-like behaviour when inside a host cell. Other biologists say they are alive in a very minimal way. WAEC traditionally says viruses are not classified as fully alive, because they fail the test for some characteristics.

This is the kind of edge case that earns higher-level marks if you can discuss it carefully.

Why this matters

WAEC will ask questions like:

• "State the seven characteristics of living things."
• "Explain why a rock is not alive."
• "What is the difference between a living thing and a dead thing?"
• "Is a virus alive? Justify your answer."

Knowing MRS GREN cold gives you all the points. Knowing what each characteristic means and being able to give examples earns full marks.

A note on what "dead" means

A dead thing is different from something that was never alive. A dead leaf was once alive — it grew, photosynthesised, etc. Now it does not. It has lost the seven characteristics.

A rock, on the other hand, was never alive. It does not have a "death" — it's just non-living.

WAEC sometimes asks for the distinction. Three categories:

1. Living — shows all seven characteristics now
2. Dead — used to show them, but does not anymore
3. Non-living — never showed them; never was alive

Summary

• Living things show seven characteristics, remembered as MRS GREN.
• The seven: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, Nutrition.
• Anything that shows ALL seven is alive. Anything that shows none is non-living. Anything that used to show them but does not now is dead.
• Plants AND animals show all seven, though they may look different. Don't dismiss plants.
• Edge cases like viruses are debated — they fail the test for some characteristics, so are usually not classified as fully alive.
• A fire mimics some characteristics but fails the test — it does not respire and has no cells.
• The next seven lessons cover each characteristic in detail.