Still, yet alive

The idea

Visible movement alone cannot define life; the ordered structures of living organisms keep breaking down, so organisms must constantly repair and maintain them by moving molecules, making molecular movement necessary for life.

Asleep, but alive

A dog is fast asleep. It lies still. Beside it, a plant is not visibly growing this week.

Are they alive?

• Yes, even though nothing visible moves
• No, they are not moving
• Only the dog, because it breathes

Zoom in and repair

Your eyes see nothing move. Zoom in to the molecules of its body. Press Time passes: the environment breaks some pieces. Tap each broken piece to repair it. Do it three times.

Order needs repair

Visible movement alone is not enough to define life. Living things are ordered structures that the environment keeps breaking down. So they must keep repairing them, and because everything is made of molecules, they must move molecules around all the time.

Movement too small to see is still movement.

• What we see | What is going on
• A dog asleep | Nothing moves to the eye, but molecules keep moving in to repair it
• A plant not visibly growing | Still alive: the repair work goes on
• A virus outside a cell | No molecular movement in it, so scientists argue about whether it is truly alive

Notes

Visible movement alone cannot define life; the ordered structures of living organisms keep breaking down, so organisms must constantly repair and maintain them by moving molecules, making molecular movement necessary for life.

Check yourself

A plant is not visibly growing this month. Is it alive?

Why must living things keep repairing their structures?

The wind wears down a stone statue and a sleeping dog. Which one can repair the damage by moving molecules in?

Biologists argue about whether viruses are truly alive. One reason is that, outside a cell, a virus …

• No. Nothing visible is moving. A plant that is not visibly growing is still alive. Visible movement alone is not enough to define life.
• Yes. Its molecules keep moving to repair it — correct. Yes! Visible growth is not the test. The repair work never stops.
• Only if its leaves are green. Plants with leaves of other colours are alive too. Colour is not the test.
• Because they want to grow taller. Growth needs extra material, but even a fully grown body has to be repaired.
• Because molecules like to move. Molecules do not move for fun. The movement does a job: repair and maintenance.
• The environment keeps breaking their order down — correct. Yes! If the order broke down for good, the organism would no longer be alive.
• The dog — correct. Yes! A living body keeps moving molecules to repair its order.
• The statue. The statue is not alive. Nothing inside it moves molecules in to repair it.
• Both do. Only a living body does this repair work.
• is too small to see. Many living things are too small to see. The argument is about molecular movement.
• shows no molecular movement in it — correct. Yes! Without molecular movement there is no repair work.
• is made of molecules. Every living thing and every stone is made of molecules. The question is whether they move.