All the signs together

About this video

A cat is alive and a rock is not: that much is easy. But a candle flame grows and moves, a dry seed in a jar seems to do nothing at all, and a virus can make copies of itself. Which of them are alive? Biology is the science of living things, so it has to start here.

Biologists don't use one test. They look for a set of signs of life, and a living thing, an organism, shows them all: it is built in an ordered way out of cells, it uses energy from sunlight or food, it grows and develops, it senses and responds to what is around it, it keeps its inside steady, and it reproduces, passing its instructions on to its young.

A flame grows, moves and uses oxygen, but it is not built of cells and passes no instructions on: it just spreads wherever there is fuel. A dry seed is the opposite case. It looks dead but is alive, resting and waiting for water and warmth. A date seed found at Masada, by the Dead Sea, and dated to about 2,000 years old was planted in 2005, and it sprouted and grew into a date palm.

Viruses are the hard case. A virus is far smaller than a cell and does nothing on its own; it can make copies of itself only inside a living cell, using the cell's machinery. Most biology books say viruses are not alive, but scientists still argue about it. This video goes with the lesson Is It Alive? in Biology Level 1 on Knaph.

Transcript

1. 0:00A cat is alive. A rock is not. Easy. But what about a flame, a dry seed or a virus?
2. 0:09Biologists use six signs of life. Built from cells. Uses energy. Grows. Senses and responds.
3. 0:17Keeps its inside steady. Reproduces. A living thing shows all six, together.
4. 0:27A candle flame grows, moves and uses oxygen. But it is not built of cells,
5. 0:34and it passes no instructions on. It just spreads wherever there is fuel. Not alive.
6. 0:41A dry seed seems to do nothing. But it is alive, resting, waiting for water and warmth.
7. 0:48In 2005, scientists planted a date seed about 2,000 years old. It sprouted, and grew into a palm.
8. 0:56A virus is far smaller than a cell. On its own, it does nothing.
9. 1:03It can only make copies of itself inside a living cell. Scientists still argue whether that counts as alive.
10. 1:12Alive means all the signs of life, together. One sign is not enough.