In Living or Not Living? you learned that living things grow, need food and water, and make more of their kind. Scientists use a fuller list. A thing is alive only if it shows all of these signs together.
Key idea
Every living thing (1) is made of one or more cells, (2) uses energy, (3) grows and develops, (4) responds to its surroundings, (5) keeps its insides steady (homeostasis), and (6) produces offspring that inherit its traits.
Why it works
Many nonliving things show one sign. A crystal grows. A fire uses energy and spreads. A robot responds. But none of them is made of cells, and none produces offspring that inherit its traits. Only living things show the whole set, which is why scientists test for all six.
Worked examples
Worked example: A seed in a packet
A bean seed has sat in a dry packet for a year. Is it alive?
- Is it made of cells? Yes: a seed holds a tiny plant made of cells.
- Does it use energy? Yes, slowly, while it rests.
- Can it grow, respond and make offspring? Given water, it sprouts, grows toward light, and later makes seeds of its own.
Answer: Yes. It is alive, only resting.
Worked example: Which sign is it?
You shiver on a cold day, and your body warms up. Which characteristic is that?
- Shivering is a response, but its purpose is to hold your body temperature steady.
- Keeping conditions inside the body steady is homeostasis.
Answer: Keeping its insides steady (homeostasis).
Common mistake
Judging by one sign, often movement or growth. A car moves and a crystal grows, yet neither is alive. A tree never walks, yet it is alive. Check for cells first, then the rest of the list.