The meaning of "life" [take a quiz on this topic]

Chapter One: An Introduction to Cells Cells and their history The meaning of "life" The origin of life How cells work Types of unicellular organisms

Biologists have debated for many decades over how to define what is alive and what isn't; in fact, they are still debating today. How would you define life? Would you say that anything that can grow and reproduce is alive? With that definition, fire would be alive! The "babies" are the sparks that can fly off, which can grow into a completely new fire! To prevent this sort of confusion, biologists have created a list of characteristics that distinguish animate (living) things from inanimate (non-living) things. The most important characteristics are listed below.

1. Living things are in general more organized in their structure than are inanimate objects.
2. Living things reproduce.
3. Living things grow.
4. Living things respond to changes in their surroundings.
5. Living things tend to keep their internal environment unchanged, even when the external environment changes (For example, people have a constant internal temperature of 98.6 degrees Fahrenheit, whether they're in the Sahara Desert or in Antarctica.).

Let's return to the example of fire to see why scientists do not consider it to be alive. As we mentioned, fire does reproduce and grow, so it satisfies #2 and #3 on the list. However, fire does not respond to changes in the environment; if you walk up to a fire, it behaves just as it does if you weren't there, whereas living things (for example, a dog) would exhibit a change in behavior. Also, fire is not more organized than other inanimate objects. In fact, fire is just the release of energy when something burns, and this release of energy is usually quite random. Finally, while a fire will be hot whether it's in the desert or Antarctica, the fire doesn't really have a well defined internal environment that #6 says it has to have, because fire is just a blob of energy without any real borders.