Biologists identify living things by looking for a set of shared characteristics that separate life from nonliving matter. These characteristics include organization, metabolism, growth, response to stimuli, reproduction, homeostasis, and evolution. They matter because they help scientists classify organisms, study ecosystems, and understand how cells, plants, animals, fungi, and microbes function.
A tree, a bacterium, and a human look very different, but all show the same basic signs of life.
Key Facts
- Organization means living things are made of one or more cells arranged in an orderly way.
- Metabolism is the total of all chemical reactions in an organism, including energy release and molecule building.
- Growth and development involve increases in size, cell number, or complexity controlled by genetic information.
- Homeostasis keeps internal conditions stable, such as body temperature, water balance, or blood glucose level.
- Reproduction passes genetic information to offspring, but an individual can be alive even if it cannot reproduce.
- Evolution occurs in populations over generations when inherited traits change in frequency.
Vocabulary
- Cell
- The smallest unit of life that can carry out basic life processes.
- Metabolism
- The sum of all chemical reactions that build materials and release or use energy in an organism.
- Homeostasis
- The regulation of internal conditions to keep them within a range that supports life.
- Stimulus
- A change in the environment that causes a response in an organism.
- Evolution
- The change in inherited traits of a population over many generations.
Common Mistakes to Avoid
- Saying movement alone proves something is alive is wrong because nonliving things such as rivers, clouds, and machines can move.
- Assuming all living things must reproduce individually is wrong because sterile animals and worker bees are alive even though they may not produce offspring.
- Confusing growth with simple size increase is wrong because living growth involves cells making new materials, not just swelling or accumulating matter.
- Thinking evolution happens to one organism during its lifetime is wrong because evolution describes inherited changes in populations across generations.
Practice Questions
- 1 A yeast culture starts with 200 cells and doubles every 30 minutes. How many cells are present after 2 hours?
- 2 A student observes 8 unknown samples and finds that 6 are made of cells, use energy, respond to light, and maintain internal conditions. What fraction and percent of the samples show multiple characteristics of life?
- 3 A robot can move, use stored energy, and respond to commands, but it is not made of cells and does not evolve as a population. Explain why biologists would not classify it as living.