Lesson 9

9

Definition of Life

What does it actually mean to be alive?

Living things are made of cells and share characteristics such as using energy, growing, and responding to their environment. The lesson examines these characteristics and examples, including viruses, that make life difficult to define.

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Key Ideas

• Living things share key characteristics: they use energy, grow, reproduce, and respond to their environment.
• All living things are made of one or more cells, the basic unit of life.
• Metabolism is all the chemical reactions in a body that convert food into energy.
• Homeostasis is the ability to maintain a stable internal environment despite outside changes.
• Some things, like viruses, challenge our definition of life and spark ongoing scientific debate.

Spines

• DK's Science as You've Never Seen it Before: pg. 134-135
• Mammoth Science: What is Life? pg. 34
• Alternative: How for Kids: Characteristics and Classification of Living Organisms (https://howforkids.com/characteristics-and-classification-of-living-organisms/)

Vocabulary

• Organism: Any living thing; all organisms share the basic characteristics of life.
• Cell: The smallest unit of life; all living things are made of one or more cells.
• Metabolism: All the chemical reactions in a living thing that convert food into energy and build or repair the body.
• Reproduction: The ability of a living thing to produce offspring, passing traits to the next generation.
• Homeostasis: The ability to maintain a stable internal environment despite changes in the outside world.
• Adaptation: A feature or behavior that helps an organism survive, developed through evolution.

Discussion Questions

1. Can you name one living thing and one nonliving thing near you? How do you know the difference?
   Sample Answer: A plant is living because it grows and needs water; a rock is nonliving because it doesn't eat, grow, or reproduce.
2. What does it mean to 'respond to the environment'?
   Sample Answer: Living things notice changes around them and react, like a plant turning toward sunlight or an animal running from danger.
3. What does 'made of cells' mean, and why is it a defining trait of life?
   Sample Answer: Cells are the smallest units that can carry out life's basic functions. Every known living thing is made of at least one cell.
4. Why are viruses so difficult to classify as living or nonliving?
   Sample Answer: Viruses can reproduce and carry genetic information, but only inside a host cell. They can't grow or reproduce on their own, which puts them in a gray zone.

Digging Deeper

1. Could a sophisticated robot ever qualify as "alive" by the scientific definition? Why or why not?
   Sample Answer: Probably not. While a robot can move and respond, it can't reproduce, grow, or evolve, and it isn't made of cells. It would fail most checklist items.