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9
Definition of Life
What does it actually mean to be alive?

What makes something alive? It's harder to answer than you might think! In this lesson, your learner will explore what scientists use to decide if something is living. They'll discover that all living things share certain characteristics: they grow, use energy, and respond to their world. By understanding what life is, your learner will develop a deeper appreciation for the amazing diversity of living things on Earth.

S2S_Lesson09 by Selene

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. They are the basic unit of life.
  • Earth is full of living things from the tiniest bacteria to the biggest whale!
  • Some things (like viruses) challenge our definition of life, sparking ongoing scientific debate.


Spines
  • DK's Science as You've Never Seen it Before: pg. 134-135
  • Mammoth Science: What is Life? pg. 34

✏️  Notebooking Activity
Create a mnemonic device to help remember the seven characteristics of life. 

Timeline PageLabel the next page in your timeline “The Definition of Life.” The workbook prompt asks learners to draw a shallow Precambrian ocean with tiny, bumpy stromatolite mounds rising from the seafloor.

Discussion Questions
  1. Can you name one living thing and one nonliving thing near you right now? 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 considered 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.
Digging Deeper
  • 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.
  • 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.

SCIENTIST SPOTLIGHT: Carl WoeseCarl Woese was an American microbiologist who completely rewrote our understanding of life on Earth. For most of scientific history, life was divided into two categories: things with a cell nucleus (eukaryotes) and things without one (prokaryotes). In the 1970s, Woese discovered through careful analysis of genetic sequences that there was a third, entirely separate domain of life that no one had recognized before: the Archaea. These ancient microbes are so fundamentally different from bacteria that they represent a whole new branch on the tree of life. The scientific establishment dismissed and mocked his work for years. He was eventually vindicated, but he was never awarded a Nobel Prize, which many scientists consider a significant oversight.

Videos
Digging Deeper Activity:
Draw the tree of life as scientists understood it before Woese, and then draw it again after his discovery. What changed? Why does it matter for our definition of life that Archaea exist as a separate domain?

Vocabulary
  • Organism — Any living thing; from the smallest bacteria to the largest whale, all organisms share the basic characteristics of life.
  • Cell — The smallest unit of life; all living things are made of one or more cells that carry out life's basic functions.
  • 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 on to the next generation.
  • Homeostasis — The ability of a living thing to maintain a stable internal environment despite changes in the outside world.
  • Stimulus — A change in an organism's environment that causes a response; living things detect and respond to stimuli.
  • Adaptation — A feature or behavior that helps an organism survive in its environment, developed through evolution.

Sources

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