The science of life
Biology · 9th grade
What makes something alive
Today: Biologists call something alive when it has a whole set of traits together, and no single trait is enough.
About 45 minutes to teach
Teach the idea
Read this together:Looking for Life on Mars
257 words
On July 20, 1976, a NASA spacecraft called Viking 1 set down on a wide plain on Mars named Chryse Planitia. A second lander, Viking 2, touched down farther north about seven weeks later, on September 3. Each one carried a small laboratory built to answer a hard question: is anything alive in the Martian soil?
The scientists could not count on seeing a creature. They had to decide ahead of time what living things do that nonliving things don't, and then test for it. One trait they chose was metabolism: living things take in food and give off gas. In a test called the Labeled Release experiment, the lander added a few drops of nutrient broth to a scoop of soil. Some of the carbon atoms in the broth were radioactive, so if anything in the soil ate the broth and gave off gas, the lander could detect that carbon in the gas.
The first results came back at the end of July, and they looked like life. Gas came off the soil. But another instrument found no carbon-based molecules in the soil at all, which is strange for a place where something lives.
Many scientists came to think the gas came from unusual chemistry in the Martian soil, not from living things. Gilbert Levin, the scientist who led the experiment, argued for decades afterward that it had found life. The question has never been fully settled, and it taught a lasting lesson: one sign of life, by itself, is not enough to call something alive.
Biology is the study of living things, and it starts with a question that sounds easy: what counts as alive? A dog is alive and a rock is not, but the hard cases are where the definition gets tested. Biologists answer with a list of traits, and a living thing has all of them together.
Living things are made of one or more cells. A bacterium is a single cell; a person is tens of trillions of them.
Living things use energy. They take in food or make it, the way a plant makes sugar from sunlight, and use that energy to run the chemical reactions inside them. All of those reactions together are called metabolism.
Living things grow and develop, and they respond to their surroundings. A seedling bends toward a sunny window. The dark centers of Ethan's eyes shrink in bright light.
Living things keep their insides steady while the outside changes. Biologists call that homeostasis. A body sweats when it is hot and shivers when it is cold, and both keep its temperature close to the same number.
Living things reproduce, and they pass their instructions, written in DNA, on to their offspring.
The list matters because some nonliving things show one or two traits. A fire grows and uses fuel. A salt crystal grows in salty water. Neither is made of cells and neither carries DNA, so neither is alive. One trait by itself proves nothing, and the read-aloud, Looking for Life on Mars, shows scientists learning that the hard way in 1976.
Do it together
1. Ask Ethan to sort five things into alive and not alive, with a reason for each: a houseplant, a candle flame, a seed in a packet, a cloud, and a cut flower in a vase. Write the answers down and keep them; Ethan will come back to them at the end.
2. Go back to Looking for Life on Mars, to the part about the Labeled Release experiment. Ask which trait of life the scientists were testing for, and why one result was not enough to settle the question.
3. Go through the traits of life together, one at a time. For each one, ask Ethan for an example from the kitchen, the yard, or Ethan's own body, the way a seedling bends toward a window or a body sweats in the heat.
4. Return to the five things from the start. Ask Ethan to check each one against the whole list and change any answer that no longer holds. The seed and the cut flower are the hard ones, so let Ethan argue them out before you move on.
Activity
Together: Is the yeast alive? Set out three clear cups or jars and label them A, B, and C. Ethan measures 1/2 cup of warm tap water, about as warm as a bath, into each. Into cup A, Ethan stirs 1 teaspoon of dry yeast and 1 teaspoon of sugar. Into cup B, 1 teaspoon of yeast only. Into cup C, 1 teaspoon of sugar only. Stand a ruler beside each cup. Every 5 minutes for 20 minutes, Ethan measures the height of the foam in millimeters and records it in a table. While waiting, Ethan smells each cup and looks for bubbles rising. Last, Ethan opens the lab notebook and writes the first entry: the date, the question (is dry yeast alive?), what was done, the foam heights, which traits of life the results show, and which traits this test could not show.
Tip If the list feels like a lot to hold, have Ethan make a short memory line from the first letter of each trait and say it twice. If Ethan moves quickly, ask whether a virus would pass the test, and keep the answer in the lab notebook to check later in the year.
On paper
Worksheet
What Makes Something Alive
The worksheet, as it prints
Biology · Worksheet
What Makes Something Alive
Answer each question in a sentence or two, using what you learned today.
Name: Ethan
Score: ____ / 8
1. Name four traits biologists look for to decide whether something is alive.
2. What is metabolism?
3. A fire grows and uses fuel. Why is a fire not alive?
4. What is homeostasis? Give one example from your own body.
5. In Looking for Life on Mars, which trait of life did the Labeled Release experiment test for?
6. Why did many scientists decide the Mars results did not show life?
7. Which cup made the most foam, and what does that show about the yeast?
8. What would happen if you stirred twice as much sugar into cup C?
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