🦠 🔬 🌱 🧬
Lesson

What Is Life?

What do bacteria, oak trees, mushrooms, and humans all have in common? Let's investigate.

🔍
Driving Question
If a virus can replicate, respond to its environment, and even evolve, what's missing that keeps it from being truly alive?
🔬 Learning Science Focus 🖼️ Dual Coding 🏗️ Scaffolding 🗂️ Concept Formation ✅ Retrieval Practice ⚖️ Load Management
Learning Goals

What You'll Be Able to Do

By the end of this lesson, you will be able to:

🧬
Describe the characteristics that all living organisms share, from energy use to homeostasis.
6.MS-LS1-1
🦠
Evaluate whether something is alive (like a virus) by checking it against the characteristics of life.
6.MS-LS1-1
🔬
Compare unicellular and multicellular organisms, and prokaryotic and eukaryotic cells.
6.MS-LS1-1
🧫
Explain why the cell is the smallest unit that can carry out the functions of life, and why a virus falls short.
6.MS-LS1-1
📚 Instructional Design
Why this section exists
  • Name what students should be able to do by the end.
  • Set the target before any content begins.
Cognitive science
  • Goal setting
  • Advance organizers
Bloom's / DOK
  • Understand to Analyze
  • DOK 1 to 3
Accessibility considerations
  • Plain "I can" statements
  • Standard code shown for reference
  • Short, scannable cards
Key Vocabulary

Vocabulary to Know

Choose a card to see what each word means.

📚 Instructional Design
Why this section exists
  • Front-load the terms students will meet in the reading.
  • Lower the language barrier before the science begins.
Cognitive science
  • Pre-teaching vocabulary
  • Reduced extraneous load
Bloom's / DOK
  • Remember to Understand
  • DOK 1
Accessibility considerations
  • One card open at a time
  • Click to reveal, no hover
  • Plain, short definitions with a jump link
Engage · Investigation

What Makes Something Alive?

Here are five things. Some are alive. Some are not. One is genuinely debated by scientists. Click each card, what do you think?

🔥
Fire
Alive?
Not Alive
Grows and uses energy; but has no cells, can't reproduce, and doesn't maintain homeostasis. Just chemistry.
🦠
Virus
Alive?
Debated
Can replicate and evolve; but only by hijacking another cell. Has no cells of its own.
🤖
Robot
Alive?
Not Alive
Responds and moves, but has no cells, doesn't grow, and cannot reproduce.
🌱
Seed
Alive?
Alive
Looks dormant; but it's made of cells and can grow, reproduce, and respond to its environment.
🔬
Bacterium
Alive?
Alive
One cell. Eats, grows, responds, reproduces. All six characteristics, check.
🤔
Make a prediction: Before scientists reveal their rule, what do you think best decides whether something counts as alive?
Here's the idea

The best answer is B. Watch the trap: fire uses energy, a robot reacts, a seed just sits there, yet only some of these are alive. No single ability settles it. Living things do many jobs at once, and that is exactly the problem scientists had to solve.

Scientists ran into the same problem. To solve it, they identified six characteristics that every living thing shares. If something meets all six, it's alive. Miss even one, and it doesn't qualify.
The Six Characteristics
01 🦠
Made of Cells
Every living organism is built from at least one cell. A unicellular organism runs entirely on a single cell; a multicellular organism is built from many cells working together.
02 ⚡
Obtain and Use Energy
Living things need fuel to power every life process. Plants capture energy from sunlight; animals break down food. Without a usable energy source, life stops.
03 🌱
Grow and Develop
Organisms increase in size and change in form over their lifetime: from a tiny seed into a towering tree, or from a single fertilized cell into a full human body.
04 👶
Reproduce
Living organisms can produce offspring. Reproduction can be sexual (two parents combine genetic material) or asexual (one parent makes a copy of itself).
05 👁️
Respond to Stimuli
Organisms detect and react to changes in their environment. A plant bends toward light. A bacterium swims away from a toxin. Any change that triggers a response is called a stimulus.
06 ⚖️
Maintain Homeostasis
Living things constantly regulate their internal conditions to stay stable, your body shivers to generate heat and sweats to cool down. This internal balancing act is called homeostasis.
📚 Instructional Design
Why this section exists
  • Open with a sorting challenge: alive, not alive, or debated.
  • Surface what students already believe before defining life.
Cognitive science
  • Curiosity gap
  • Prior knowledge activation
  • Phenomenon-based learning
Bloom's / DOK
  • Understand
  • DOK 2
Accessibility considerations
  • Click to reveal, no hover
  • Concrete, familiar examples
  • Short prompt on each card
Explore · Cell Theory

The Discovery of Cells

Once scientists agreed on the six characteristics, a deeper question emerged: what are living things actually made of? Scientists eventually discovered the answer through centuries of microscope observations, forming the foundation of modern biology.

For most of human history, cells were completely invisible, too small to detect without magnification. The invention of the microscope in the 1600s changed that, and with it, our entire understanding of what life actually is.

NO MATTER HOW DIFFERENT THEY LOOK, Bacterium Oak Tree Mushroom Human ALL MADE OF CELLS
I 🔬
All living things are made of cells
From the smallest bacterium to the largest whale, every organism on Earth is built from one or more cells. There are no known exceptions.
II 🧱
The cell is the basic unit of life
A cell is the smallest unit that can carry out all the processes of life on its own, taking in energy, responding to the environment, and reproducing.
III 🔁
All cells come from existing cells
Cells don't appear from nothing. Every new cell is produced when an existing cell divides, connecting every living thing to an unbroken chain of life stretching back billions of years.
KEY IDEA: Cell

A cell is the smallest unit that can carry out all the functions of life on its own. Every living thing is built from at least one, and nothing smaller than a cell can stay alive by itself. Hold onto this, it is the reason a virus lands in the "debated" column.

These three statements make up Cell Theory, one of the most important frameworks in all of science. Understanding cells means understanding life. The next question scientists asked: are all cells the same?
📚 Instructional Design
Why this section exists
  • Establish that all living things are made of cells.
  • Show how centuries of microscope work built Cell Theory.
Cognitive science
  • Cause-and-effect modeling (tools drove the discovery)
  • Dual coding with the convergence diagram
  • Signaled Key Idea reveal (cell) marks the load-bearing term
  • Coherent narrative
Bloom's / DOK
  • Understand to Analyze
  • DOK 2
Accessibility considerations
  • Labeled diagram paired with text
  • Short paragraphs
  • Key terms defined in place
Explore · Cell Organization

One Cell or Many?

Once scientists confirmed that all living things are made of cells, a surprising discovery followed: some organisms run their entire lives on just a single cell. Others are built from trillions. Both are completely alive.

KEY IDEA: Unicellular and Multicellular

A unicellular organism runs its whole life on one cell. A multicellular organism is built from many cells that specialize and share the work. The count of cells does not decide whether something is alive, having cells at all is what matters.

Unicellular
One Cell, Every Job
One cell performs every life function itself.
Bacteria · Amoeba · Paramecium · Yeast
Multicellular
Many Cells, Shared Jobs
Specialized cells work together in tissues and organs.
Humans · Oak Trees · Jellyfish · Mushrooms
Big idea: Multicellular organisms survive because different cells specialize for different jobs. This is called division of labor, instead of one cell doing everything, many cells share the work.
Feature Unicellular Multicellular
Number of cells One cell only Many cells, sometimes trillions
How it works One cell handles everything, eating, moving, reproducing Cells specialize and work together in tissues and organs
Examples Bacteria, amoeba, paramecium, yeast Humans, oak trees, mushrooms, jellyfish
Cell type Can be prokaryotic or eukaryotic Always eukaryotic
Complexity Simpler, one cell carries every life function More complex, division of labor between specialized cell types
Visible to naked eye? Usually not Usually yes
A single amoeba eats, moves, senses its environment, and reproduces, all with one cell. This raises the next question: if cells can be so different in number, are they also different in structure?
📚 Instructional Design
Why this section exists
  • Show life can run on one cell or on trillions.
  • Make clear both are fully alive.
Cognitive science
  • Comparison and contrast
  • Concrete to abstract
  • Dual coding with the side-by-side visual
  • Signaled Key Idea reveal (unicellular and multicellular)
Bloom's / DOK
  • Understand to Analyze
  • DOK 2
Accessibility considerations
  • Side-by-side comparison cards
  • Short, parallel bullet lists
  • Labeled visuals
Explore · Cell Types

Two Major Cell Types

Not all cells are built the same way. Scientists discovered that the single biggest structural difference (the one that separates all of life into two camps) is where a cell stores its instruction manual.

Every cell carries DNA, the instructions for how to build and run a living thing. But where that DNA is stored is what makes all the difference.
WHERE IS THE DNA? EUKARYOTIC cytoplasm NUCLEUS DNA enclosed plants · animals · fungi · protists PROKARYOTIC cytoplasm ✕ no nucleus DNA floating freely bacteria
KEY IDEA: Prokaryotic and Eukaryotic

A eukaryotic cell keeps its DNA sealed inside a nucleus. A prokaryotic cell has no nucleus, so its DNA floats freely in the cytoplasm. Where the DNA sits is the single feature that sorts all cells into these two types.

Feature Eukaryotic Prokaryotic
DNA location Enclosed inside a membrane-bound nucleus Floating freely in the cytoplasm, no nucleus
Has a nucleus? Yes No
Examples Plants, animals, fungi, protists (amoeba, paramecium) Bacteria
Unicellular or multicellular? Both are possible Always unicellular
Relative size Generally larger Generally smaller
📚 Instructional Design
Why this section exists
  • Split all of life into two cell types by one structural difference.
  • Anchor that difference to where the cell stores its DNA.
Cognitive science
  • Comparison and contrast
  • Cause-and-effect modeling (nucleus or no nucleus)
  • Dual coding with the comparison table
  • Signaled Key Idea reveal (prokaryotic and eukaryotic)
Bloom's / DOK
  • Understand to Analyze
  • DOK 2
Accessibility considerations
  • Side-by-side comparison table
  • One key difference highlighted
  • Key terms defined in place
Quick Recall

Brain Check

Pull these ideas back from memory before we return to the virus question.

Quick Recall · 1 of 3
Just a quick brain check before we move on. Not graded.
Most scientists say a virus is not fully alive. What is the main reason?
Quick Recall · 2 of 3
An amoeba does everything it needs to stay alive with just one cell. What is it?
Quick Recall · 3 of 3
A cell has no nucleus, and its DNA floats freely in the cytoplasm. What type of cell is it?
📚 Instructional Design
Why this section exists
  • Pull three key ideas back from memory (cells, cell number, cell type) before the virus question.
  • Catch gaps early with a low-stakes check that spans the Explore sections.
Cognitive science
  • Retrieval practice
  • Generation effect
Bloom's / DOK
  • Understand to Apply
  • DOK 1 to 2
Accessibility considerations
  • Ungraded and low stakes
  • Immediate feedback on each item
  • Three short, plainly worded questions
Explain · Back to Viruses

Back to Viruses

This lesson began with a question about viruses. Now you have the framework, the six characteristics and the cell requirement. Let's apply it.

Putting the whole lesson together
1 Life is defined by six characteristics. To be alive, something has to meet every one, not just some.
2 The very first characteristic is cells. Every living thing is made of at least one cell, and the cell is the smallest unit that can carry out the functions of life.
3 Cells come in two types, prokaryotic and eukaryotic, and life can run on one cell or on trillions. Either way, there is always at least one cell.
4 A virus has no cell of its own. It borrows a host cell to do everything. So it fails the first characteristic before we even reach the other five.
The whole arc points to one line: the cell is the threshold a virus cannot cross. That is what is missing when a virus can replicate, respond, and evolve, yet still is not counted as truly alive.
Viruses CAN...
●Carry genetic material (DNA or RNA)
●Replicate, inside a host cell
●Evolve over time through mutation
●Respond to host cell chemistry
Viruses CANNOT...
●Build or maintain cells of their own
●Reproduce independently; they need a host
●Carry out life processes without hijacking another cell
●Maintain homeostasis

So, are viruses alive?

Scientists have debated this question for decades. Where do you stand?

You're in good company, some scientists argue viruses should qualify as a form of life, given that they carry genetic material, replicate, and evolve. But most biologists currently classify viruses as nonliving, primarily because they have no cells of their own. Without a host cell to hijack, a virus can't carry out any life processes independently. For most scientists, the cell is the minimum unit of life, and viruses don't meet that threshold.
That's where most scientists currently land, and the central reason is cells. Viruses have no cells of their own. They can only replicate by hijacking the machinery of a living cell. On their own, they can't obtain energy, grow, respond, or maintain homeostasis. The debate isn't completely closed, but the dominant view in biology is clear: without cells, something cannot be considered truly alive.
📚 Instructional Design
Why this section exists
  • Return to the opening virus question with the full framework.
  • Synthesize the whole arc (six characteristics, cells, cell types) into one criteria recap before the verdict.
  • Reason from the criteria to classify a genuinely debated case.
Cognitive science
  • Schema building
  • Misconception checking (is a virus alive?)
  • Evidence-based reasoning
Bloom's / DOK
  • Understand to Analyze
  • DOK 3
Accessibility considerations
  • Two-column can and cannot comparison
  • Criteria stated plainly
  • Key terms defined in place
Evaluate · Quiz

What Is Life? Quiz

10 questions covering characteristics of life, cell theory, cell types, and viruses. Answer every question, then submit.

Your score will not be sent Your score will be sent to your teacher
Before You Begin
This info is required to submit your quiz results to your teacher.
Please enter your name.
Please select your teacher.
Please select your block.
0 / 10 selected
🧠 Show Your Thinking

Scientists don't just know the answer. They explain their thinking.

Write your own explanation first. Then submit your work to compare your thinking with a model answer.

In one sentence, explain what a virus is missing that keeps it from being truly alive. Use the word cell.

One strong way to say it A virus is not made of a cell of its own, so it cannot carry out the functions of life on its own, and because the cell is the smallest unit of life, a virus falls short of being truly alive. If your sentence names the missing cell and why that matters, you have it.
📚 Instructional Design
Why this section exists
  • End the lesson with the student constructing the central idea in their own words, not selecting it.
  • Give the one place where the student generates rather than clicks.
Cognitive science
  • Generation effect and self-explanation
  • Elaboration and organization of knowledge
  • Self-check reveal for comparison, ungraded
Bloom's / DOK
  • Understand to Analyze
  • DOK 3
Accessibility considerations
  • One-sentence response keeps the writing load low
  • Model answer provided to self-check against
  • Submitted with the quiz, never scored separately

🔍 The Question You Came In With You started this lesson asking: "If a virus can replicate, respond to its environment, and even evolve, what's missing that keeps it from being truly alive?" The answer is cells. A virus has none of its own, and the cell is the smallest unit of life. If you can say that, you have answered it.
📚 Instructional Design
Why this section exists
  • Check understanding across characteristics, cells, and viruses.
  • Offer practice mode and a classroom submission mode.
  • Close the score board with a mystery loop that restates the driving question.
Cognitive science
  • Retrieval practice
  • Feedback loops
  • Narrative closure (return to the opening question)
Bloom's / DOK
  • Understand to Apply
  • DOK 1 to 2
Accessibility considerations
  • Answer explanations provided
  • Plausible, evenly placed options
  • Immediate feedback
Go Further

More Learning

The lesson is just the beginning. Push the definition of life further by exploring viruses, cells, and the edge cases that blur the line between living and nonliving.

🔍
The Gray Zone
Viruses, prions, fire, things that blur the line between living and nonliving. Where does life actually begin?
Investigate
🦠
Are Viruses Alive?
Dive deep into the case of viruses; they replicate, evolve, and infect, but fail the cell test. Are they alive?
Explore
📚 Instructional Design
Why this section exists
  • Offer optional ways to push the definition of life further.
  • Let students apply the framework to investigations and games.
Cognitive science
  • Interest-driven extension
  • Transfer
Bloom's / DOK
  • Apply to Analyze
  • DOK 2 to 3
Accessibility considerations
  • Optional and self-paced
  • No penalty for skipping
  • Clear, labeled cards
Connections

Connections

If what it means to be alive caught your attention, here are related ideas worth exploring.

🦠
Cell Types
Every living thing is built from cells. See why a bacterium and a human brain cell look nothing alike, and how cells specialize for different jobs.
Explore
🫀
Body Systems
Living things take in energy, remove waste, and respond to the world. See the teams of organs that carry out those life functions inside your body.
Explore
🧬
Biological Evolution
Reproducing and passing on traits is one of the signs of life. See how those traits shift across many generations and reshape entire species.
Explore