What do bacteria, oak trees, mushrooms, and humans all have in common? Let's investigate.
Here are five things. Some are alive. Some are not. One is genuinely debated by scientists. Click each card — what do you think?
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.
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.
| 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 |
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.
| 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 |
This lesson began with a question about viruses. Now you have the framework — the six characteristics and the cell requirement. Let's apply it.
So — are viruses alive?
Scientists have debated this question for decades. Where do you stand?
Every term below appeared in this lesson. Click any pill to jump to where it was explained.
10 questions covering characteristics of life, cell theory, cell types, and viruses. Answer every question, then submit.
The lesson is just the beginning — push the definition further, test your skills, or explore the edge cases.