All Living Things Are Made of Cells
Lumi peers through the eyepiece of a bright compound microscope in a sunlit lab, grinning as a thin slice of onion skin comes into sharp focus, revealing its tightly packed rectangular cells like a mosaic of tiny rooms.
- Explain the three core statements of the cell theory in your own words.
- Identify whether an organism is unicellular or multicellular using a given description or image.
- Compare a prokaryotic cell to a eukaryotic cell by naming one key structural difference.
- Predict whether a newly discovered organism must be made of cells, and justify your reasoning using cell theory.
Key terms
- Cell theory
- The principle that all living things are made of cells which arise from pre-existing cells.
- Unicellular
- Made of a single cell that performs every function needed to stay alive.
- Multicellular
- Made of many cells that specialize and cooperate, like the trillions in your body.
- Prokaryote
- A cell with no membrane-bound nucleus, so its DNA floats freely in the cytoplasm.
- Eukaryote
- A cell with a true membrane-bound nucleus and other membrane-bound organelles.
How Cell Theory Was Built
Cell theory was not discovered in a single flash; it was assembled over two centuries as microscopes improved. Hooke first named the empty 'cells' he saw in cork, and Leeuwenhoek revealed that pond water teemed with living single cells. Schleiden and Schwann then generalized that plants and animals alike are made of cells, and Virchow supplied the capstone: cells only come from other cells. Together these contributions show that a scientific theory is a well-supported explanation built from many independent observations, not a guess.
Prokaryotes Versus Eukaryotes
Although every living thing is cellular, cells come in two great categories. Prokaryotic cells, such as bacteria, lack a membrane-bound nucleus, so their single loop of DNA sits directly in the cytoplasm. Eukaryotic cells — found in animals, plants, fungi, and protists — wrap their DNA inside a true nucleus and also contain membrane-bound organelles like mitochondria. Both types still share a cell membrane and the ability to reproduce, so the nucleus, not size or movement, is the defining difference between them.
Worked examples
Decide whether a virus counts as a living organism under cell theory.
- State the rule: cell theory says every living organism is made of one or more cells.
- Examine the candidate: a virus is a strand of nucleic acid inside a protein coat, with no cell membrane, cytoplasm, or organelles.
- Compare: because a virus is not built from any cell, it fails the most basic criterion of cell theory.
Answer: A virus is not a living organism under cell theory because it is not made of cells.
Activity
Drag each organism or object into the correct bin: Unicellular, Multicellular, or Not a Living Thing.
Practice
Decide whether a single yeast cell is unicellular or multicellular and justify your answer.
State the three statements of cell theory and name the scientist who added the third.
Common mistakes to avoid
- Cells can form spontaneously from non-living matterEvery cell arises only from a pre-existing cell, which is the third statement of cell theory.
- Only animals and plants are made of cellsAll organisms, including bacteria, fungi, and protists, are made of cells, not just animals and plants.
Check your understanding
Which statement is part of the Cell Theory?
A student argues: 'Viruses are alive because they reproduce.' Using cell theory, what is the strongest counter-argument?
What is the key structural difference between a prokaryotic cell and a eukaryotic cell?
Scientists discover a tiny organism in a deep-sea vent. It is unicellular, has DNA floating freely in its cytoplasm, and reproduces by splitting in two. How should it be classified?
Recap
Cell theory holds that all living things are made of one or more cells, that the cell is life's basic unit, and that every cell comes from a pre-existing cell, which is why viruses are not classified as living.
Reflect
If you found a new object on another planet, what single question would tell you whether it is alive?