Cell Theory and Cell Types
The cell is where life happens. Every living thing is built from cells, and understanding the basic rules about cells, plus the two great types they come in, unlocks much of biology. These ideas are foundational, which is exactly why tests return to them again and again.
This lesson covers cell theory and the difference between the two main cell types.
Cell theory states that all living things are made of cells, that the cell is the basic unit of life, and that all cells come from existing cells. Cells come in two main types: prokaryotic cells, which have no nucleus, and eukaryotic cells, which keep their DNA inside a nucleus.
What is cell theory?
Cell theory is one of the great unifying ideas in biology, and it rests on three points: every living organism is made of one or more cells; the cell is the smallest unit that counts as alive; and all cells arise from cells that already exist. That last point ended the old belief that life could spring from nonliving material on its own.
Cell theory was built over time from the work of many scientists using early microscopes. It is why biologists say that to understand life, you must understand the cell.
What is the difference between prokaryotic and eukaryotic cells?
The biggest divide among cells is whether they have a nucleus. Prokaryotic cells have no nucleus; their DNA floats freely in the cell. They are small and simple, and bacteria are the classic example. Eukaryotic cells are larger and more complex, keeping their DNA inside a membrane-bound nucleus and containing many specialized parts called organelles. Plants, animals, fungi, and protists are all made of eukaryotic cells.
| Feature | Prokaryotic | Eukaryotic |
|---|---|---|
| Nucleus | No | Yes |
| Size | Small | Larger |
| Organelles | Few | Many |
| Example | Bacteria | Plant and animal cells |
What do all cells share?
Despite their differences, every cell has some common features: a cell membrane that surrounds it and controls what enters and leaves, cytoplasm that fills it, DNA that carries instructions, and ribosomes that build proteins. These shared parts are the minimum equipment for life.
Watch: A Short Video Lesson
Mr Exham Biology walks through this skill clearly in a few minutes. It is a helpful companion to the reading above:
A routine for cell questions
- Recall the three parts of cell theory.
- To classify a cell, ask first whether it has a nucleus.
- No nucleus means prokaryotic (bacteria); a nucleus means eukaryotic.
- Eukaryotic cells are larger and have many organelles.
- Remember the features every cell shares: membrane, cytoplasm, DNA, ribosomes.
Practice questions
- State the three main ideas of cell theory.
- Which cell type has no nucleus?
- Give an example of a prokaryotic organism.
- Name two structures found in all cells.
- Do plant and animal cells have a nucleus?
- True or false: cells can arise from nonliving material on their own.
Answers:
- All living things are made of cells; the cell is the basic unit of life; all cells come from existing cells.
- Prokaryotic.
- Bacteria.
- Any two of: cell membrane, cytoplasm, DNA, ribosomes.
- Yes; they are eukaryotic.
- False. Cell theory says all cells come from existing cells.
Where this fits
Cell theory is the foundation for everything about the cell and its parts, and it explains why life is organized the way it is. From here, the next step is how materials move in and out of a cell through its membrane. All topics live on the ASVAB General Science Learning Hub.
Recommended Prep Books
These study guides and practice books help you keep building momentum as you prepare:
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