The cell theory says the cell is the basic unit of life. Some living things are just one cell. Others are built from trillions.
Key idea
A unicellular organism is one cell that does every job of life: bacteria, amoebas, paramecia and yeast. A multicellular organism is made of many cells that specialize, each kind doing certain jobs: trees, mushrooms, insects and people.
Why it works
A single cell must take in food, get rid of wastes, respond and divide all by itself, and it does. In a multicellular body, cells share the work. A red blood cell carries oxygen, a nerve cell carries signals and a muscle cell contracts. Each depends on the others for everything else.
Worked examples
Worked example: Bigger body, bigger cells?
An elephant is much bigger than a mouse. Are its cells bigger?
- Cells must stay small to move materials in and out quickly.
- Most animal cells are a similar, tiny size.
- The elephant is bigger because it has far more cells.
Answer: No. It has many more cells of about the same size.
Worked example: Who does the job?
Which cell in a plant takes in water from the soil?
- Root hair cells grow long, thin extensions into the soil.
- That shape gives a lot of surface to take in water.
Answer: Root hair cells.
Common mistake
Thinking a single-celled organism is too simple to be alive, or that big organisms have big cells. A paramecium does every job of life on its own. An elephant's cells are about the size of a mouse's; it simply has more.