One cell or millions?

The idea

All living organisms are made of cells; unicellular organisms consist of only one cell, while multicellular organisms are made of many cells that work together.

A tiny speck and a giant tree

A bacterium is far too small to see. A banyan tree is huge. Both are alive.

What do you think they share?

• Both are made of cells
• Nothing; they are completely different
• Only the tree is made of cells

Look inside

Choose an organism. Then look inside it to count its cells.

Cells are life’s building block

All living organisms are made of cells. A unicellular organism is just one cell. A multicellular organism is made of many cells that work together.

A single cell can be a complete organism. In a multicellular body, many cells share the work.

• Organism | Cells | Kind
• Bacterium | 1 | unicellular
• Yeast | 1 | unicellular
• Banyan tree | millions | multicellular
• Fish, bird, human | millions | multicellular

Notes

All organisms are made of cells. Unicellular organisms are one cell; multicellular organisms are many cells working together.

Check yourself

Which of these is made of only one cell?

In a multicellular organism the cells…

A friend says, “A bacterium is only one cell, so it is not a real living organism.” Is that right?

• Yeast — correct. Yes. Yeast is unicellular.
• Fish. A fish has millions of cells.
• Bird. A bird has millions of cells.
• Banyan tree. A tree has millions of cells.
• do different jobs and work together — correct. Yes. The cells cooperate.
• live alone and ignore one another. They work as a team.
• all stop working when the organism is big. They keep working together.
• No. One cell is enough to be a living organism — correct. Yes. A unicellular organism does every job of life with one cell.
• Yes, only many-celled things are alive. Unicellular organisms like bacteria and yeast are alive.
• Yes, because it is too small. Size does not decide whether something is alive.