Primitive nucleus, true nucleus

The idea

Prokaryotic cells (1–10 µm, usually unicellular) lack a well-defined nucleus and membrane-bound organelles; eukaryotic cells (10–100 µm, unicellular or multicellular) have a well-defined nucleus and several membrane-bound organelles.

Two kinds of cell

A bacterium and a plant cell are both cells, but they are built very differently.

What is the biggest difference you would expect?

• One has a well-defined nucleus and organelles, the other does not
• One has no membrane at all
• They are exactly the same inside

Compare the cells

Tap each cell. Then show them to scale.

Pro- and eu-karyotic

Prokaryotic cells (1–10 µm, usually unicellular) have no well-defined nucleus and no membrane-bound organelles. Eukaryotic cells (10–100 µm, unicellular or multicellular) have a well-defined nucleus and several membrane-bound organelles.

“Pro” means primitive and “eu” means true; “karyon” means nucleus. A bacterial cell is prokaryotic; plant and animal cells are eukaryotic.

• Prokaryotic | Eukaryotic
• Well-defined nucleus | absent | present
• Typical diameter | 1 to 10 µm | 10 to 100 µm
• Organisms | usually unicellular | unicellular or multicellular
• Membrane-bound organelles | absent | present

Notes

Prokaryotic cells (1–10 µm) have no well-defined nucleus or membrane-bound organelles; eukaryotic cells (10–100 µm) have a nucleus and membrane-bound organelles.

Check yourself

A bacterial cell is…

Which cells have membrane-bound organelles?

A typical bacterium is 5 µm across and a typical eukaryotic cell is 50 µm. How many times wider is the eukaryotic cell?

Answer: 10

50 ÷ 5 = 10, so about ten times wider.

• prokaryotic — correct. Yes. It has no well-defined nucleus.
• eukaryotic. Eukaryotic cells have a well-defined nucleus.
• neither. It is one of the two kinds.
• Eukaryotic cells — correct. Yes.
• Prokaryotic cells. They have none.
• Neither. Eukaryotic cells have several.