What is the difference between prokaryotic and eukaryotic cells?

Prokaryotic cells are the simplest and most ancient types of cells on Earth. The name 'prokaryote' comes from Greek roots meaning 'before the nut' or 'before the nucleus'. They are entirely single-celled organisms, primarily making up the domains Bacteria and Archaea. Because they lack a nucleus and membrane-bound organelles, all of their internal reactions happen together in the cytoplasm. They are also quite small, typically measuring between $0.1$ and $5.0 \mu m$ in diameter.

Eukaryotic cells are much more complex and generally much larger, measuring between $10$ and $100 \mu m$ in diameter. 'Eukaryote' translates to 'true nucleus', which is their defining feature. They house their DNA inside a double-membrane structure called the nucleus. Beyond the nucleus, they also contain many other membrane-bound organelles, such as mitochondria (for energy production) and the endoplasmic reticulum (for building proteins). Plants, animals, fungi, and protists are all made of eukaryotic cells.

Despite their differences, both cell types share four fundamental structures. Every cell has a plasma membrane that acts as an outer boundary. They all contain cytoplasm, the jelly-like substance filling the cell. They both use DNA as their genetic blueprint. Finally, both types have ribosomes, which are tiny factories responsible for building proteins.

A common mistake is thinking that because prokaryotes do not have a nucleus, they do not have DNA. Prokaryotes definitely have DNA! It is simply bunched up in an open region of the cell called the nucleoid, rather than being enclosed in a membrane. Another area of confusion is the cell wall. Students sometimes think only plant cells (eukaryotes) have cell walls. In reality, almost all prokaryotic cells have a cell wall too, though it is made of different chemical compounds than plant cell walls.