Organisms whose cells each have a membrane-bound nucleus are called eukaryotes.

Here's why:

* Eukaryotes are organisms whose cells have a true nucleus enclosed within a membrane. This nucleus contains the cell's genetic material (DNA).

* Prokaryotes, on the other hand, are organisms whose cells lack a membrane-bound nucleus. Their DNA is located in a region called the nucleoid, but it's not enclosed by a membrane.

Examples of eukaryotes include:

* Animals

* Plants

* Fungi

* Protists

Let me know if you have any other questions!