Eukaryotic cells are found in plants, animals, fungi, and protists. They generally have a nucleus—an organelle surrounded by a membrane called the nuclear envelope—where DNA is stored. There are a few exceptions to this generalization, such as human red blood cells, which don’t have a nucleus when mature. Eukaryotic cells are typically larger than prokaryotic cells, ranging from around 10 to 100 μm in diameter. While many eukaryotes consist of multiple cells, there are also single-celled eukaryotes.