Eukaryotes are organisms whose cells have a nucleus enclosed within a nuclear envelope. Eukaryotic cells are cells that contain a nucleus and organelles, and are enclosed by a plasma membrane. Eukaryotes are organisms whose cells have a nucleus enclosed within a nuclear envelope. Overview of eukaryotic cells and how they differ from prokaryotic cells (nucleus, organelles, and linear chromosomes). Eukaryote, any cell or organism that possesses a clearly defined nucleus. In addition to the nucleus, eukaryotic cells may contain several other types of organelles, which may include mitochondria, chloroplasts, the endoplasmic reticulum,.