What type of cellular structure is characterized by having a membrane-enclosed nucleus?

Eukaryotic cells are defined by their complex structure, which includes a membrane-enclosed nucleus. This characteristic sets them apart from prokaryotic cells, which do not have a defined nucleus; instead, their genetic material is located in a nucleoid region that is not separated by a membrane. Eukaryotic cells also contain various membrane-bound organelles, such as mitochondria and the endoplasmic reticulum, contributing to their ability to perform specialized functions within multicellular organisms.

This distinction is crucial in biology, as it underlies the organization and complexity seen in higher life forms, including plants, animals, and fungi, which all possess eukaryotic cells. The presence of a nucleus allows for more sophisticated regulation of gene expression and cellular processes compared to their prokaryotic counterparts. Understanding this fundamental aspect of cell biology is key to grasping more complex topics in the field.