What characteristic do both prokaryotic and eukaryotic cells share?

Both prokaryotic and eukaryotic cells share the characteristic of having a cell membrane. The cell membrane is a fundamental structure that surrounds the cell, providing a barrier that separates the interior of the cell from the external environment. This semi-permeable membrane is essential for maintaining homeostasis by regulating the passage of substances in and out of the cell, facilitating communication with the surroundings, and supporting cellular integrity.

In contrast, the presence of a nucleus is a characteristic exclusive to eukaryotic cells; prokaryotic cells do not have a nucleus. Similarly, organelles, such as mitochondria and endoplasmic reticulum, are found in eukaryotic cells but not in prokaryotic cells, which typically have a simpler structure without membrane-bound organelles. The cell wall is also a feature that varies; while many prokaryotic cells (like bacteria) have a cell wall, eukaryotic cells may or may not have one, depending on the organism (plants have cell walls, while animal cells do not).

Thus, the cell membrane is the shared characteristic that defines both types of cells.