Which structure is not found in prokaryotic cells?

Prokaryotic cells are characterized by their lack of a defined nucleus. Instead of having a membrane-bound nucleus, the genetic material in prokaryotes is located in a region called the nucleoid, which is not surrounded by a nuclear membrane. This is a fundamental distinction between prokaryotic cells and eukaryotic cells, which do have a nucleus to contain their DNA.

In contrast, prokaryotic cells do contain a plasma membrane, cytoplasm, and ribosomes. The plasma membrane acts as a barrier to regulate the entry and exit of substances, the cytoplasm is the gel-like substance that fills the cell, and ribosomes are essential for protein synthesis, present in both prokaryotes and eukaryotes. Thus, the presence of all these structures, apart from a nucleus, confirms that the defining feature of prokaryotic cells is indeed the absence of a true nucleus.