Which cellular structure is absent in prokaryotic cells?

Prokaryotic cells are characterized by a lack of a defined nucleus, and this is primarily due to the absence of a nuclear membrane. In eukaryotic cells, the nuclear membrane encloses the nucleus, which contains the cell's genetic material. In contrast, prokaryotes, such as bacteria, do not have membrane-bound organelles, including a nucleus. Instead, their genetic material is located in a region called the nucleoid, which is not separated from the cytoplasm by a membrane.

The other cellular structures mentioned do exist in prokaryotic cells. Prokaryotic cells possess a cell membrane that serves as a barrier and regulates the movement of substances in and out of the cell. They also contain cytoplasm, which is the gel-like substance within the cell where various cellular processes occur. Additionally, prokaryotic cells contain ribosomes, which are essential for protein synthesis, although they differ slightly in size and composition from those in eukaryotic cells.