Final answer:
The structure not found in prokaryotic cells is the nucleus, as their genetic material is localized in a nucleoid region, not a defined nucleus.
Step-by-step explanation:
The structure that is not found in prokaryotic cells, but is found in eukaryotic cells is the nucleus. While prokaryotic cells do have ribosomes, a cell membrane, and a cell wall, they do not have a defined nucleus. Instead, their genetic material is localized in a region known as the nucleoid. Ribosomes are universal to all cells and are the sites of protein synthesis for both prokaryotic and eukaryotic organisms. It's important to remember that the cell wall, while commonly associated with prokaryotic cells, is also found in some eukaryotic cells, particularly in plants and fungi.