Which structure can be found in both types of cells but is not membrane-bound?

Ribosomes are non-membrane-bound structures found in both prokaryotic and eukaryotic cells. They serve as the sites of protein synthesis, translating messenger RNA (mRNA) into polypeptide chains. Ribosomes can be found freely floating in the cytoplasm or attached to the endoplasmic reticulum in eukaryotic cells, whereas prokaryotic cells have them dispersed throughout the cytoplasm. Their presence in both cell types, along with the fact that they do not have a surrounding membrane, makes ribosomes distinct in this context.

In contrast, structures like mitochondria and the nucleus are membrane-bound organelles specific to eukaryotic cells, while capsules are protective layers commonly associated with prokaryotic cells but not found in eukaryotic cells. Thus, the unique characteristic of ribosomes being present in both types of cells and lacking a membrane confirms their correct identification in this question.