Final answer:
Both eukaryotic and prokaryotic cells contain ribosomes, the sites of protein synthesis, which is a structure not surrounded by a membrane.
Step-by-step explanation:
The structure found in both eukaryotic and prokaryotic cells is the ribosome. Ribosomes are essential for protein synthesis and are not surrounded by a membrane, distinguishing them from other organelles present in eukaryotic cells that are typically encased in a membrane. While other structures like mitochondria, the endoplasmic reticulum, and the Golgi apparatus are exclusive to eukaryotic cells, ribosomes represent a fundamental component shared across all forms of life, indicative of a common origin.
Ribosomes are found in both eukaryotic and prokaryotic cells. Like a prokaryotic cell, a eukaryotic cell has a plasma membrane, cytoplasm, and ribosomes, but a eukaryotic cell is typically larger than a prokaryotic cell, has a true nucleus (meaning its DNA is surrounded by a membrane), and has other membrane-bound organelles that allow for compartmentalization of functions.