Which structure is present in both prokaryotic and eukaryotic cells?

The cell membrane is a fundamental structure found in both prokaryotic and eukaryotic cells. It serves as a critical boundary that separates the interior of the cell from the external environment. This semi-permeable membrane is composed of a phospholipid bilayer with embedded proteins, allowing selective transport of substances into and out of the cell, thereby playing a vital role in maintaining homeostasis.

In prokaryotic cells, which lack membrane-bound organelles, the cell membrane still performs essential functions for the cell's integrity and nutrient uptake. In eukaryotic cells, the cell membrane not only maintains the cell's shape and protects its contents but also facilitates communication and signaling between cells through receptor molecules.

The other structures listed are not found in prokaryotic cells. The nucleus, mitochondria, and endoplasmic reticulum are all features of eukaryotic cells, reflecting their complexity and compartmentalization, which is not characteristic of prokaryotic organisms.