Which structure is NOT found in prokaryotic cells?

Prokaryotic cells are characterized by the absence of a membrane-bound nucleus. Instead, their genetic material, which is typically a single circular chromosome, is located in a region called the nucleoid. The presence of a nucleus is a defining feature of eukaryotic cells, which have a more complex structure, including organelles surrounded by membranes.

In contrast, prokaryotic cells do possess a cell wall, cytoplasm, and cell membrane. The cell wall is a critical structure that provides shape and protection, often made up of peptidoglycan in bacteria. Cytoplasm is the gel-like substance within the cell membrane where various cellular processes occur, and the cell membrane acts as a barrier that regulates what enters and leaves the cell, playing a vital role in maintaining homeostasis. Hence, the nucleus is the correct answer as it is not found in prokaryotic cells.