What organelle distinguishes a eukaryotic cell from a prokaryotic cell? Describe the role this organelle plays in eukaryotic cells.
The nucleus's presence within a cell distinguishes the cell as being eukaryotic, meaning "possessing a true nucleus," versus being prokaryotic, meaning "lacking a true nucleus." Whereas eukaryotic nuclear materials are bounded by a membrane, called the nuclear membrane, prokaryotic nuclear materials are not. The nuclear membrane insures that the interior of the nucleus is isolated from a cell's cytoplasm, allowing two different environments to be maintained. The membrane has two layers that enclose a distinct space in between, and although marked by large pores, the membrane is very selective, permitting only certain substances to enter or leave the nucleus. In addition, the space contained within the double-layered nuclear membrane is continuous at points with the endoplasmic reticulum, a membrane-enclosed structure that pervades the entire cytoplasm and provides a means for nuclear materials to reach all parts of the cell.
