Final answer:
The plasma membrane, inner nuclear membrane, and the ER are continuous primarily due to their shared phospholipid composition, forming a fluid mosaic with embedded proteins that can be synthesized at the rough ER and then transported throughout the cell.
Step-by-step explanation:
The plasma membrane, inner nuclear membrane, and the endoplasmic reticulum (ER) are continuous for a reason related to the underlying structural organization of the cell. The common factor that makes these membranes continuous is the shared phospholipid composition. All these membranes are a fluid mosaic of proteins attached to or embedded in a phospholipid bilayer.
The rough ER is continuous with the nuclear envelope and plays a critical role in protein synthesis. Proteins are produced at the ribosomes found on the surface of the rough ER and may become embedded in its membrane. The segments of the ER membrane can then bud off to form transport vesicles, which ultimately fuse with the Golgi apparatus, carrying the proteins to their next destination.
Similarly, the continuity allows for the integral membrane proteins, some of which are synthesized in the rough ER, to become part of the plasma membrane. Thus, their phospholipid and protein composition is the main reason for the continuity between these structures within the cell.