Final answer:
A peripheral membrane protein synthesized in the lumen of the ER will end up on the outside of the plasma membrane due to the orientation maintained during membrane fusion.
Step-by-step explanation:
In the process of protein synthesis and cell membrane assembly, if a peripheral membrane protein is synthesized within the lumen of the endoplasmic reticulum (ER), it is initially positioned on the lumenal side of the ER membrane. As the protein travels through the endomembrane system via vesicles, it maintains the same orientation. Therefore, when a vesicle fuses with the plasma membrane to deliver its contents, a peripheral membrane protein that was synthesized in the lumen of the ER would end up on the outside of the plasma membrane. This spatial orientation is due to the mechanism of membrane fusion, where the outer leaflet of a vesicle's lipid bilayer becomes continuous with the outer leaflet of the plasma membrane.