Final answer:
Peripheral membrane proteins synthesized in the ER end up on the outside of the plasma membrane due to the orientation during vesicle transport and fusion with the cell membrane.
Step-by-step explanation:
If a peripheral membrane protein were synthesized inside the lumen of the endoplasmic reticulum (ER), it would be destined to end up on the outside of the plasma membrane. This is due to the process of protein synthesis and transport across the cell membrane. As the protein is synthesized within the ER, it is packaged into a vesicle that eventually fuses with the Golgi apparatus for further modification. As the vesicle progresses through the Golgi and finally to the cell surface, the vesicle merges with the plasma membrane in such a way that the lumenal side of the vesicle becomes continuous with the outside of the cell membrane. Peripheral proteins can be found on either the cytoplasmic side or the outer surface of the membrane, attached either to integral proteins or to phospholipids. When these proteins are synthesized in the ER, the orientation of the proteins is preserved throughout their transport, ensuring that proteins intended for the outer surface end up on the external side of the membrane when the vesicle containing them fuses with the plasma membrane. This preserves their functional orientation, which is crucial for their roles, such as acting as enzymes, as structural attachments for fibres of the cytoskeleton, or as part of the cell's recognition sites.