Final answer:
Peripheral membrane proteins synthesized in the ER lumen are directed to the outside of the plasma membrane via vesicles that pass through the Golgi apparatus. Proteins meant for the inner surface of the plasma membrane are synthesized by free ribosomes in the cytosol.
Step-by-step explanation:
If a peripheral membrane protein is synthesized in the lumen of the endoplasmic reticulum (ER), it is initially part of the pathway that secretes proteins either to the plasma membrane or outside the cell. Following the secretory pathway, the protein will be modified and packaged into vesicles that fuse with the Golgi apparatus for further processing. As it passes through the Golgi apparatus, additional modifications may occur. Eventually, vesicles containing the protein bud off the Golgi and move towards the plasma membrane.
When these vesicles fuse with the plasma membrane, proteins that were inside the vesicles (and thus originally in the ER lumen) will end up on the outside of the plasma membrane. This mechanism ensures that proteins meant to function outside the cell or on its surface are properly localized. Conversely, peripheral proteins that bind to the inner surface of the plasma membrane are synthesized by free ribosomes in the cytosol and directed to their proper location after translation without entering the ER or Golgi pathway.
These peripheral proteins perform various functions, including enzymatic activities, structural support, and playing a role in cell recognition. Their specific positioning is crucial for these functions, with those on the inner surface often interacting with elements within the cell, such as the cytoskeleton, and those on the outer surface interacting with the cell's external environment.