Final answer:
Proteins synthesized in the RER are transported to the Golgi apparatus, where they are modified and then sorted to their final destinations, such as lysosomes or the cell surface. Proteins destined to become part of organelles or to be secreted from the cell never return to the cytosol after synthesis.
Step-by-step explanation:
Proteins synthesized in the rough endoplasmic reticulum (RER) are packaged into transport vesicles, which then fuse with the Golgi apparatus on its cis face. Inside the Golgi apparatus, proteins undergo further modifications such as glycosylation and phosphorylation. These modifications often include a signal sequence that dictates the final destination of the protein. For instance, the addition of mannose-6-phosphate is a signal for proteins to be directed to lysosomes.
Proteins move through the Golgi from the cis face to the trans face, where they are sorted and packaged into vesicles that will eventually fuse with either the plasma membrane, releasing their contents outside the cell, or with other organelles. This process of sorting and modifying proteins is essential for their proper function and is a keystone in cellular organization and operation.
It is vital to understand that proteins in the RER that are not destined to remain there will not return to the cytosol; they are sequestered from the cytosol from the moment of synthesis. Thus, in response to the student's question, the correct answer is D) They are sorted for degradation in the lysosome, if that is their final determined destination after post-translational modification.