What kind of proteins are synthesized by ribosomes attached to the rough endoplasmic reticulum?

Ribosomes attached to the rough endoplasmic reticulum (RER) are primarily responsible for synthesizing proteins that are destined for secretion or for use in the cell membrane. This is because the rough ER is involved in the production and packaging of polypeptides that will be either exported outside of the cell or incorporated into cellular membranes.

Membrane proteins include integral proteins that span the membrane and peripheral proteins that are attached to the surface. Secretory proteins, on the other hand, are synthesized in such a way that they are transported out of the cell, typically for functions like signaling or as enzymes that act outside the cell.

The other options focus on types of proteins that are either typically synthesized in free ribosomes within the cytoplasm or do not specifically align with the roles of the RER. Structural proteins and enzymes may sometimes be synthesized in the RER, but they are not exclusively produced there, as many structural proteins are found in the cytoplasm or other organelles, and different types of enzymes can be synthesized in various locations depending on their function. Transport proteins can also be synthesized in various locations, not specifically tied to the rough ER.

Thus, the correct identification of membrane and secretory proteins as the primary products of ribosomes on