Which type of endoplasmic reticulum is involved in protein synthesis?

The rough endoplasmic reticulum (RER) is directly involved in protein synthesis due to the presence of ribosomes on its cytoplasmic surface. These ribosomes are essential for translating messenger RNA (mRNA) into polypeptide chains, which then enter the RER for further folding and modification. The rough appearance is attributed to these ribosomes, which are vital for the synthesis of proteins that are either secreted from the cell, incorporated into the cell’s plasma membrane, or sent to an organelle.

Ribosomes attached to the RER translate mRNA into specific proteins, allowing for the proper post-translational modifications to occur within the endoplasmic reticulum. This complex process ensures that proteins are correctly folded and modified before they are transported to their final destinations within or outside the cell.

The other options do not play a direct role in protein synthesis. The smooth endoplasmic reticulum lacks ribosomes and is primarily involved in lipid synthesis and detoxification processes. Glycocalyx refers to a carbohydrate-rich structure on the cell's surface, which is important for cell recognition and signaling rather than protein synthesis. The cell membrane itself serves as a barrier and a site for various cell signaling processes but does not synthesize proteins.