Final answer:
While both rER and cytoplasmic ribosomes participate in protein synthesis, they are composed of different subunits: the 80S ribosomes found in eukaryotic cells dissociate into 60S and 40S subunits, whereas prokaryotic 70S ribosomes consist of 50S and 30S subunits.
Step-by-step explanation:
The ribosomes in the rough endoplasmic reticulum (rER) and in the cytoplasm, although both essential for protein synthesis, are not composed of the same subunits. To clarify, the ribosomes attached to the rough endoplasmic reticulum in eukaryotic cells are 80S ribosomes, which dissociate into a large 60S subunit and a small 40S subunit. In comparison, prokaryotic cells have 70S ribosomes consisting of a 50S large subunit and a 30S small subunit. Additionally, eukaryotic mitochondria and chloroplasts contain 70S ribosomes similar to those in prokaryotes.
All of these ribosomal units, whether 80S or 70S, are involved in the process of synthesizing proteins by translating mRNA into polypeptide chains; however, despite their similarities in function, they differ in structure, size, and the rRNA and proteins they contain.