1. Ribosomes: These are the sites of protein synthesis. They are found in the cytoplasm of all cells, and also in the rough endoplasmic reticulum (RER) of eukaryotic cells. Ribosomes are essentially protein factories, where amino acids are assembled into polypeptide chains according to the instructions encoded in messenger RNA (mRNA).
2. Rough endoplasmic reticulum (RER): This network of membranes in eukaryotic cells is studded with ribosomes. When proteins are synthesized on ribosomes attached to the RER, they often enter the lumen (inner space) of the RER for folding, modification, and transport.
So, in summary, protein synthesis itself happens on ribosomes, but the process can be further enhanced and regulated in the context of the RER in eukaryotic cells.
