The ER lumen is the internal aqueous compartment of the endoplasmic reticulum. A ribosome attached to its membrane sits on the cytosol side, not inside the ER lumen. This distinction separates the machinery making a protein from the compartment receiving it.[ref]
The Endoplasmic Reticulum. Molecular Biology of the Cell, 4th edition by Bruce Alberts and others; Membrane-bound Ribosomes Define the Rough ER, Figure 12-36: cytosolic ribosomes and lumenal delivery
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