Proteins Are Formed by the Granular Endoplasmic Reticulum. The granular portion of
the endoplasmic reticulum is characterized by large numbers of ribosomes attached to the outer
surfaces of the endoplasmic reticulum membrane. As discussed in Chapter 3, protein molecules are
synthesized within the structures of the ribosomes. The ribosomes extrude some of the synthesized
protein molecules directly into the cytosol, but they also extrude many more through the wall of the
endoplasmic reticulum to the interior of the endoplasmic vesicles and tubules, into the endoplasmic matrix.
