The endoplasmic reticulum exists in two forms within a cell. The rough endoplasmic reticulum (RER) gets its “rough” appearance from numerous ribosomes studded across the cytoplasmic surface of its membrane, while the smooth endoplasmic reticulum (SER) lacks these ribosomes and has a smooth-looking membrane.

Because ribosomes are the sites where messenger RNA is translated into polypeptide chains, the RER — being covered in ribosomes — functions as the principal site of protein synthesis in the cell, particularly for proteins destined for secretion, the cell membrane, or other organelles. The SER, by contrast, is mainly involved in lipid synthesis and detoxification, not protein synthesis.

Having circular self-replicating DNA describes mitochondria and chloroplasts, thylakoid membranes with photosynthetic pigments describe chloroplasts, and cristae with oxidative phosphorylation describe mitochondria — none of these are features of the RER, which is specifically distinguished by its ribosome-studded, protein-synthesising membrane.