Rough Endoplasmic Reticulum

The rough endoplasmic reticulum (rough ER) is a series of flattened membrane sacs studded with ribosomes, usually located near the nucleus. Ribosomes on the rough ER synthesize proteins that enter the ER lumen, where they are folded and modified (for example, by adding sugar groups). The rough ER also produces membranes (it makes phospholipids and membrane proteins), and it packages proteins into vesicles for transport to the Golgi apparatus.

• The rough ER appears -rough- due to ribosomes attached to its cytoplasmic surface. These ribosomes make proteins destined for secretion, insertion into the cell membrane, or delivery to lysosomes.
• Cells that secrete a lot of protein (like antibody-producing plasma cells or pancreas cells that secrete digestive enzymes) have an extensive rough ER to handle the high protein output.
• Rough ER is continuous with the nuclear envelope (outer membrane of the nucleus). This proximity allows mRNA from the nucleus to be readily translated by the ER-bound ribosomes.

• Organelle identification - If an electron micrograph or description mentions membranes with ribosomes attached, that-s the rough ER (not to be confused with the Golgi, which has no ribosomes).
• Function questions - -Which organelle is directly responsible for the initial modification and folding of secretory proteins-- Correct answer: rough ER (ribosome translates the protein into the ER lumen for folding).
• Comparison trap - Be ready to distinguish rough ER vs smooth ER: e.g., a question might give functions (protein synthesis vs lipid synthesis) or presence/absence of ribosomes and ask which ER type is described.