Practice question

Endoplasmic reticulum is continuous reticulum of sheets and tubules emanating from outer nuclear membrane, enclosing single lumen bounded by phospholipid bilayer containing translocon Sec61 for cotranslational import. Rough ER domain densely studded with 80S ribosomes docking via SRP receptor synthesizes secretory and membrane proteins that fold aided by chaperone BiP, protein disulfide isomerase PDI forming disulfides, and undergo initial N-glycosylation by oligosaccharyltransferase. Smooth ER abundant in hepatocytes and steroidogenic cells lacks ribosomes, houses lipid synthesis enzymes including HMG-CoA reductase for cholesterol, phosphatidylcholine synthesis, plus cytochrome P450 family for drug metabolism and calcium pump SERCA storing calcium released via IP3 receptors for signaling. ER exists exclusively in eukaryotes where endomembrane system compartmentalizes; prokaryotes lack internal organelles exporting proteins directly across plasma membrane. Classification as simple single-membrane vesicle like lysosome understates its network complexity, paired cisternae forming double membrane appearance around lumen, contact sites with mitochondria and plasma membrane for lipid exchange. Moreover claim of prokaryotic exclusivity reverses reality.