Practice question

Smooth ER forms anastomosing tubules without ribosomes enriched in enzymes for lipid and xenobiotic metabolism. In hepatocytes it contains cytochrome P450 monooxygenases, NADPH-cytochrome P450 reductase, UDP-glucuronosyltransferases and sulfotransferases that hydroxylate, reduce and conjugate lipophilic drugs, environmental pollutants and endogenous steroids to make them water soluble for biliary or renal excretion. Chronic exposure to phenobarbital induces proliferation of smooth ER increasing detoxification capacity, observed by proliferation of membranes in electron micrographs. Additionally it hosts de novo synthesis of cholesterol, phospholipids, ceramides and steroid hormones from cholesterol via StAR-mediated delivery to mitochondria. Calcium ATPase SERCA pumps calcium into lumen, particularly extensive in sarcoplasmic reticulum of muscle enabling release during excitation-contraction coupling. ATP synthesis via oxidative phosphorylation occurs exclusively in mitochondrial cristae, RNA transcription nuclear, histone modification nuclear, so detoxification defines hallmark smooth ER role linking metabolism to protection. Integration with cell cycle kinases, calcium signaling and mechanical cues ensures coordinated remodeling during growth, migration and differentiation.