smooth endoplasmic reticulum
/ smooth en-doh-PLAZ-mik reh-TIK-yoo-lum /
Right next door to the bumpy, ribosome-studded rough ER lies a stretch of the same membrane network with no ribosomes attached, so it looks smooth and tube-like. This is the smooth endoplasmic reticulum, and despite sharing a name with its rough neighbor, it does a completely different set of jobs — mostly chemistry rather than protein-building.
The smooth ER is a network of membrane tubules continuous with the rough ER but lacking surface ribosomes. Its specialties include making lipids and steroids (such as the building blocks of membranes and steroid hormones), detoxifying harmful chemicals and drugs (especially in liver cells), and storing calcium ions that the cell can release as a signal. In muscle cells a specialized version called the sarcoplasmic reticulum stores the calcium that triggers each contraction.
Because its tasks vary so much by cell type, the smooth ER is abundant in places that match those tasks: the liver (detox and fat handling), the ovaries and testes (steroid hormone production), and muscle (calcium control). A common confusion is to think smooth and rough ER are two different organelles — they are really one continuous system whose local appearance and function depend on whether ribosomes happen to be attached.
When someone drinks alcohol regularly, their liver cells grow more smooth ER to handle the detox work — a measurable, reversible change in cell architecture.
The amount of smooth ER adapts to the chemical workload the cell faces.
"Smooth" refers only to the absence of ribosomes, not to a different membrane; smooth and rough ER are physically one connected organelle.