What does ER (of a cell) stand for?

“ER” stands for endoplasmic reticulum, a membrane network inside eukaryotic cells. Its name describes its location and appearance: “endoplasmic” means within the cytoplasm, while “reticulum” means a small net.

The endoplasmic reticulum connects with the outer membrane of the nucleus and forms flattened sacs and branching tubules. Rough ER is covered with ribosomes, which build proteins that may be secreted, inserted into membranes, or sent to organelles such as the Golgi apparatus.

Smooth ER lacks ribosomes and performs different jobs, including lipid synthesis, steroid production, detoxification, and calcium storage. Cells specialize their ER according to their work: liver cells contain abundant smooth ER, while protein-secreting cells have extensive rough ER.

A common mix-up is treating the ER as one uniform structure. Rough and smooth ER are connected parts of the same organelle, but their ribosomes give them distinct roles. The ER is also not the Golgi apparatus: the ER makes and begins processing many molecules, while the Golgi modifies, sorts, and packages them.