A network of membrane-bound tubes and sacs that extends throughout the cytoplasm, serving as a highway for transporting materials and a site for protein and lipid synthesis. It exists in two forms: rough ER (with ribosomes) and smooth ER (without ribosomes).
From Greek 'endon' (within), 'plasma' (formed thing), and Latin 'reticulum' (little net). The term reflects its appearance as a net-like structure within the cell's cytoplasm, first described in detail by electron microscopy in the 1940s.
The endoplasmic reticulum is like the cell's postal service and manufacturing district combined! The rough ER makes proteins destined for export, while the smooth ER produces lipids and detoxifies harmful substances - it's why liver cells are packed with smooth ER.
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