endoplasmic reticulum (ER), Membrane system within the cytoplasm of a eukaryotic cell (see eukaryote), important in the synthesis of proteins and lipids. The ER usually makes up more than half the membrane of the cell and is continuous with the outer membrane of the nuclear envelope (see nucleus). There are two distinct regions of ER: the rough ER, or RER (so called because of the protein-synthesizing ribosomes attached to it), and the smooth ER (SER), which is not associated with ribosomes and is involved in the synthesis of lipids and the detoxification of some toxic chemicals.
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