The endoplasmic reticulum (ER) is a complex, membrane-bound
labyrinth of flattened sheets, sacs, and tubules that branches and
spreads throughout the cytoplasm. The ER is continuous from the
nuclear envelope to the plasma membrane and is a
series of channels that helps various materials to circulate
throughout the cytoplasm. It also is a storage unit for enzymes and
other proteins and a point of attachment for ribosomes. ER with
attached ribosomes is rough ER, and ER without
attached ribosomes is smooth ER. Smooth ER is the
site for lipid production, detoxification of a wide variety of organic
molecules, and storage of calcium ions in muscle cells. Most cells
contain both types of ER, although the relative proportion varies
among cells.
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