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The Endoplasmic Reticulum

The endoplasmic reticulum (ER) is a network of interconnecting tubes that begins at the nuclear envelope and runs out into the cytoplasm. Two types exist in the cell:
  • Rough ER: Ribosomes are attached to the cytoplasmic side of the ER. The rough ER often looks as though it is “stacked” in the cell. Polypeptides assembling on the ribosomes will be threaded into the ER if they contain a specific sequence of amino acids called a signal sequence. Once inside the ER, proteins become modified through the addition of oligosaccharides. Rough ER is very abundant in cells that secrete large amounts of proteins.
  • Smooth ER: Unlike the rough ER, the smooth ER is not studded with ribosomes. This organelle plays a role in lipid synthesis.

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