endoplasmic reticulum An organelle that begins at the nucleus and curves through the cytoplasm. In rough er (with many ribosomes on its cytoplasmic side), many new polypeptide chains acquire specialized side chains Smooth er(with no attached ribosomes) is a site of lipid synthe pl ynt lesSendoplasmic reticulum An organelle that begins at the nucleus and curves through the cytoplasm. In rough ER (with many ribosomes on its cytoplasmic side), many new polypeptide chains acquire specialized side chains. Smooth ER (with no attached ribosomes) is a site of lipid synthesis