EUGENE M. MCCARTHY, PHD Google+ Profile
The endoplasmic reticulum (ER) is a large, complex organelle present in eukaryotic cells, the site of lipid and glycoprotein synthesis. The ER lies on the external surface of the nuclear envelope. There are two distinct endoplasmic reticula of which the ER is itself composed, the rough (ribosome studded) ER and the smooth (ribosome-free) ER, respectively abbreviated rER and sER (see the videos on this page to learn about the differences between the rER and sER). Like all other cellular organelles, ERs are membrane-bounded.
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