Ribosomes in eukaryotes can either be free, which means they are moving freely in the cytoplasm, or bound, which means they are affixed to the endoplasmic reticulum or the outer nuclear envelope.
The huge network of linked, membranous, infolded, and twisted sacks that are found in the cytoplasm of the cell is known as the reticulum (rough ER)
Ribosomes on the surface of rough ER give it a rough appearance.
The ER membrane is directly connected to the ribosome that is generating the protein.
Rough endoplasmic reticulum, often known as rough ER, is a phrase used to describe places where these membrane-bound ribosomes coat the ER surface.
Hence, the structure directly outside the nucleus that has ribosomes bound to it is called endoplasmic reticulum.
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