Why do ribosomes associate with rough endoplasmic reticulum but not other membranes?

Question

As far as I am aware, all eukaryotic membranes consist of a lipid bilayer. Smooth endoplasmic reticulum (ER) and rough ER are distinguished by the presence of ribosome on the rough ER surface, but are apparently otherwise similar. So what causes ribosomes to associate with the membrane of rough ER but not that of smooth ER. Or do ribosomes convert smooth ER to rough ER by binding to their membranes?

Answer 1

Hi there! Hopefully I could clear this up for you!

Both the smooth ER and the rough ER carry out similar functions where they help in the production and storage of proteins. The main difference is that the rough endoplasmic reticulum has ribosomes on its surface and the smooth endoplasmic reticulum doesn't. The RER has ribosomes on its surface for protein synthesis and packaging them. The smooth ER (as mentioned before, does not have proteins attached) have a tube-like appearance. They store proteins and lipids.

To summarize, the RER makes and packages proteins whereas the SER stores proteins.

Moreover, there are membrane-bound proteins and free ribosomes.

Membrane-bound proteins are bound to the endoplasmic reticulum and the proteins produced are made to be exported out of the cell. The free ribosomes create proteins for the cell itself.

Answer 2

You are correct in stating that all eukaryotic membranes are made of lipid bilayers, but remember that membranes are much more than just the bilayer. There are many proteins, channels, receptors, sugars, and various lipids associated with each membrane. The membrane that makes up the Rough ER contains receptor molecules that recognize both ribosomes and sequences near the N-terminus of newly synthesized proteins. So it's not that ribosomes can adhere to any lipid bilayer, but rather the ribosomes and proteins that are being synthesized by it are attracted to and bind to receptors on the rough ER membrane.

Hope this helps!