Why doesn't smooth endoplasmic reticulum have ribosomes? And why does rough endoplasmic reticulum?

Question

What do rough endoplasmic reticula have that smooth endoplasmic reticula don't that makes ribosomes appear on it's surface?

Please include sources.

Answer 1

it is a good question. i wrote my honors paper in cell biology about that. It is believed that the high curvature membrane tubules which are characteristic of smooth ER makes it difficult for ribosomes to attach to it while the flat sheets of the rough ER makes it easier for ribosomes to attach to the ER. Now this is still new science, so it is still not proven, but it does make sense. One thing to point out though is that smooth ER still has some ribosomes attached but there are not nearly as many of them on smooth ER.  

Answer 2

The rough ER has receptor pores on its surface, which are where the ribosomes dock in order to translate the elongating polypeptide chain through into the ER lumen. The smooth ER is not involved in protein synthesis (it performs lipid synthesis, and other functions), and does not have these receptor/pores.

As to why the RER has these pores, and the SER does not ... I don't know. It could be the curvature, as suggested by AosM - but I imagine it is probably due to a different phospholipid composition of the membrane itself. Different phospholipids interact with different transmembrane proteins (like the pore/receptors), and their structure also gives the membranes they are in a different morphology (more tubular or more lamellar, etc.).

So a different phospholipid composition would both give the SER and RER a different morphology, and result in different transmembrane proteins being found there.

Another possibility is the interactions of different motor proteins and cytoskeletal components. The different motor proteins and cytoskeletons of the cell are responsible for keeping the various membrane-bound organelles in their respective positions - by channeling different substances to different locations in the cell. If the SER components are all transported to one location, and the RER components (including the ribosomes, and their pore/receptors) to another location, then the two membrane systems will be distinct from each other.