Which type of endoplasmic reticulum is responsible for the synthesis of proteins?

The endoplasmic reticulum (ER) is a crucial organelle within eukaryotic cells, and it comes in two types: rough and smooth. Rough ER is embedded with ribosomes on its cytoplasmic surface, giving it a "rough" appearance under a microscope. These ribosomes are the sites where protein synthesis occurs. As proteins are synthesized, they are threaded into the interior of the rough ER, where they can undergo folding and modification before being transported to their next destination, such as the Golgi apparatus or other cellular sites.

The presence of ribosomes is what differentiates rough ER from smooth ER, which lacks ribosomes and is predominantly involved in lipid synthesis and metabolic processes rather than protein production. Other terms like skeletal ER or vascular ER do not represent widely recognized structures in cellular biology, further confirming that the rough ER is indeed the correct answer when discussing protein synthesis.