###### Rough endoplasmic reticulum
The rough endoplasmic reticulum is a site of protein synthesis; its
cytosolic surface is studded with ribosomes (Fig. 1.5E). Ribosomes only
bind to the endoplasmic reticulum when proteins targeted for secretion
begin to be synthesized. Most proteins pass through its membranes and
accumulate within its cisternae, although some integral membrane proteins, e.g. plasma membrane receptors, are inserted into the rough
endoplasmic reticulum membrane, where they remain. After passage
from the rough endoplasmic reticulum, proteins remain in membranebound cytoplasmic organelles such as lysosomes, become incorporated
into new plasma membrane, or are secreted by the cell. Some carbohydrates are also synthesized by enzymes within the cavities of the rough
endoplasmic reticulum and may be attached to newly formed protein
(glycosylation). Vesicles are budded off from the rough endoplasmic
reticulum for transport to the Golgi as part of the protein-targeting
mechanism of the cell.