Sandpaper like structure dotted with ribosomes concerned with protein synthesis?

The structure you're describing is the rough endoplasmic reticulum (RER). It is characterized by its sandpaper-like appearance due to the numerous ribosomes attached to its surface, which play a crucial role in synthesizing proteins. These proteins are typically destined for secretion, incorporation into cellular membranes, or for use within lysosomes. The RER is essential for the proper folding and post-translational modification of these proteins.