Rough endoplasmic reticulum (RER) appears rough under the microscope because of the attachment of ribosomes to its surface. The rough appearance is due to the numerous ribosomes that are bound to the RER. Ribosomes are cell organelles responsible for protein synthesis. They can be found either freely floating in the cytoplasm or attached to the surface of the RER.

The RER is a network of membrane-bound tubules and sacs involved in various functions, including protein synthesis, processing, and sorting. The ribosomes on the RER synthesize proteins that are destined to be secreted from the cell or embedded in the cell membrane. These ribosomes attached to the RER contribute to the rough appearance when viewed under the microscope.

Centrioles, plastids, and lysosomes are not involved in the attachment of ribosomes to the RER and therefore do not contribute to its rough appearance. Centrioles are involved in cell division, plastids are involved in photosynthesis in plant cells, and lysosomes are responsible for the digestion of cellular waste materials.