The rough endoplasmic reticulum (RER) is characterized by the presence of ribosomes on its surface, whereas the smooth endoplasmic reticulum (SER) does not exhibit this feature.

The RER and SER are two distinct types of endoplasmic reticulum (ER), forming an extensive network of membranous tubules within the cytoplasm of eukaryotic cells. Both structures are essential for cellular function, but they differ significantly in their architecture and specific roles.

The RER earns its name from its rough texture, which results from the ribosomes attached to its cytoplasmic surface. These ribosomes are responsible for synthesizing proteins, which are subsequently transported into the lumen of the RER. Inside the lumen, these proteins undergo various modifications before being packaged into transport vesicles. These vesicles then ferry the proteins to the Golgi apparatus for additional processing and sorting. The RER is particularly plentiful in cells that are heavily engaged in protein synthesis, such as pancreatic cells.

In contrast, the SER has a smooth appearance due to the absence of ribosomes on its surface. The functions of the SER are diverse and can vary depending on the specific cell type. It plays a critical role in lipid and steroid hormone synthesis, detoxifying harmful substances, and storing calcium ions. For instance, in liver cells, the SER is instrumental in the detoxification of drugs and alcohol. In muscle cells, the SER, often referred to as the sarcoplasmic reticulum, is crucial for storing and releasing calcium ions necessary for muscle contraction.

In summary, both the RER and SER are vital components of the cell’s endomembrane system, yet they possess distinct structures and functions. The RER, with its ribosome-laden surface, is primarily involved in protein synthesis and processing, whereas the SER, lacking ribosomes, is responsible for lipid synthesis, detoxification, and calcium ion storage.