What is the function of the rough endoplasmic reticulum?

• A. To store calcium ions
• B. To synthesize and process lipids
• C. To synthesize and process proteins
• D. To break down toxins

Answer: (C)

The rough endoplasmic reticulum (RER) is primarily involved in the synthesis and processing of proteins. This organelle is studded with ribosomes on its cytoplasmic surface, which gives it a "rough" appearance. These ribosomes are the sites of protein synthesis, where messenger RNA (mRNA) is translated into polypeptide chains.

As proteins are synthesized on the ribosomes, they are translocated into the lumen of the rough endoplasmic reticulum, where they undergo folding and post-translational modifications, such as glycosylation, which is the addition of carbohydrate groups. This is crucial for the proper functioning of many proteins, as their structure often determines their activity and interactions. After processing in the rough endoplasmic reticulum, proteins are packaged into vesicles for transport to their next destination, which could be the Golgi apparatus for further modification or secretion outside the cell.

In contrast, the other functions listed are not associated with the rough endoplasmic reticulum. Storing calcium ions is primarily the function of the smooth endoplasmic reticulum, which also plays a role in lipid synthesis rather than protein synthesis. Breaking down toxins is typically carried out by the liver and various organelles like per