Final answer:
The rough endoplasmic reticulum (RER) synthesizes and packages proteins for export due to its studded ribosomes, whereas the smooth endoplasmic reticulum (SER) is involved in lipid synthesis and other metabolic processes and cannot synthesize proteins.
Step-by-step explanation:
The key functional differences between the rough endoplasmic reticulum (RER) and the smooth endoplasmic reticulum (SER) are based on their structures and the presence or absence of ribosomes. The RER is called 'rough' because of its ribosomes, which synthesize proteins that are either secreted from the cell, incorporated into the cell membrane, or sent to the lysosomes. In contrast, the SER, which lacks ribosomes, is involved in lipid and steroid hormone synthesis, metabolism of carbohydrates, detoxification of drugs and poisons, and storage of calcium ions, but it is not involved in protein synthesis.
Therefore, the correct difference between the functions of RER and SER is that the RER can synthesize and package proteins for export, while the SER cannot. Instead, the SER has roles in lipid synthesis and handling various other metabolic processes.