Final answer:
The rough ER synthesizes proteins, while the smooth ER is involved in lipid and carbohydrate metabolism and detoxification. Therefore, the correct function of the endoplasmic reticulum is option 4.
Step-by-step explanation:
The endoplasmic reticulum (ER) is a crucial organelle in eukaryotic cells known for its role in processing and sorting proteins. This complex structure is divided into two distinct regions: the rough endoplasmic reticulum (RER), which is associated with protein synthesis due to the presence of ribosomes on its surface, and the smooth endoplasmic reticulum (SER), which is involved in lipid and carbohydrate metabolism, detoxification, and calcium ion storage. Proteins synthesized by the RER are packaged into transport vesicles to be sent to the Golgi apparatus for further modification and sorting for their final destinations within the cell or for secretion.