Final answer:
The endoplasmic reticulum is responsible for the synthesis of lipids and proteins; the rough ER synthesizes proteins, while the smooth ER is involved in lipid synthesis, among other functions.
Step-by-step explanation:
The endoplasmic reticulum (ER) is a critical cellular organelle involved in several important functions related to protein synthesis and lipid metabolism. Recalling the structure of the ER, we have two distinct regions: the rough endoplasmic reticulum (RER) and the smooth endoplasmic reticulum (SER). The RER is notable for its studded ribosomes on the surface, which are engaged in the synthesis of proteins, often those that are destined to be part of the cell membrane or secreted out of the cell. These proteins are collected in transport vesicles which then carry them to their next destination, such as the Golgi apparatus for further modification.
In contrast, the SER, with its smooth appearance due to the absence of ribosomes, fulfills roles like lipid and carbohydrate synthesis, detoxification of medications and poisons, and storing calcium ions. Given this information, the function of the ER that matches the student's question is the synthesis of lipids and proteins.