Which of the following is a function of the endoplasmic reticulum?

The endoplasmic reticulum (ER) plays a crucial role in the synthesis and transport of proteins within the cell. It is a network of membranous tubules and sacs that is involved in the production of proteins and lipids. The rough endoplasmic reticulum, which is studded with ribosomes, is specifically responsible for synthesizing proteins that are either secreted from the cell, incorporated into the cell's plasma membrane, or sent to an organelle. After synthesis, these proteins are often transported to other parts of the cell or outside of it through vesicles.

The smooth endoplasmic reticulum, on the other hand, is involved in lipid synthesis and detoxification processes, but it is not primarily focused on protein synthesis. This specialization of the ER highlights its essential functions in maintaining cellular structure and facilitating communication through protein transport. Other processes like photosynthesis, energy production, and cell division are carried out by different organelles or systems within the cell, thus not being functions of the endoplasmic reticulum.