What distinguishes rough ER from smooth ER?

Rough endoplasmic reticulum (Rough ER) is characterized primarily by the presence of ribosomes on its cytoplasmic surface, which gives it a "rough" appearance under a microscope. These ribosomes are sites of protein synthesis, allowing the Rough ER to play a crucial role in producing proteins that are either secreted from the cell, incorporated into the cell's plasma membrane, or sent to an organelle. This is in contrast to smooth endoplasmic reticulum (Smooth ER), which does not have ribosomes associated with its surface, resulting in a smoother appearance. Instead, Smooth ER is involved in lipid synthesis, detoxification processes, and calcium ion storage, thus fulfilling different cellular functions.

The other choices relate to aspects that do not specifically distinguish these two types of ER. While Smooth ER is involved in lipid synthesis, that does not apply to the rough ER's defining features. Both types of ER are membrane-bound organelles, so the absence of membranes is not a distinguishing factor. Additionally, photosynthesis occurs in chloroplasts, not in either type of endoplasmic reticulum, making the involvement in photosynthesis irrelevant when comparing the two.