What are the two types of endoplasmic reticulum?

The two types of endoplasmic reticulum are indeed smooth and rough. The smooth endoplasmic reticulum (SER) is characterized by the absence of ribosomes on its surface, which gives it a smooth appearance. This type of ER is involved in the synthesis of lipids, metabolism of carbohydrates, detoxification of drugs and poisons, and storage of calcium ions.

In contrast, the rough endoplasmic reticulum (RER) is studded with ribosomes, making its surface appear rough. This ribosome presence is crucial for protein synthesis, as the RER is primarily responsible for the production of proteins that are either secreted from the cell or incorporated into the cell's membranes.

Thus, the distinction between smooth and rough ER is based on their structure and the specific roles they play in cellular functions, making this answer the most accurate choice.