Advanced Placement (AP) Biology Practice Exam

Which type of endoplasmic reticulum lacks ribosomes?

1. Smooth ER

2. Rough ER

3. Nuclear ER

4. Granular ER

The correct answer is: Smooth ER

The smooth endoplasmic reticulum (ER) is distinguished by the absence of ribosomes on its cytoplasmic surface. Unlike the rough endoplasmic reticulum, which is studded with ribosomes that give it a "rough" appearance and is primarily involved in protein synthesis and processing, the smooth ER is involved in various functions such as lipid synthesis, detoxification of drugs and poisons, and calcium ion storage. The lack of ribosomes is crucial for its specific functions, allowing it to focus on the synthesis of lipids and metabolism of carbohydrates without the interference from ribosomal activity. In contrast, the rough ER, which contains ribosomes, plays a major role in the synthesis of proteins destined for secretion or for use in membranes. The nuclear ER is not widely recognized as a distinctive type of ER and typically refers to the endoplasmic reticulum associated with the nuclear envelope. Granular ER is another term that is sometimes used to describe rough ER because of the ribosomes, which further highlights why smooth ER is the correct choice for this question.