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

• A. Calcium storage, protein synthesis and lipid metabolism
• B. Cellular respiration
• C. Genetic information storage
• D. Protein folding only

Answer: (A)

The endoplasmic reticulum performs multiple important tasks that keep cells functioning, including handling proteins, lipids, and calcium signals. The rough region, studded with ribosomes, is where many proteins destined for secretion, for the cell membrane, or for certain organelles are synthesized and threaded into the ER lumen to be folded and processed. The smooth region focuses on making lipids and participates in detoxification processes. In addition, the ER lumen stores calcium ions, which are released to help regulate various cellular signals, including muscle contraction in muscle cells. Putting these roles together, calcium storage, protein synthesis, and lipid metabolism together describe the full range of the ER’s functions.

Other options miss important parts: cellular respiration takes place mainly in mitochondria; genetic information storage is in the nucleus (and mitochondria); and while protein folding occurs in the ER, it isn’t the only function, so saying “protein folding only” is too narrow.