**Question:** Which of the following is not the function of smooth endoplasmic reticulum?

A. Protein synthesis
B. Calcium storage
C. Phospholipid synthesis
D. Lipid synthesis

**Core Concept:** Smooth endoplasmic reticulum (SER) is a specialized type of endoplasmic reticulum (ER) found in eukaryotic cells. SER plays a crucial role in various cellular processes, including protein synthesis, calcium storage, phospholipid synthesis, and lipid synthesis.

**Why the Correct Answer is Right:** Smooth endoplasmic reticulum is involved in the synthesis of proteins, storage of calcium, and synthesis of phospholipids and lipids. These functions are essential for maintaining cellular homeostasis, cellular signaling, and overall cell function.

**Why Each Wrong Option is Incorrect:**

A. Protein synthesis: Smooth endoplasmic reticulum is involved in the synthesis of proteins, so this option is incorrect.
B. Calcium storage: While SER does not exclusively store calcium, it does play a crucial role in calcium regulation and signaling, making this option incorrect.
C. Phospholipid synthesis: Smooth endoplasmic reticulum is involved in phospholipid synthesis, making this option incorrect.
D. Lipid synthesis: Smooth endoplasmic reticulum is involved in lipid synthesis, making this option incorrect.

**Clinical Pearl / High-Yield Fact:** Smooth endoplasmic reticulum and rough endoplasmic reticulum work together to facilitate various cellular processes, including protein synthesis, calcium regulation, phospholipid and lipid synthesis, and calcium storage. Understanding the role of ER in these functions is essential for understanding cellular biology, cellular signaling, and cellular homeostasis.