You're on the right track! The smooth ER (endoplasmic reticulum) is indeed involved in regulating calcium levels in muscle cells, breaking down toxins in liver cells, and synthesizing membrane lipids.

However, the specific term for "making membrane lipids" is lipid biosynthesis.

Here's a breakdown of the smooth ER's functions you mentioned:

* Regulating calcium levels in muscle cells: Smooth ER stores and releases calcium ions (Ca²⁺), which are essential for muscle contraction.

* Breaking down toxins in liver cells: The smooth ER contains enzymes that detoxify harmful substances, such as drugs and alcohol, in the liver.

* Lipid biosynthesis: Smooth ER synthesizes various types of lipids, including phospholipids, cholesterol, and steroid hormones, which are essential components of cell membranes.

So, the complete sentence would be:

"Smooth ER has several functions including regulating calcium levels in muscle cells, breaking down toxins in liver cells, and making membrane lipids called lipid biosynthesis."