Answer;
-In the smooth ER
In certain kinds of muscle cells, calcium ions are stored in the smooth ER.
Step-by-step explanation;
-The smooth endoplasmic reticulum is the part of the endoplasmic reticulum (ER) which does not have ribosomes bound to it, thus not exhibiting the bumpy appearance shown by the rough endoplasmic reticulum.
-The smooth ER is important in the synthesis of lipids, such as cholesterol and phospholipids, which form all the membranes of the organism. In addition it is important for the synthesis and secretion of steroid hormones from cholesterol and other lipid precursors.