Final answer:
The smooth endoplasmic reticulum (SER) sequesters Ca2+ from the cytosol in muscle cells.
Step-by-step explanation:
The organelle in muscle cells that sequesters Ca2+ from the cytosol is the smooth endoplasmic reticulum (SER). The SER is a region of the endoplasmic reticulum that does not have ribosomes attached to it. It performs various functions including lipid synthesis, detoxification of certain chemicals, and storage of calcium ions.