Final answer:
The sarcoplasmic reticulum stores calcium ions that trigger muscle contractions and requires integral membrane proteins to actively transport calcium ions into it.
Step-by-step explanation:
In muscle cells, a specialized SER called the sarcoplasmic reticulum stores calcium ions that are needed to trigger the coordinated contractions of the muscle cells.
The sarcoplasmic reticulum must have integral membrane proteins that can actively transport calcium ions into the sarcoplasmic reticulum. This is important for the storage and release of calcium ions during muscle contraction.