The organelle responsible for Ca storage in muscle cells is the sarcoplasmic reticulum (SR).

Here's why:

* Calcium's role in muscle contraction: Calcium ions (Ca²⁺) play a crucial role in muscle contraction. When a muscle receives a signal to contract, Ca²⁺ is released from the SR, which triggers the interaction between actin and myosin filaments, leading to muscle shortening.

* SR structure and function: The SR is a network of interconnected membrane-bound sacs that encircles each muscle fiber. It acts like a storage compartment for Ca²⁺. When a nerve impulse arrives at a muscle fiber, it triggers the release of Ca²⁺ from the SR into the cytoplasm.

* Calcium re-uptake: After contraction, the SR actively pumps Ca²⁺ back into its lumen, restoring the resting state and preparing the muscle fiber for the next contraction.

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