In the structure of a skeletal muscle fiber, which organelle stores calcium ions ($C^2$) and releases them to initiate contraction?

• A. Sarcoplasmic reticulum
• B. Transverse (T) tubules
• C. Mitochondria
• D. Golgi apparatus

Answer: (A)

Verified step by step guidance

1. Understand the role of calcium ions (Ca^{2+}) in muscle contraction: Calcium ions are essential for initiating the contraction process in skeletal muscle fibers by interacting with the contractile proteins.
2. Identify the organelles involved in calcium storage and release within the muscle fiber: The main candidates include the sarcoplasmic reticulum, transverse (T) tubules, mitochondria, and Golgi apparatus.
3. Recall the function of the sarcoplasmic reticulum: It is a specialized form of the endoplasmic reticulum in muscle cells that stores calcium ions and releases them when the muscle fiber is stimulated.
4. Understand the role of transverse (T) tubules: These structures help propagate the action potential deep into the muscle fiber but do not store calcium ions themselves.
5. Conclude that the organelle responsible for storing and releasing calcium ions to initiate contraction is the sarcoplasmic reticulum.