Which structure within a skeletal muscle fiber stores calcium ions necessary for muscle contraction?

• A. Myofibril
• B. Sarcolemma
• C. Sarcoplasmic reticulum
• D. T-tubule

Answer: (C)

Verified step by step guidance

1. Understand the role of calcium ions in muscle contraction: Calcium ions are essential for initiating the interaction between actin and myosin filaments, which is the basis of muscle contraction.
2. Identify the structure responsible for storing calcium ions: Within a skeletal muscle fiber, the sarcoplasmic reticulum is a specialized organelle that stores and releases calcium ions when needed.
3. Differentiate between the given options: The myofibril is the contractile unit of the muscle, the sarcolemma is the plasma membrane of the muscle fiber, and the T-tubule is involved in transmitting the action potential into the muscle fiber. None of these structures store calcium ions.
4. Focus on the sarcoplasmic reticulum: This organelle surrounds the myofibrils and has a high concentration of calcium ions, which are released into the cytoplasm during muscle contraction.
5. Conclude that the sarcoplasmic reticulum is the correct answer: It is the structure within a skeletal muscle fiber that stores calcium ions necessary for muscle contraction.