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

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

Answer: (D)

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 leads to muscle contraction.
2. Identify the structure responsible for storing calcium ions: Within a muscle fiber, calcium ions are stored in a specialized organelle called the sarcoplasmic reticulum.
3. Differentiate the sarcoplasmic reticulum from other structures: The sarcoplasmic reticulum is a network of membranous tubules that surrounds the myofibrils, distinct from the sarcolemma (the muscle cell membrane) and T-tubules (which transmit electrical signals).
4. Understand the release mechanism: When a muscle fiber is stimulated, the sarcoplasmic reticulum releases calcium ions into the cytoplasm, triggering the contraction process.
5. Conclude the correct answer: Based on the function and location, the sarcoplasmic reticulum is the structure within a muscle fiber that stores calcium ions necessary for muscle contraction.