Which organelle in skeletal muscle fibers stores calcium ions necessary for muscle contraction?

• A. Nucleus
• B. Sarcoplasmic reticulum
• C. Mitochondrion
• D. Golgi apparatus

Answer: (B)

Verified step by step guidance

1. Step 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. Step 2: Identify the organelle responsible for storing calcium ions in skeletal muscle fibers. The sarcoplasmic reticulum is a specialized form of the endoplasmic reticulum found in muscle cells, and its primary function is to store and release calcium ions.
3. Step 3: Compare the functions of the other organelles listed in the problem. The nucleus contains genetic material and regulates cellular activities, the mitochondrion produces ATP for energy, and the Golgi apparatus processes and packages proteins. None of these are directly involved in calcium storage for muscle contraction.
4. Step 4: Recognize that the sarcoplasmic reticulum is uniquely adapted for calcium storage and release. It has specialized channels and pumps, such as the calcium ATPase pump, to regulate calcium ion concentration within the muscle fiber.
5. Step 5: Conclude that the correct answer is the sarcoplasmic reticulum, as it is the organelle responsible for storing calcium ions necessary for muscle contraction.