In which of the following structures, `Ca2^+` is sequestered when muscles are relaxed?

Options

A. Cell membrane
B. Sarcoplasmic reticulum
C. Mitochondia
D. Golgi body

Answer

To solve the question regarding where `Ca2^+` ions are sequestered when muscles are relaxed, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Muscle Structure**:
- Skeletal muscles consist of muscle fibers, which contain myofibrils made up of sarcomeres. Each sarcomere contains two types of filaments: thin filaments (actin) and thick filaments (myosin).

**Hint**: Remember the basic structure of muscle fibers and the role of actin and myosin.

2. **Role of Calcium Ions in Muscle Contraction**:
- During muscle contraction, calcium ions (`Ca2^+`) play a crucial role. They bind to troponin, a protein associated with actin, which leads to the exposure of binding sites for myosin on actin filaments.

**Hint**: Focus on the function of calcium ions in muscle contraction and their interaction with troponin.

3. **Muscle Relaxation**:
- When the muscle is relaxed, the binding sites on actin are hidden by troponin. The calcium ions are not present in the cytoplasm; instead, they are stored in a specific structure within the muscle cell.

**Hint**: Consider where calcium ions are stored when the muscle is not contracting.

4. **Identify the Storage Structure**:
- The structure responsible for sequestering calcium ions in muscle cells is the sarcoplasmic reticulum. This organelle is a specialized form of the endoplasmic reticulum that stores calcium ions.

**Hint**: Recall the name of the organelle that stores calcium ions in muscle cells.

5. **Conclusion**:
- Therefore, when muscles are relaxed, `Ca2^+` ions are sequestered in the sarcoplasmic reticulum.

### Final Answer:
The correct answer is **sarcoplasmic reticulum** (Option B).