Exposure of active sites on thin filaments is due to

Options

A. chloride ions
B. Calcium ions
C. sodium ions
D. magnesium ions

Answer

**Step-by-Step Solution:**

1. **Understanding the Structure of Thin Filaments:**
- Thin filaments in muscles are primarily composed of the protein actin. Actin is organized into two strands (F-actin) that are helically wound around each other.

2. **Role of Tropomyosin and Troponin:**
- Alongside actin, there are other proteins such as tropomyosin and troponin. Tropomyosin wraps around the actin filaments and covers the active sites where myosin can bind.
- Troponin is a complex of proteins that is attached to tropomyosin and helps regulate the interaction between actin and myosin.

3. **Active Sites Masking:**
- The active sites on the actin filaments are initially masked by the tropomyosin due to the presence of troponin. This prevents myosin from binding to actin, which is essential for muscle contraction.

4. **Role of Calcium Ions:**
- When a muscle cell is stimulated, calcium ions (Ca²⁺) are released from the sarcoplasmic reticulum into the cytoplasm.
- The increase in calcium ion concentration leads to calcium binding to the troponin complex.

5. **Exposure of Active Sites:**
- The binding of calcium ions to troponin causes a conformational change in the troponin-tropomyosin complex. This change shifts tropomyosin away from the active sites on actin, exposing them.

6. **Conclusion:**
- The exposure of active sites on thin filaments is primarily due to the binding of calcium ions to troponin, which removes the masking effect of tropomyosin.

**Final Answer:**
The exposure of active sites on thin filaments is due to calcium ions.

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