What is the role of `Ca^(2+)` in muscle contraction ?

Options

A. It binds to tropomyosin, enabling troponin to reveal binding sites for cross bridges
B. It binds to tropoin, enabling tropomyosim to reveal binding sties for cross bridges
C. It binds to tropomyosin, enabling troponin to reveal
D. It binds to troponin, enabling tropomyosin to reveal

Answer

### Step-by-Step Solution:

1. **Introduction to Calcium in Muscle Contraction**: Calcium ions (Ca²⁺) play a crucial role in the process of muscle contraction. They act as signaling molecules that trigger the contraction mechanism in muscle fibers.

2. **Interaction with Proteins**: Calcium interacts primarily with two proteins in muscle fibers: troponin and tropomyosin. These proteins are associated with the actin filaments, which are essential for muscle contraction.

3. **Binding to Troponin**: When a muscle cell is stimulated by a nerve impulse, calcium ions are released from the sarcoplasmic reticulum into the cytoplasm of the muscle cell. The Ca²⁺ binds to troponin, a regulatory protein that is part of the actin filament complex.

4. **Conformational Change**: The binding of Ca²⁺ to troponin causes a conformational change in the troponin-tropomyosin complex. This change moves tropomyosin away from the binding sites on actin filaments.

5. **Exposure of Binding Sites**: Once tropomyosin is moved, the binding sites on actin are exposed. This allows myosin heads (the thick filament) to attach to the actin filaments, forming what are known as cross bridges.

6. **Cross Bridge Cycle**: The formation of cross bridges is essential for muscle contraction. Once the myosin heads attach to actin, they pull the actin filaments inward, which shortens the muscle fiber and leads to contraction.

7. **Role of ATP**: ATP is also necessary for muscle contraction. It provides the energy required for the myosin heads to detach from actin after the power stroke and re-cock for another cycle of contraction.

8. **Conclusion**: In summary, the role of Ca²⁺ in muscle contraction is to bind to troponin, which leads to the exposure of binding sites on actin, allowing for cross bridge formation and subsequent muscle contraction.