Calcium ions bring about muscle contraction through

Options

A. Activation of myosin ATP-ase
B. Exposing active sites of actin filaments
C. Both A and B
D. Mobilisation of food reserve.

Answer

**Step-by-Step Solution:**

1. **Understanding Muscle Structure**: Muscle fibers contain myofibrils, which are made up of repeating units called sarcomeres. Sarcomeres consist of thick filaments (myosin) and thin filaments (actin), arranged in a pattern that creates light and dark bands.

2. **Role of Regulatory Proteins**: Actin filaments are associated with regulatory proteins, specifically troponin and tropomyosin. Troponin is a globular protein composed of three subunits:
- Troponin I (inhibitory)
- Troponin C (calcium binding)
- Troponin T (binds to tropomyosin)

3. **Initiation of Muscle Contraction**: When an action potential is generated in a muscle fiber, calcium ions are released from the sarcoplasmic reticulum, which serves as a storage site for calcium.

4. **Calcium Binding to Troponin**: The released calcium ions bind to troponin C. This binding causes a conformational change in the troponin complex.

5. **Movement of Tropomyosin**: The conformational change in troponin leads to the movement of tropomyosin away from the binding sites on the actin filaments. Normally, tropomyosin blocks these sites, preventing myosin from binding to actin.

6. **Exposing Active Sites**: As tropomyosin moves away, the binding sites on the actin filaments are exposed, allowing myosin heads to attach to actin.

7. **Formation of Cross-bridges**: The binding of myosin to the exposed active sites on actin forms cross-bridges, which is essential for muscle contraction.

8. **Shortening of Sarcomeres**: Once the myosin heads bind to actin, they pull the actin filaments inward, causing the sarcomeres to shorten, which results in muscle contraction.

9. **Conclusion**: Therefore, calcium ions bring about muscle contraction primarily through the exposure of active sites on actin filaments (Option 2).

**Final Answer**: Calcium ions bring about muscle contraction through the exposure of active sites of actin filaments.

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