Which of these statements about the molecular structure of myofilaments is ture?

Options

A. ATPase is found on troponin
B. Tropomoyosin has a binding site for `Ca^(2+)`
C. Troponin bind to the rodike portion of myosin
D. The head of the myosin molecule binds to an active site on actin

Answer

To determine which statements about the molecular structure of myofilaments are true, let's analyze each statement based on our understanding of muscle structure and function.

### Step-by-Step Solution:

1. **Understanding Myofilaments**: Myofilaments are the contractile proteins found in muscle cells. There are two main types: thick filaments (myosin) and thin filaments (actin).

2. **Analyzing the Statements**:
- **Statement 1**: "ATPase is found on troponin."
- **Evaluation**: This statement is incorrect. ATPase is an enzyme that is found in the myosin heads, not on troponin.

- **Statement 2**: "Tropomyosin has a binding site for calcium ions."
- **Evaluation**: This statement is also incorrect. Tropomyosin does not have a binding site for calcium ions; instead, it is troponin that binds calcium ions.

- **Statement 3**: "Troponin binds to the rod-like portion of myosin."
- **Evaluation**: This statement is incorrect. Troponin does not bind to the myosin; it binds to calcium ions and is associated with tropomyosin on the actin filaments.

- **Statement 4**: "The head of the myosin molecule binds to the active site on actin."
- **Evaluation**: This statement is correct. The myosin heads bind to the active sites on actin filaments during muscle contraction, forming cross-bridges.

3. **Conclusion**: The only true statement among the options provided is that "the head of the myosin molecule binds to the active site on actin."

### Final Answer:
The correct statement about the molecular structure of myofilaments is: **The head of the myosin molecule binds to the active site on actin.**

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