The functional unit of contraction is the protion of the myofibril between :

Options

A. Two I - bands
B. A and I bands
C. Two Z - lines
D. Z and A bands

Answer

### Step-by-Step Solution:

1. **Understanding Myofibrils**: Myofibrils are the basic rod-like units of a muscle cell, and they are composed of repeating sections called sarcomeres.

2. **Identifying the Sarcomere**: The sarcomere is defined as the segment of myofibril that is between two Z-lines (or Z-discs). It is the functional unit of muscle contraction.

3. **Z-lines**: The Z-line is a structure that marks the boundary of each sarcomere. When a muscle contracts, the sarcomeres shorten, pulling the Z-lines closer together.

4. **Options Analysis**: The question provides several options regarding the portion of the myofibril that constitutes the functional unit of contraction:
- A. Between two I-bands
- B. Between A and I-band
- C. Between two Z-lines
- D. Between Z and A-band

5. **Correct Answer**: Among the given options, the correct answer is **C. Between two Z-lines**. This is because the sarcomere, which is the functional unit of contraction, is defined by the space between two Z-lines.

6. **Conclusion**: Therefore, the functional unit of contraction in the myofibril is the segment located between two Z-lines, known as the sarcomere.