A sarcomere is best described as a

Options

A. (1) movable structural unit within a myofibril bounded by H zones.
B. (2) fixed structural unit within a myofibril bounded By Z lines.
C. (3) fixed structural unit within a myofibril bounded by A bands.
D. (4) movable structural unit within a myofibnl bounded by Z lines.

Answer

**Step-by-Step Solution:**

1. **Definition of Sarcomere**: A sarcomere is a structural unit of a myofibril in striated muscle. It is the basic contractile unit of muscle tissue.

2. **Location**: Sarcomeres are located between two Z-lines (or Z-discs). These lines mark the boundaries of each sarcomere.

3. **Structure**: Each sarcomere contains various proteins, including actin and myosin, which are responsible for muscle contraction.

4. **Movability**: Sarcomeres are not fixed; they can shorten and lengthen during muscle contraction and relaxation. This characteristic makes them a movable structure.

5. **Conclusion**: Therefore, a sarcomere can be best described as a movable structural unit within a myofibril, bounded by Z-lines.

**Final Answer**: A sarcomere is best described as a movable structural unit within a myofibril bounded by Z-lines.

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