Select the structral and functional unit of a muscle fibre :

Options

A. Sarcolemma
B. Sarcomere
C. Sarcoplasmic reticulum
D. Myofibril

Answer

To determine the structural and functional unit of a muscle fiber, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Options**: The question provides four options: sarcolemma, sarcomere, sarcoplasmic reticulum, and myofibrils. We need to evaluate each option to identify which one is the structural and functional unit of a muscle fiber.

2. **Evaluate Sarcolemma**: The sarcolemma is the cell membrane of a muscle fiber. While it is an important component of muscle cells, it does not serve as the structural and functional unit of muscle contraction. Therefore, this option is not correct.

**Hint**: Remember that the sarcolemma is just a membrane; it does not perform the primary functions of muscle contraction.

3. **Evaluate Sarcomere**: The sarcomere is defined as the basic structural and functional unit of a muscle. It is the segment of a myofibril that lies between two Z-lines and is responsible for muscle contraction. This makes it a strong candidate for our answer.

**Hint**: Think of the sarcomere as the building block of muscle tissue, similar to how a cell is the building block of life.

4. **Evaluate Sarcoplasmic Reticulum**: The sarcoplasmic reticulum is an organelle that stores calcium ions and plays a role in muscle contraction, but it is not the structural and functional unit itself. It supports muscle function but does not directly perform the contraction.

**Hint**: Consider the role of the sarcoplasmic reticulum as a helper in muscle contraction rather than the main unit.

5. **Evaluate Myofibrils**: Myofibrils are long, rod-like structures made up of many sarcomeres arranged in series. While they are important for muscle contraction, they are not the smallest unit that performs the contraction.

**Hint**: Myofibrils are made up of multiple sarcomeres, so they cannot be the basic unit.

6. **Conclusion**: After evaluating all options, the sarcomere is identified as the structural and functional unit of a muscle fiber. Therefore, the correct answer is option B: Sarcomere.

### Final Answer:
The structural and functional unit of a muscle fiber is the **sarcomere**.