Question

The contractile unit of muscle fibers is:

• A. Muscle fiber
• B. Sarcomere
• C. Myofibril
• D. Sarcosome

Answer 1

The correct option is B

Sarcomere

The correct option is option B.

Explanation for correct option:

1. The unit of muscle contraction is called a sarcomere.
2. It is formed of $\frac{1}{2}$I-band + one full A-band + another $\frac{1}{2}$I-band.
3. I bands are formed of actin filaments and A bands are formed of myosin filaments.
4. During muscle contraction, the actin filaments from both ends slide between the thick myosin filaments.
5. The sarcomere as well as the whole muscle filament shortens in size.

Explanation for incorrect options:

Option A:

1. A muscle cell is an unbranched and cylindrical fiber.
2. It is 10-100$\mu m$thick and 10-40mm long.
3. It is covered by a thin plasma membrane called sarcolemma.

Option C:

1. It is about 1-3$\mu m$in diameter.
2. It is a combination of actin and myosin filaments.
3. Light bands are composed of actin and dark bands are composed of myosin.
4. The light and dark bands are arranged alternatively throughout the length o the myofibril.

Option D:

1. It is a large specialized mitochondrion.
2. It is found in striated muscle fiber.

Final answer: The contractile unit of muscle fibers is the sarcomere.