Which of the following best describes the sarcomere in the context of the sliding filament theory?

• A. The sarcomere is the basic contractile unit of muscle fiber, defined by the area between two Z lines.
• B. The sarcomere is a type of connective tissue that surrounds muscle fibers.
• C. The sarcomere is a protein that binds to calcium ions to initiate muscle contraction.
• D. The sarcomere is a neurotransmitter released at the neuromuscular junction.

Answer: (A)

Verified step by step guidance

1. Understand the context of the sliding filament theory, which explains how muscles contract by the sliding of actin and myosin filaments within the muscle cell.
2. Identify the role of the sarcomere in muscle contraction. The sarcomere is the segment of a myofibril in striated muscle, defined by the area between two Z lines.
3. Recognize that the sarcomere is the basic contractile unit of muscle fiber, where the sliding of actin and myosin filaments occurs, leading to muscle contraction.
4. Differentiate the sarcomere from other components mentioned: it is not a type of connective tissue, a protein that binds calcium, or a neurotransmitter.
5. Conclude that the correct description of the sarcomere in the context of the sliding filament theory is that it is the basic contractile unit of muscle fiber, defined by the area between two Z lines.