A Level Biology Practice Exam

What is a sarcomere?

1. The largest unit of muscle

2. The smallest contractile unit of muscle

3. The connective tissue surrounding muscles

4. The chemical that triggers muscle contraction

The correct answer is: The smallest contractile unit of muscle

A sarcomere is indeed recognized as the smallest contractile unit of muscle tissue. It is a structural unit found within striated muscle fibers, specifically skeletal and cardiac muscles. Sarcomeres are organized in series along the length of myofibrils, which are the contractile threads in muscle cells. Each sarcomere is bordered by Z discs and contains organized arrangements of myofilaments: thick filaments made predominantly of myosin and thin filaments composed mainly of actin. The interaction between these myofilaments facilitates muscle contraction through the sliding filament model, where the sarcomere shortens when the thick and thin filaments slide past one another. The other options focus on different aspects of muscle structure or function but do not define a sarcomere. The largest unit of muscle would refer to the entire muscle or muscle group rather than the sarcomere. Connective tissue surrounding muscles pertains to fascia or epimysium, which encapsulates muscle fibers but is separate from the contractile unit itself. The chemical that triggers muscle contraction, such as calcium ions, relates to the physiological processes involved in contraction but does not define the structure of a sarcomere.