Final answer:
The sarcomere is the contractile unit of striated muscle that extends from one Z line to the next, essential for muscle contraction and found within the myofibril.
Step-by-step explanation:
The sarcomere, or contractile unit of striated muscle, extends from one Z line to the next. It is the basic functional unit of skeletal and cardiac muscles, vital for muscle contraction. Within the sarcomere, actin and myosin protein filaments slide over one another to produce a shortening of the sarcomere and result in muscle contraction. This structure contributes to the striated appearance of skeletal muscle fibers due to the arrangement of myofilaments in the muscle fiber. The sarcomere is bundled within the myofibril, which is composed of many sarcomeres that collectively contract, leading to the shortening of muscle cells.