Final answer:
Muscle fibers are primarily made up of sarcomeres, which are the basic contractile units of muscle. The arrangement of these sarcomeres gives muscles their striated appearance, enabling contraction through the interaction of actin and myosin filaments.
Step-by-step explanation:
Muscle fibers are essentially composed of many smaller units called sarcomeres, which are the most basic functional units of a muscle. These sarcomeres are organized within myofibrils. Each skeletal muscle fiber is a bundle of myofibrils, and these myofibrils are composed of the protein filaments actin and myosin. The unique arrangement of sarcomeres within muscle fibers gives skeletal muscle tissue its characteristic striated appearance.
Skeletal muscle fibers are long, multinucleated cells with a plasma membrane known as the sarcolemma and a cytoplasm termed the sarcoplasm. These fibers contain myofibrils that are packaged into orderly units. Sarcomeres within the myofibrils are responsible for muscle contraction, as myosin and actin filaments slide past each other to shorten and generate force. This mechanism occurs in a repeating pattern along the length of the fiber, contributing to their striated pattern.