Biological Systems MCAT Practice Exam 2025 – Complete Study Resource

The Z line, or Z disk, is a key structural component in a sarcomere, which is the basic functional unit of muscle tissue in striated muscles. Its primary role is to anchor the thin filaments, primarily composed of actin, from adjacent sarcomeres. This ensures that as muscle fibers contract, the thin filaments slide over the thick filaments (composed of myosin) in a coordinated and organized manner, allowing for effective muscle contraction.

The Z lines define the borders of each sarcomere and create a repeating pattern that gives skeletal muscle its striated appearance. By connecting the thin filaments across neighboring sarcomeres, the Z line plays an essential role in maintaining the structural integrity and functional connectivity of the muscle fiber during contraction and relaxation cycles.

The other functions mentioned in the options do not accurately describe the role of the Z line. For instance, the connection of thick filaments is not its primary function, as thick filaments are anchored to the M line, while calcium binding sites are located on troponin, which is part of the thin filament structure. Additionally, myosin head attachment occurs at the actin-binding sites during muscle contraction, not directly related to the Z line's function in connecting