Final answer:
ATP is the molecule responsible for the formation and breakage of cross bridges in crossbridge cycling, which is essential for muscle contraction.
Step-by-step explanation:
The molecule that allows cross bridges to be made and broken during the crossbridge cycling is adenosine triphosphate (ATP). During the cycling process, a myosin head attaches to an actin filament while adenosine diphosphate (ADP) and inorganic phosphate (Pi) are bound to it. A power stroke occurs, which moves the actin filament and shortens the sarcomere. ATP then binds to the myosin head, causing it to detach from the actin filament and allowing the muscle to relax. The hydrolysis of ATP to ADP and Pi then provides the energy for the myosin head to return to a cocked position, ready for the next cycle. The crossbridge cycling is crucial for muscle contraction and is influenced by the presence of calcium ions (Ca¹²) which trigger the process.