Describe the roles of calcium ions and ATP in the contraction of a myofibril (5 marks)

Calcium ions, which diffuse in from the sarcoplasmic reticulum, cause tropomyosin to shift which reveals the myosin binding site located on the actin filament allowing the myosin head to bind. A molecule of ATP attatched to the myosin head is the hydrolysed to ADP and inorganic phosphate. The energy released from the hydrolysis causes the myosin head to bend which pulls the myosin along the actin filament. A new molecule of ATP binds to the myosin head which causes the head to detach from the actin filament