Explain the roles of ATP and calcium ions in muscle contraction. (4 marks)

The hydrolysis of ATP provides the energy necessary to break the cross-bridges formed between actin and myosin heads. The hydrolysis of ATP is also used to provide the energy for the power stroke, which is when the myosin heads swing forward causing the sarcomere to shorten. 

Calcium ions bind to tropomyosin, this changes its tertiary structure. Consequently, tropomyosin moves away from the actin-myosin binding sites, this allows cross bridges to form. Calcium ions also activate the enzyme ATP hydrolase, this allows ATP to be hydrolysed, which provides the energy for contraction.