Final answer:
The substance that can be added to cause actin and myosin to unbind is ATP, which is crucial for detaching myosin from actin during muscle relaxation.
Step-by-step explanation:
The substance that could be added to the beaker to make actin and myosin unbind is ATP (Adenosine Triphosphate). In the context of muscle contraction, ATP binding causes myosin to release actin, allowing them to detach from each other. This process begins with the binding of ATP to the myosin head, which then undergoes a conformational change to a 'cocked' high-energy position. Subsequent hydrolysis of ATP to ADP and inorganic phosphate (Pi) by the enzyme ATPase prepares myosin for the next cycle of muscle contraction or relaxation.
It is important to note that while calcium ions play a crucial role in allowing the actin-binding sites to be exposed by binding to proteins like troponin and tropomyosin, it is the hydrolysis of ATP that is directly responsible for the detachment of myosin from actin, ultimately causing muscle relaxation once the calcium ions are removed and actin-binding sites are covered again.