Final answer:
In the absence of ATP, muscles become rigid and unable to relax even if Ca2+ is present. This condition is known as rigor, which occurs as myosin heads cannot detach from actin without ATP. Thus, ATP is vital for both muscle contraction and relaxation.
Step-by-step explanation:
If you were to add Ca2+ to muscle tissue without ATP, the muscle would not be able to contract or relax properly. This is because muscles require both calcium ions (Ca2+) and adenosine triphosphate (ATP) for cross-bridge cycling, which is the process that leads to muscle contraction and relaxation. Without ATP, even though calcium ions may bind to the regulatory proteins (troponin and tropomyosin) allowing for the formation of cross-bridges, the myosin heads would not be able to detach from the actin filaments after the power stroke, leading to a state known as rigor. This is because ATP is responsible for breaking the link between the myosin heads and actin filaments during relaxation.
In the absence of ATP, muscles become rigid and locked in a contracted state, unable to relax, which is why ATP is crucial for muscle contraction and relaxation. Muscles in this state are referred to as being in rigor mortis, which is commonly seen shortly after death when ATP synthesis has ceased. This emphasizes the importance of ATP in muscle function, as without it, muscles lose their ability to undergo successful cross-bridge cycling and cannot return to their relaxed state.