If the cell runs out of ATP, what would be the state of the sarcomere?
(A) Myosin is bound to actin in the high-energy conformation.
(B) Myosin is alone in the high-energy conformation.
(C) Myosin is bound to actin in the low-energy conformation.
(D) Myosin is alone in the low-energy conformation.

The unit of contraction within skeletal muscle cells is called the sarcomere. A sarcomere contracts when the filamentous protein myosin stretches into a high-energy conformation and binds to the filamentous protein actin. When the myosin returns to its low-energy, relaxed conformation, actin is pulled, and the sarcomere contracts. The following steps relate ATP to each step of this process.
1- Myosin binds to actin (ADP is attached).
2- Myosin returns to low-energy conformation (ADP is released).
3- Myosin releases actin (ATP binds).
4- Myosin stretches to high-energy conformation (ATP is hydrolyzed).