Our example explains that the events of the crossbridge cycle as they relate to actin and myosin are numbered below. Separately, events as they relate to ATP are labeled ABC, but they are not necessarily in the correct order. We need to match the steps related to actin to the steps related to ATP by matching each letter to the correct numbered step. Note that not all numbered steps correspond to a step related to ATP. So here, numbered 1, 2, 3, 4, we have those steps of the crossbridge cycle, focusing on the interactions between myosin and actin. On the right, labeled ABC, we have what's happening with the chemical energy that's powering these steps, namely the ATP, the ADP, and the inorganic phosphate.
First, we have step 1, where the myosin head binds to the actin. This occurs when the myosin head, already cocked and loaded with chemical energy, encounters exposed actin once the tropomyosin moves out of the way. This binding does not directly correspond to any of the ATP-related steps listed. I'll note this with a dash, indicating no direct correspondence.
Next is the power stroke in step 2, during which the myosin head pulls on the actin, resulting in movement. This corresponds to step C, where the ADP and inorganic phosphate are released from the myosin. While the release of these molecules may not happen simultaneously with the power stroke, they are part of powering that movement. I'll assign letter C to this line for step 2.
In step 3, the myosin head releases from the actin, a process necessary for the repetition of the cycle. This occurs when ATP binds to the myosin head, facilitating its release from actin. Thus, step 3 corresponds to letter A, the binding of ATP to the myosin head.
Finally, in step 4, the myosin head moves into the cocked position. This movement requires the transfer of chemical energy from ATP to the myosin head, which occurs through hydrolysis of the ATP into ADP and inorganic phosphate, denoted as step B. Thus, letter B is assigned to this fourth line.
Understanding these steps and how they are powered by chemical energy, which is then converted into mechanical energy, is crucial. Familiarize yourself with this process, and try more problems to reinforce your understanding.