Final answer:
Actin filaments will glide toward their minus end in the presence of ATP. This is due to the cross-bridge cycle of muscle contraction in which the myosin heads bind to actin and perform a power stroke, moving the actin inwards. ATP is essential for myosin head detachment and continuation of the cycle.
Step-by-step explanation:
If myosin II heads are attached to a glass slide and actin filaments are allowed to bind to them, the filaments will glide on the surface toward their minus end in the presence of ATP. This occurs because of the cross-bridge cycle of muscle contraction, where the myosin heads bind to the actin filament with ADP and phosphate (Pi) bound. Following the power stroke, ADP and Pi are released, which leads to the inwards movement of the actin filament towards the sarcomere center.
The myosin heads initially attach to the actin filament in a state loaded with ADP and Pi. Once myosin binds to actin, it expends the stored energy from ADP and Pi in a conformational change termed the power stroke. This stroke moves the actin filament approximately 10 nm towards the M line, which accounts for the muscle contraction.
In the presence of ATP, the myosin head will detach from the actin allowing for the cycle to continue. In contrast, GTP is not involved in this process, and in the presence of only ADP without ATP, the myosin heads are unable to complete the power stroke and release from the actin filaments.