Final answer:
The free head of the myosin molecule is capable of interacting with thin filaments thanks to its ability to bind to specific sites on actin, which are made accessible during muscle contraction when troponin and tropomyosin react to calcium ions. This binding is a key part of the cross-bridge cycle which drives muscle contraction using energy from ATP.
Step-by-step explanation:
The free head of the myosin molecule is able to interact with the thin filaments because it can bind to specific binding sites on the thin filament, which is primarily composed of the protein actin and supplemented by tropomyosin and troponin. When muscle relaxation occurs, tropomyosin blocks these binding sites to prevent interaction, but during contraction, calcium ions bind to troponin, causing a conformational change that moves tropomyosin away and makes the sites accessible. The myosin head then binds to actin, performing a power stroke that pulls the thin filament past the thick filament by hydrolyzing ATP, which provides the necessary energy for these interactions.
This entire process is part of the cross-bridge cycle, essential for muscle contraction, where the myosin head binds to actin, detaches after the power stroke, and attach again, similar to oars rowing a boat. The flexibility of myosin heads, which can assume different stable conformations, facilitates their attachment-detachment movement required for muscle contraction. This movement, coordinated across millions of sarcomeres in muscle fibers, enables the muscle to contract effectively.