Final answer:
The absence of ATP causes myosin heads to remain attached to actin filaments, leading to a condition of muscle stiffness known as rigor mortis. This rigid state occurs as myosin heads are unable to complete the cross-bridge cycle without ATP to detach from the actin filaments.
Step-by-step explanation:
The phenomenon where myosin heads remain attached to actin filaments due to a lack of ATP is known as rigor mortis. During the cross-bridge cycle, ATP is a crucial molecule that allows myosin heads to detach from the binding sites on actin filaments. When ATP is present, a myosin head will bind to actin, release inorganic phosphate (Pi), and perform a power stroke that pulls the actin filament towards the M-line of the sarcomere. After this, the myosin head will release ADP and bind a new ATP molecule, which results in the detachment of the myosin head from actin, allowing the cycle to repeat.
In the absence of ATP, myosin heads cannot detach, causing continuous binding, which leads to muscle stiffness or rigor mortis in deceased organisms. This state of permanent cross-bridge attachment without ATP is important clinically and has implications for understanding muscle contractility and related pathologies.