Question

What is the correct sequence of four steps of the contraction cycle in muscles?
A. Power stroke
B. Attachment of myosin to actin
C. ATP hydrolysis
D. Detachment of myosin from actin
Choose the most appropriate answer from the options given below:

• A. A, B, C, D
• B. A, C, B, D
• C. C, B, A, D
• D. C, B, D, A

Hint:

Remember the two key roles of ATP in the cycle:

\textbf{Splitting ATP (Hydrolysis)} \(\rightarrow\) Energizes/Cocks the myosin head.
\textbf{Binding of new ATP} \(\rightarrow\) Causes detachment of myosin from actin. Rigor mortis occurs after death because there is no new ATP to cause detachment. \end{itemize}

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The muscle contraction cycle, also known as the cross-bridge cycle, is a sequence of molecular interactions between actin and myosin filaments that drives muscle contraction.

Step 2: Detailed Explanation of the Sequence:
The cycle begins with the myosin head prepared for binding. The key steps are:
1. C. ATP hydrolysis: Myosin possesses an ATP binding site and ATPase activity. ATP is hydrolyzed into ADP and inorganic phosphate (Pi), releasing energy that "cocks" the myosin head into a high-energy state.
2. B. Attachment of myosin to actin: The energized myosin head binds to an available site on the actin filament, forming a cross-bridge.
3. A. Power stroke: The release of Pi from the myosin head initiates the power stroke. The myosin head pivots, pulling the actin filament toward the sarcomere's center (M line). ADP is subsequently released.
4. D. Detachment of myosin from actin: A new ATP molecule binds to the myosin head, causing it to detach from the actin filament. The cycle restarts with the hydrolysis of this new ATP (returning to step C).

Step 3: Final Answer:
The correct order of these events is C \(\rightarrow\) B \(\rightarrow\) A \(\rightarrow\) D, corresponding to option (C).