Question

Arrange the order of events in the cross-bridge cycle:
(i) The myosin head pulls the actin filament towards the M-line.
(ii) The myosin head releases ADP and phosphate.
(iii) The myosin head hydrolyses ATP.
(iv) The myosin head binds actin.

• A. i, ii, iii, iv
• B. i, iii, ii, iv
• C. ii, iii, i, iv
• D. iii, iv, i, ii

Answer 1

The correct option is D iii, iv, i, ii

The cross bridge cycle is the sequence of events in the thick and thin filaments of a muscle that result in its contraction. It starts with the myosin head binding and hydrolysing ATP into ADP and phosphate. The myosin head then binds the actin filament, and using the energy of ATP hydrolysis, it pulls the actin filament towards the M-line, i.e., the centre of the sarcomere. Next, the myosin head releases the bound ADP and phosphate. Then, it binds a fresh ATP molecule and the cycle continues, as long as there are abundant calcium ions in the sarcoplasm.