During muscle relaxation, the ______ covers the active site on actin.

Correct option is D

Ans: (d) Tropomyosin Sol. During muscle relaxation, tropomyosin covers the active sites on the actin filaments, preventing the binding of myosin heads, and thus inhibiting contraction. Tropomyosin is a regulatory protein that, along with troponin, controls the interaction between actin and myosin, which is crucial for muscle contraction.
Explanation of each option:
· (a) Myosin: Myosin is the thick filament of the muscle, which interacts with actin during contraction. However, it does not cover the active site on actin during relaxation. Instead, it binds to the active site during contraction to produce movement.
· (b) Titin: Titin is a structural protein that helps maintain the alignment of the thick and thin filaments and provides elasticity to the muscle. It does not cover the active site on actin; rather, it stabilizes the sarcomere.
· (c) Troponin: Troponin is a complex of proteins (troponin C, I, and T) that work with tropomyosin to regulate muscle contraction. While troponin is involved in the movement of tropomyosin in response to calcium ions, it itself does not cover the active site on actin. Troponin binds to calcium ions and causes tropomyosin to shift, exposing the active sites on actin for myosin binding during contraction.