Q.Differentiate between the thick and the thin filament of a sarcomere in terms of the proteins present in each and their arrangement.

[!TLDR]

The thick filament is built of myosin (with a projecting ATPase head); the thin filament is built of actin plus the regulatory proteins tropomyosin and troponin.

[!ANSWER]

Thick = myosin only; thin = actin + tropomyosin + troponin.

The thick filament is made purely of many molecules of myosin, each with a rod-shaped tail (which bundles together to form the shaft of the filament) and a globular head that projects outward and has ATPase activity -- it is this head that forms cross-bridges with actin and hydrolyses ATP to power the power stroke. The thin filament, by contrast, is built of three different proteins together: a backbone of F-actin (a double helix of polymerised G-actin monomers), the rod-shaped tropomyosin lying along the actin groove, and the troponin complex (subunits C, I and T) attached at intervals along tropomyosin. So while the thick filament is a single-protein structure, the thin filament is a three-protein assembly in which actin provides the structural backbone while tropomyosin and troponin serve a purely regulatory function, controlling access to actin's myosin-binding sites.

[!ANSWER]

Thick = myosin only; thin = actin + tropomyosin + troponin.

List the protein(s) making up each filament separately, noting that the thick filament has one protein (myosin) while the thin filament has three (actin, tropomyosin, troponin).

Assuming the thin filament is made of actin alone, forgetting that tropomyosin and troponin are also structurally part of it, even though they play a regulatory rather than a load-bearing role.