Biology · Locomotion and Movement · NEET
The thick filament is made of the protein myosin. One thick filament is a polymer of many small units called meromyosins. So the order is: many meromyosins join to form one myosin (thick) filament. NCERT: 'Each myosin (thick) filament is also a polymerised protein. Many monomeric proteins called Meromyosins constitute one thick filament.'
Each meromyosin has two parts. The globular head with a short arm is the heavy meromyosin (HMM). The long tail is the light meromyosin (LMM). The HMM (head) is the working part: it projects outward at a regular distance and angle, it binds actin, and it carries the ATPase site. The LMM (tail) only forms the backbone of the filament. Simple rule: HMM = Head, LMM = Long tail.
A cross-bridge is the physical connection formed when the myosin head binds to the exposed active site on the actin (thin) filament. It looks like a bridge crossing between the thick and thin filaments. NCERT: 'the myosin head now binds to the exposed active sites on actin to form a cross bridge.' The head then pulls the actin, and this is what shortens the sarcomere.
The myosin head has an ATPase enzyme site. It hydrolyses ATP and uses that energy to bind actin and pull it. After the pull, the head releases ADP and Pi. Then a new ATP binds to the head, and this breaks the cross-bridge so the head can detach and repeat the cycle. So ATP is needed both to power the pull and to release the cross-bridge.
The HMM part, meaning the head and the short arm, projects outwards at a regular distance and angle from the rest of the filament. This outward-pointing head is what reaches across to the actin filament to make the cross-bridge. The LMM tail stays inside the filament backbone and does not point out.
Myosin is the THICK filament. Actin is the THIN filament. NCERT states actin filaments are thinner and myosin filaments are thicker, so they are 'commonly called thin and thick filaments respectively'. A quick memory trick: Myosin has more letters and is Massive (thick); Actin is thinner.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
HMM is heavy meromyosin (the globular head with a short arm). LMM is light meromyosin (the long tail). Both are parts of one meromyosin unit of the thick filament.
No. Troponin and tropomyosin belong to the THIN (actin) filament. The thick filament is only myosin (meromyosins). This is a common NEET mix-up.
For NCERT-level NEET, remember the myosin head has an actin-binding site and an ATP-binding (ATPase) site. It binds actin to form the cross-bridge and uses ATP energy to pull.
During the sliding filament theory, the myosin heads of the thick filament form cross-bridges and pull the thin filaments toward the centre of the A band. This shortens the sarcomere and causes contraction. Without the myosin head, no pull and no contraction.
No. In contraction the A band (which holds the thick filaments) stays the same length. The filaments do not shorten; the thin filaments only slide over the thick filaments, so the I band shrinks instead.
