Q.Distinguish between actin and myosin.

Skeletal muscle shortens because two kinds of protein filament interact: the thin filament, built mainly of actin, and the thick filament, built of myosin.

The thin filament is a double helix of two chains of F-actin, each chain itself a string of globular G-actin monomers. Lying along the groove of this helix is the fibrous protein tropomyosin, and at regular intervals along the tropomyosin sits the troponin complex, made of three subunits — troponin T (binds tropomyosin), troponin I (inhibits actin-myosin binding at rest) and troponin C (binds calcium ions). Together, tropomyosin and troponin regulate whether the myosin-binding sites on actin are exposed or masked.

The thick filament is built from many myosin molecules bundled together. Each myosin molecule has a globular head region, together called heavy meromyosin (HMM), which has ATPase activity and can bind actin, and a long rod-like tail, called light meromyosin (LMM), which packs together with other myosin tails to form the shaft of the thick filament. The heads project outward from the shaft at intervals as “cross-arms” or cross-bridges, ready to reach across and grip an adjacent thin filament.

So the key differences are: actin is thinner, found mainly in the I-band, carries no enzymatic activity of its own, and its myosin-binding sites are normally masked by tropomyosin; myosin is thicker, found in the A-band, has ATPase activity in its head, and it is myosin that actively binds actin and generates force, not the reverse.

Not every muscle fibre is built the same way metabolically. Red fibres (slow, oxidative fibres) are rich in the oxygen-storing pigment myoglobin (which gives them their colour) and in mitochondria, are well supplied with blood capillaries, and contract slowly but can sustain activity for long periods without tiring — they are suited to posture and endurance work. White fibres (fast, glycolytic fibres) contain much less myoglobin and fewer mitochondria, are generally thicker, and rely on anaerobic glycolysis for quick bursts of ATP; they contract rapidly and powerfully but fatigue much sooner. Most human skeletal muscles are a mixture of both fibre types in proportions suited to the muscle's job.