Q.Write the difference between:

(a) Actin vs Myosin

• Actin forms the thin filament; myosin forms the thick filament.
• On actin the myosin-binding sites are normally masked by troponin; the myosin head carries ATPase, binds ATP and grips actin.

(b) Red vs White muscles

• Red fibres are rich in myoglobin (reddish) with many mitochondria and work aerobically; white fibres have little myoglobin (pale), few mitochondria but abundant sarcoplasmic reticulum and work anaerobically.

(c) Pectoral vs Pelvic girdle

• The pectoral girdle attaches the forelimbs; each half is a clavicle and a scapula, with a glenoid cavity for the humerus. …

Each pair below is set out as a difference between two related structures.

(a) Actin and Myosin. These are the two contractile proteins of muscle, but they differ in build and role.

• Actin is the thinner of the two proteins and forms the thin filament; myosin is stouter and forms the thick filament.
• The thin filament is a pair of F-actin strands (each a polymer of G-actin) wound together, alongside tropomyosin and troponin; the thick filament is built of many meromyosin units.
• On the resting actin filament the sites where myosin would attach are masked by a subunit of troponin. Myosin, by contrast, has a globular head that acts as an ATPase, carries binding sites for ATP, and has active sites that fit onto actin — so it is myosin that reaches across and pulls.

(b) Red and White muscles. These two kinds of muscle fibre differ mainly in how much myoglobin they carry and how they obtain energy.

• Red fibres have a high content of myoglobin, the red oxygen-storing pigment, which gives them a reddish colour; white fibres have very little myoglobin and so look pale or whitish.
• Red fibres contain plenty of mitochondria that use the stored oxygen to make ATP, so they are aerobic muscles; white fibres have few mitochondria but a high amount of sarcoplasmic reticulum, and they depend on an anaerobic process for energy.

(c) Pectoral and Pelvic girdle. Both girdles attach limbs to the axial skeleton, and each is made of two halves, but they serve different limbs and are built differently.

• The pectoral girdle attaches the forelimbs (upper limbs) to the axial skeleton; the pelvic girdle attaches the hind limbs (lower limbs). …