Question

Describe the structure of actin and myosin filaments.
(The length Sarcomere is 2.5 μm (. Actin rod = 1 μm) myosin = 1.5 μm 1 Myosin filament & 1 Actin filament is surrounded by 3 Myosin filaments.)

Answer 1

Hint: These are the major components found in the muscle cells and most of the cytoplasm consists of myofibrils which are formed of actin and myosin filament.

Complete answer:
First of all, you should know what actin and myosin are. Myosin and actin are the major components in the muscle cells and these cells are always elongated slender and spindle-shaped fibre like cells due to which they are known as muscle fibres.
Skeletal muscles are the bundles of muscle fibres which are the single large cells formed by the fusion of multi individual cells and most of the cytoplasm consists of myofibrils which are formed of actin and myosin filaments. Actin and myosin are the two types of filaments where myosin is thick filament of 15 nm in diameter and actin is thin filament of 7nm in diameter.
Each microfibril is an organised chain which consists of contractile units which are called sarcomeres and it is responsible for striated appearance of skeleton and cardiac muscle. Sarcomere is approximately 2.3 μm long and the end of each sarcomere is defined by the Z disc and within each sarcomere, there are dark bands alternated with light bands. Dark bands are called A bands because they are anisotropic whereas light bands are called I bands because they are isotropic.
I band contains only thin filaments i.e., actin filaments while A band contains thick filaments i.e., myosin filaments and both these actin and myosin filaments overlap in the peripheral regions of A band while the middle region which is called H band contains only myosin filament.
There are two additional proteins which contribute to sarcomere structure and stability are titin and nebulin where titin is an extremely large protein.

Note: Muscle fibres possess a large number of myofibrils which are formed by actin and myosin filaments. In the striated muscles, there is transverse striation shown where there is alternate light and dark bands are seen. Light bands are called I bands whereas dark bands are called A band. A band contains thick myosin filaments whereas I band contains thin actin filaments.