Scleroprotein comprising a peptide chain made up of some 1050 amino acids. Three of these chains are intertwined in the form of a triple helix. When these helixes are further intertwined, collagen fibrils are produced; these are then stabilized by cross-linking them into a three-dimensional network. This structure renders collagen insoluble in water

Contractile proteins are proteins that mediate sliding of contractile fibres (contraction) of a cell’s cytoskeleton, and of cardiac and skeletal muscle. Heart and muscle contractile fibres consist of bundles of actin polymers that slide alongside each other by the activity of the motor protein myosin and associated contractile proteins such as troponin and titin.

Following are the correct statements about the mechanism of muscle contraction: 
(i) Acetylcholine is released when the neural signal reaches the motor end plate.
(ii) Repeated activation of the muscles can lead to lactic acid accumulation.
So, the correct answer is (B).

The region between two Z lines is called a sarcomere; sarcomeres can be considered the primary structural and functional unit of muscle tissue. Ultrastructure of a group of myofibrils, showing the sarcoplasmic reticulum and transverse tubules, which constitute the two membrane systems within a muscle fibre.

The ‘A’ and ‘I’ bands are arranged alternately throughout the length of the myofibrils. The light bands contain actin(thin filament) and are called I-band or Isotropic band, whereas the dark band called ‘A’ or Anisotropic band contains myosin(thick filament). In the centre of each ‘I’ band is an elastic fibre called ‘Z’ line which bisects it. The thin filaments are firmly attached to the ‘Z’ line. The portion of the myofibril between two successive ‘Z’ lines is considered as the functional unit of contraction and is called a sarcomere 
Utilizing the energy from ATP hydrolysis, the myosin head now binds to the exposed active sites on actin to form a cross bridge. 
This pulls the attached actin filaments towards the centre of ‘A’ band.
The ‘Z’ line attached to these actins are also pulled inwards thereby causing a shortening of the sarcomere, i.e., contraction. Hence the ‘I’ bands get reduced, whereas the ‘A’ bands retain the length.
 The central part of thick filament, not overlapped by thick filament is the H-zone.

Cardiac muscle (also called heart muscle or myocardium) is one of three types of vertebrate muscles, with the other two being skeletal and smooth muscles. It is an involuntary, striated muscle that constitutes the main tissue of the walls of the heart.

In some plant cells there is a rapid rotatory cytoplasmic movement, which is limited to the peripheral parts of the cell next to the cell wall chloroplasts and granules move in this stream. This movement may be increased by light

During muscular contraction myosin cross bridges heads. Myosin is not only an actin binding protein, it is also an ATPase (an enzyme) which is activated by Ca²⁺ and Mg²⁺ ions. Hence, ATPase enzyme is located in myosin.

Adenosine Triphosphate (ATP) is the immediate source of (chemical) energy for muscle contraction. Very little ATP is stored in muscle fibres (= 'muscle cells'), only enough to power muscle contractions for a few seconds. The ATP in muscles must be constantly replenished as it is used for various processes.

Smooth muscles are plain, non- striated, involuntary or unstriped muscles due to the absence of striations. These occur in the walls of hollow internal organs, in capsules of lymph glands, spleen etc, in iris and ciliary body of eyes, skin dermis, penis and other accessory genitalia etc.