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(a) Shortening of muscle is due to the sliding of actin filament over myosin filament. According to sliding filament theory, the cross bridges take energy from ATP molecules and get rotated inward. Thus sarcomeres contract as Z-lines come closer. Due to contraction of sarcomeres finally whole muscle fibre contracts. During relaxation link between cross bridges and actin filaments get broken and cross bridge rotate outward. Thus sarcomeres expand as Z-lines get apart resulting in increase in the length of the mucle fibre.

(b) Functions of skeletal system :

1. It provides mechanical support.

2. It protects the essential organs as heart, brain, spinal cord etc.

3. It plays role in breathing.

4. It helps in locomotion.

5.  It checks entry of food into glottis at the time of swallowing of food.

(c) Red muscle fibres contain myoglobin which stores oxyen as oxymyoglobin and it also has more number of mitochondria. Due to sufficient supply of oxygen little lactic acid is formed. These muscles contract slowly and remain in the contracted state for a long time. TDue to more energy and less lactic acid these muscles do not fatigue in less time. 

White muscle fibres have no myoglobin. So, lactic acid is formed in them. Moreover, their contraction is fast, thus they get fatigued.