Answer:
Formation of
cross-bridge between the actin and myosin filament help muscle to contract.
(i) An ATP molecule
joins the active site on myosin head of myosin myofilament. These heads
contains an enzyme, myosin ATPase that along with and ions catalyses the
breakdown of ATP
Energy
(ii)The
energy is transferred to myosin head which energises and straightens to join an
active site on actin myofilament, forming a cross-bridge.
(iii) The
energised cross-bridges move, causing the attached actin filaments to move towards
the centre of A-band. The Z-line is also pulled inwards causing shortening of
sarcomere, i'.e., contraction. It is clear from the above explanation that
during contraction A-bands retain the length, while 1-bands get reduced.
(iv) The
myosin head releases ADP and Pi, relaxes to its low energy state. The head detaches
from actin myofilaments when new ATP molecule joins it and cross-bridge are
broken.
(v) In
repeating cycle, the free head cleaves the new ATR The cycles of cross-bridge formation
and breakage is repeated causing further sliding.
(vi) Muscle
relaxation occurs after contraction when the calcium ions are pumped back to the
sarcoplasmic cisternae, thus, blocking the active sites on actin myofilaments.
The Z-line returns to original position, i.e., relaxation.
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