Question

What two free-floating molecules are absolutely needed to power muscle contraction? (Hint: calcium ions is not the answer)

Answer 1

ADP (Adenosine diphosphate) and Pi (inorganic phosphate group) are the free floating molecules required for muscle contraction.

• The skeletal muscle fibers are made up of two types of myofilaments, namely-thick and thin filament.
• Thick filament is made up of the protein 'myosin' and the thin filament is made up of the protein 'actin'.
• Each myosin filament is made up of units called 'meromyosin'. Each unit consists of a head, arm and a tail.
• Each actin filament consists of two filamentous actin wound helically around each other. Along these filaments, another pair of filaments made up of protein called 'tropomysosin' are wound. At regular intervals, a protein called troponin are found along the filaments.
• When an impulse or action potential reaches the neuromuscular junction, a neurotransmitter called 'acetyl choline' is released.
• This generates an action potential in the sarcolemma (cell membrane of muscle fiber cell) , which spreads throughout the muscle fibre. This results in the release of calcium ions into the sarcoplasm( cytoplasm of muscle fiber cell) .
• The calcium ions now bind to the subunit of troponin attached to tropomysosin. Now, the calcium ion - troponin complex moves away from tropomysosin.This exposes the active site for binding of myosin head.
• ATP hydrolysis occurs and energy is released. The energy is used by the myosin head to bind to the exposed active site and form a cross bridge.
• The cross bridge pulls the actin filaments towards the center of sarcomere (basic unit of muscle fiber), thereby shortening the length of sarcomere.
• Following ATP hydrolysis, the ADP and Pi are then reutilized to form new ATP, which again binds to myosin head and the process continues.