Which of the following statements best describe(s) the fate(s) of ATP in the mus

Question

Which of the following statements best describe(s) the fate(s) of ATP in the muscle cell during exercise?

A. ATP is used to run the electron transport chain in order to produce more ATP for muscle contraction. B. ATP is hydrolyzed by myosin heads and liberated energy is used to generate the power stroke. C. ATP is either hydrolyzed by myosin heads and liberated energy is used to generate the power stroke or ATP is used by the calcium-ATPase pumps on the sarcoplasmic reticulum of skeletal muscle D. ATP is used by the calcium-ATPase pumps on the sarcoplasmic reticulum of skeletal muscle. E. All of the above are possible fates of ATP. F. None of the above are possible fates of ATP.

Explanation / Answer

Which of the following statements best describe(s) the fate(s) of ATP in the muscle cell during exercise?

The correct option is C. ATP is either hydrolyzed by myosin heads and liberated energy is used to generate the power stroke or ATP is used by the calcium-ATPase pumps on the sarcoplasmic reticulum of skeletal muscle.

Note: When someone is exercising, his/her muscles are contracting and relaxing continuously to compensate his/her actions. High energy is required continuously and the excess energy is supplied from blood glucose burning and generation of ATP.

This ATP is mainly used to contract the muscle in a process called 'Cross Bridge Cycling'. In this cycle the sarcomere is contracted as sliding filament model.

Sarcomere is made of actin myofilaments, myosin myofilaments and other accessory proteins. Myosin heads can bind to adjacent actin filament when ADP is bound to them. The contraction cycle start when an ATP replaces bound ADP on myosin head making the myosin to detach from actin. An enzyme called ATPase then catalyzes the ATP hydrolyzation, breaking the bound ATP down to ADP and inorganic phosphate. The energy released during this hydrolization, changes the myosin filaments angle, so it can extend to a cocked position. Myosin finds a new open site of actin to bind in its high energy state (bound ADP and Pi). When a calcium ion binds to a protein complex called troponin, it exposes a binding site for myosin on actin. The myosin quickly binds to the open site as soon as it is released and forms a cross-bridge. Once the myosin binds, the Pi is released, making a conformational change in myosin head, producing a less energetic state. During this conformational change myosin moves through a ‘power stroke’, pulling the actin filament towards the M-line and making a contraction. Then again the bound myosin returns to low energy state with ADP still attached to its head. Again when a ATP replaces ADP, it breaks the cross bridge. Myosin then returns to its original cocked position by a release stroke. This occurs continuously as the muscle is not contracted to desired position.

Muscle once contracted also needs to be released. For that the sarcoplasmic calcium are withdrawn by sarcoplasmic reticulum, the calcium storage of muscle. For that Ca++gates have to open and the calcium needs to be transported inside sarcoplasmic reticulum against a concentration gradient. So an active transport is required. For that an ATPase helps calcium transporter to take up the calciums back as ATP is hydrolized.

So this excitation and contraction coupling arises as neuronal stimulation make release calcium inside the sarcoma, which start the cycle.

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