Question

Which of the following indicates that a muscle fiber is relaxed?

ATP molecules are bound to the heads of the myosin filaments.

Myosin heads are attached to actin filaments, forming cross bridges.

Calcium ions are bound to troponin, causing the protein to change shape.

Z lines are pulled toward the center of the sarcomere.

Answer 1

ATP molecules are bound to the heads of the myosin filaments

In relaxed form of the sarcomere, ATP is bound to its site on the myosin heads. It is when the ATP is hydrolyzed to ADP and Pi that the mysoin forms a cross ridge with the actin fiber. Upon hydrolysis the ADP + Pi exits the site and another ATP binds. This again causes the myosin head to disengage the actin fiber and the cycle is repeated.

Explanation:

During contraction of muscle calcium ions are usually released from their sarcoplasmic reticulum where they bind to troponin enabling tropomyosin to be exposed such that myosin can bind. Contraction of sarcomere causes the z lines to move closer together.

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