The cross bridge cycle is a series of molecular events that occur after excitation of the sarcolemma. What is a cross bridge? View Available Hint(s) The cross bridge cycle is a series of molecular events that occur after excitation of the sarcolemma. What is a cross bridge? ATP bound to a myosin head Troponin bound to tropomyosin Calcium bound to troponin A myosin head bound to actin

D

Question:

The cross bridge cycle is a series of molecular events that occur after excitation of the sarcolemma. What is a cross bridge? View Available Hint(s) The cross bridge cycle is a series of molecular events that occur after excitation of the sarcolemma. What is a cross bridge?

A. ATP bound to a myosin head

B. Troponin bound to tropomyosin

C. Calcium bound to troponin

D. A myosin head bound to actin

Explanation:

The answer is D

A myosin head bound to actin

During step 2 of the contractile cycle, that is, the hydrolysis of ATP which provide energy for the myosin head to rotate and re-attach to actin

- ATP-binding site on myosin head closes around ATP and hydrolyzes it to ADP + Pi.

- ADP and Pi remain bound to myosin as energy released by ATP hydrolysis rotates the myosin head until it forms a 90 degree angle with the long axis of the filament

- In this position, myosin binds to a new actin that is 1-3 molecules away from where it started

- This cross bridge is weak because tropomyosin is covering the binding site

- Myosin has stored potential energy -- waiting for Ca signal

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