Question

13. Alphydrolysis = ADP + P Look at all the myosin binding sites on the actin filament. Does it appear that each site lines up perfectly with a myosin head? a. Myosin can only bind to a binding site that is at the tip of a helix in the actin filament (like the one in Phase 1 of the model). After the power stroke (pulling Phase) and detachment is the myosin head lined up with a binding site at the tip of the filament? b. Can this particular myosin head bind to the actin filament for the next stroke? c. Look at the feet of these children. Are they all able to pull at exactly the same time? d. As a group, explain how myosin pulling on actin works kind of like kids playing tug- of-war.

Answer 1

13. a) The steps of mucsle contraction are :-

- ATP Hydrolysis

The first step in the contraction cycle, the myosin head includes an ATP-binding site and ATPase (a catalytic enzyme which hydrolyzes ATP into ADP + a phosphate group or Pi). This reaction breaks down the high energy bond found between the Phosphate groups of ATP, energizing the myosin head. The ADP and Pi are still attached to the myosin head.

- Attachment of myosin to acting to form cross-bridges

The 2nd step of the contraction cycle, the energized myosin head attached to the myosin-binding site on the actin filament, and releases the previously attached ADP + Pi. The myosin head attached to actin forms a cross-bridge.

- Power stroke

The 3rd step of the contraction cycle, after the cross-bridge forms, the power stroke occurs. The site where the ADP is still bound opens. As a result, the cross-bridge rotates, and the ADP is released. The cross-bridge generates force as it rotates, sliding the thin filament over the thick filament, toward the M line of the sarcomere.

- Detachment of myosin from actin

The final stage of the contraction cycle. After the power stroke, the cross-bridge remains firmly attached to actin until it binds another ATP molecule. As ATP binds to the ATP-binding site on the myosin head, the myosin head detaches from actin. The contraction cycle repeats from the beginning of the contraction cycle steps again as long as ATP is available and Ca2+ ion levels near the thin filament is sufficiently high

b) Yes ,the myosin head binds to the actin filament for next stroke

c) No , these childrens are not able to pull exactly at the same time because there muscle contraction cycle are not occuring at the same time .

d) In tug of war , the children whose muscle contraction starts pulls strongly while those whose muscle contraction cycle is not started are not able to pull efficiently .