Question

The myosin cross-bridge cycle________.

1. requires ATP hydrolysis
2. involves interaction of myosin heads and the actin filament
3. cannot occur if Ca⁺⁺ is present
4. a and b
5. all of the above

Answer 1

During a muscle contraction
Thick (myosin) filaments and thin (actin) filaments slide across one another. Z-bands move closer together causing the sarcomere to shorten.I bands appear narrow.

When a muscle is relaxed, tropmyosin covers the binding sites on actin. During a contraction, Calcium binds to troponin. A molecule of ADP and Phosphate remains attached to myosin from the previous contraction.
Tropomyosin is repositioned, exposing the myosin binding sites on actin filaments. Myosin heads bind to actin filaments. The phosphate is released.

Myosin heads spring forward Power Stroke pulling the actin filaments.
ADP is released from Myosin.
Myosin is released from actin.
A new molecule of ATP binds to myosin, causing it to be released from the actin filament.
ATP is not yet broken down, but it is essential to release the crossbridges.

ATP is broken down, providing the energy to “cock" the myosin filaments (recovery stroke).
The above steps are repeated several times.

The myosin cross-bridge cycle________.

1. requires ATP hydrolysis
2. involves interaction of myosin heads and the actin filament
3. cannot occur if Ca++ is present
4. a and b
5. all of the above

hence the answer is 4 both and b , since calcium is required and the answer cannot be all the above.