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The Cross-Bridge Cycle

How is muscle contraction like active transport? Both muscle contraction and active transport rely on conformational changes in proteins. Active transport results from a change in protein shape that allows other molecules to move across a membrane. Muscle contraction is caused by changes in protein shape that move filaments past one another in opposite directions, causing the muscle to shorten. In both cases ATP molecules are broken down to ADP and inorganic phosphate to provide energy for the conformational changes.

View the animation below, then complete the quiz to test your knowledge of the concept.

1For every cross-bridge formed...
A)the energy from one ATP molecule is required to form the cross-bridge, and the energy from another ATP molecule is required for its detachment.
B)the energy from the same ATP molecule that forms the cross-bridge also detaches it.
C)the energy from the same ATP molecule that detaches the cross-bridge also forms the next one.
D)the energy from an ATP molecule is required for the calcium ion to bind to its receptor site.
E)an ATP molecule is required to attach during the power stroke to provide the necessary energy for movement.

2Which of the following is necessary for cross-bridge binding?
A)calcium binds to the actin filament.
B)calcium binds to the troponin molecule.
C)ATP binds to the actin filament.
D)ATP binds to the myosin head.
E)ATP binds to the myosin filament.

3While the muscle rests...
A)ADP and Pi are released from their attachment sites.
B)ATP binds to its attachment site.
C)ATP is released from its attachment site.
D)the Z-lines move back to their original location before contraction.
E)calcium ions are no longer present at their attachment site.

4The distance from one sarcomere to another is called the Z-line.

5The thin filament consists of actin, troponin, and tropomyosin molecules.

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