Question

question 9
10 points
during the contraction of a vertebrate skeletal muscle fiber, calcium ions _.
a bind with troponin, changing its shape so that the myosin - binding sites on actin are exposed
b break cross - bridges as a cofactor in hydrolysis of atp
c transmit action potentials from the motor neuron to the muscle fiber
d spread action potentials through the t tubules

Explanation:

• Option A: In vertebrate skeletal muscle contraction, calcium ions ($\text{Ca}^{2+}$) bind to troponin. This binding causes a conformational change in troponin, which moves tropomyosin away from the myosin - binding sites on actin. As a result, myosin heads can bind to actin and the contraction cycle (cross - bridge cycling) can proceed.
• Option B: ATP hydrolysis (where ATP is broken down into ADP and inorganic phosphate) is catalyzed by the myosin head's ATPase activity. Calcium ions do not break cross - bridges as a cofactor in ATP hydrolysis. The breaking of cross - bridges occurs when a new ATP molecule binds to the myosin head.
• Option C: Action potentials are transmitted from the motor neuron to the muscle fiber at the neuromuscular junction through the release of neurotransmitters (such as acetylcholine), not by calcium ions.
• Option D: Action potentials spread through the T - tubules (transverse tubules) due to the electrical properties of the muscle cell membrane (depolarization of the sarcolemma). Calcium ions are not responsible for spreading action potentials through the T - tubules.

Answer:

A. bind with troponin, changing its shape so that the myosin - binding sites on actin are exposed