
Concept explainers
During the contraction of a vertebrate skeletal muscle fiber, calcium ions
- A. break cross-bridges by acting as a cofactor in the hydrolysis of ATP.
- B. bind with troponin, changing its shape so that the myosin-binding sites on actin are exposed.
- C. transmit action potentials from the motor neuron to the muscle fiber.
- D. spread action potentials through the T tubules.

Introduction:
Skeletal muscles are voluntary muscles that are attached to bones by tendons. They help in the movement of various body parts in relation to each other.
Answer to Problem 1TYU
Correct answer:
During the contraction of a vertebrate skeletal muscle fiber, calcium ions bind with troponin, changing its shape so that the myosin-binding sites on actin are exposed. Therefore, option (B) is correct.
Explanation of Solution
Reasons for the correct statement:
Calcium ions (Ca2+) and actin binding proteins play an important role in muscle contraction and relaxation. Tropomyosin and troponin complex are proteins bound to thin filaments (actin filaments).
In muscle fiber at rest, a regulatory protein called tropomyosin covers the binding site of myosin protein, thereby preventing the interaction of actin and myosin.
The arrival of action potential causes release of calcium ions from the sarcoplasmic reticulum. The calcium ions bind to troponin complex causing the tropomyosin to shift away from the myosin-binding sites, thereby allowing the contraction of muscle.
Option (B) is given as “bind with troponin, changing its shape so that the myosin-binding sites on actin are exposed”.
As, “during the contraction of a vertebrate skeletal muscle fiber, calcium ions bind with troponin, changing its shape so that the myosin-binding sites on actin are exposed”, it is the right answer.
Hence, option (B) is correct.
Reasons for the incorrect statements:
Option (A) is given as “break cross-bridges by acting as a cofactor in the hydrolysis of ATP”.
Calcium ions promote the formation of cross-bridges between actin and myosin, so it is a wrong answer.
Option (C) is given as “transmit action potentials from the motor neuron to the muscle fiber”.
The acetylcholine (neurotransmitter), but not the calcium ions transmits action potential from the motor neuron to the muscle fiber, so it is a wrong answer.
Option (D) is given as “spread action potential through the T tubules”.
The binding of acetylcholine to its receptors on the plasma membrane of a muscle fiber triggers action potential which spread through the T-tubules. The calcium ions do not spread action potential through the T-tubules, so it is a wrong answer.
Hence, options (A), (C), and (D) are incorrect.
Calcium ions play an important role in muscle contraction.
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