Question 1

Which of the following best describes the role of ATP in the sliding filament mechanism of muscle contraction?

Accepted Answer

ATP hydrolysis by myosin provides the free energy for conformational changes that drive the movement of myosin along actin filaments.

Question 2

During muscle contraction, what is the immediate effect of Ca$$^{2+}$$ binding to the troponin complex?

Accepted Answer

It triggers a conformational change in the troponin-tropomyosin complex, exposing myosin-binding sites on actin.

Question 3

Which protein acts as the Ca$$^{2+}$$ sensor in the regulation of muscle contraction?

Accepted Answer

Troponin C (TnC)

Question 4

What is the molecular basis of rigor mortis, the stiffening of muscles after death?

Accepted Answer

ATP is depleted, preventing myosin heads from dissociating from actin, locking the filaments in place.

Question 5

Which of the following best explains why actin filaments do not slide backward during the myosin ATPase cycle?

Accepted Answer

A fraction of myosin heads remain attached to actin at any given time, maintaining tension and preventing backward sliding.

Question 6

Which of the following correctly matches the protein with its function in muscle contraction?

Accepted Answer

Troponin I (TnI): Inhibits myosin-actin interaction

Question 7

A mutation causes the myosin S1 head to lose its ATPase activity. What is the most likely consequence for muscle contraction?

Accepted Answer

Myosin will bind actin but will not be able to release it, leading to a rigor-like state.

Question 8

Which event directly triggers the release of Ca$$^{2+}$$ from the sarcoplasmic reticulum into the cytosol?

Accepted Answer

Arrival of a nerve impulse at the muscle cell membrane

Question 9

Which of the following best describes the 'power stroke' in muscle contraction?

Accepted Answer

The conformational change in myosin after Pi release that moves actin and myosin filaments relative to each other.

Question 10

Which of the following equations correctly represents the hydrolysis of ATP by myosin during muscle contraction?

Accepted Answer

$$ATP + H_2O \rightarrow ADP + $$HPO_4$$^{2-} + H^+$$

Question 11

What is the primary function of the sarcoplasmic reticulum in skeletal muscle cells?

Accepted Answer

To store and release Ca$$^{2+}$$ ions that regulate contraction

Question 12

Which of the following statements about the structure of myosin is correct?

Accepted Answer

Each myosin molecule contains two heavy chains and four light chains.

Question 13

A patient has a defect in the Ca$$^{2+}$$ pump of the sarcoplasmic reticulum. Which of the following would most likely be observed?

Accepted Answer

Muscle relaxation is impaired due to sustained high cytosolic Ca$$^{2+}.$$

Question 14

Which of the following best describes the arrangement of actin and myosin filaments in a sarcomere?

Accepted Answer

Thin actin filaments are anchored at the Z-disk, and thick myosin filaments are centered at the M line.

Question 15

Which of the following is a direct consequence of ATP binding to the myosin head during the contraction cycle?

Accepted Answer

Myosin head detaches from actin.

Question 16

Which of the following best explains the role of tropomyosin in muscle contraction?

Accepted Answer

Tropomyosin blocks myosin binding sites on actin in the absence of Ca$$^{2+}.$$

Question 17

Which of the following best describes the function of the myosin light chains?

Accepted Answer

They stabilize the lever arm of the myosin S1 head.

Question 18

A researcher isolates a muscle fiber and finds that, even in the presence of high ATP, contraction does not occur. Which of the following is the most likely explanation?

Accepted Answer

Ca$$^{2+} is $$not released from the sarcoplasmic reticulum, so troponin C does not bind Ca$$^{2+}$$ and tropomyosin continues to block myosin binding sites.

Question 19

Which of the following best describes the 'bind, pull, and release' mechanism of myosin during muscle contraction?

Accepted Answer

Myosin binds to actin, undergoes a conformational change (power stroke) to pull the actin filament, then releases actin upon ATP binding.

Contractile Proteins and the Molecular Mechanism of Muscle Contraction

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