BIOLOGY 251 Lecture Exam 3 | Full Questions, Correct Answers, and Rationales
| 2026/27 Update | 100% Correct – WSU.
1. In smooth-muscle cells, calcium entering the cell first binds to ______; this
complex then activates myosin light-chain kinase to initiate contraction.
A. Troponin
B. Tropomyosin
C. Titin
D. Actin
E. Calmodulin
Correct answer: E. Calmodulin
Rationale: Smooth muscle does not use troponin to regulate contraction. Calcium binds
calmodulin, and the calcium-calmodulin complex activates myosin light-chain kinase, or MLCK.
2. Cross-bridge cycling between actin and myosin in smooth muscle requires:
A. Calcium binding to troponin to expose myosin-binding sites on actin
B. Troponin moving tropomyosin away from actin
C. Tropomyosin activating troponin
D. Phosphorylation of myosin heads by myosin light-chain kinase, allowing them to bind actin
E. Troponin removing phosphate from myosin
Correct answer: D
Rationale: Activated MLCK phosphorylates the regulatory light chains of smooth-muscle
myosin. This increases myosin ATPase activity and permits cross-bridge formation with actin.
3. Which option correctly identifies the calcium sources required for complete
cardiac-muscle contraction?
A. Extracellular fluid only
B. Sarcoplasmic reticulum only
C. Extracellular fluid and sarcoplasmic reticulum
D. Mitochondria
E. Nucleus
Correct answer: C
, Rationale: Calcium entering through L-type calcium channels triggers additional calcium
release from the sarcoplasmic reticulum. This is called calcium-induced calcium release.
4. What is the function of the sodium-calcium exchanger in cardiac-muscle cells?
A. Depolarize cardiac cells to produce contraction
B. Trigger calcium release from the sarcoplasmic reticulum
C. Move calcium from extracellular fluid into the cell to initiate contraction
D. Remove calcium from the cell by transporting it into extracellular fluid, promoting relaxation
E. Activate troponin directly
Correct answer: D
Rationale: The sodium-calcium exchanger generally uses the inward sodium gradient to move
calcium out of the cardiomyocyte, reducing cytosolic calcium and supporting relaxation.
5. Which event is not part of excitation-contraction coupling in cardiac muscle?
A. An action potential travels along the cardiac-cell membrane.
B. The action potential opens voltage-gated calcium channels, allowing calcium entry.
C. Entering calcium triggers calcium release from the sarcoplasmic reticulum.
D. SR calcium channels open and release calcium toward the myofilaments.
E. Calcium binds calmodulin to initiate contraction.
Correct answer: E
Rationale: In cardiac muscle, calcium binds troponin C—not calmodulin—to move tropomyosin
and initiate cross-bridge cycling.
6. What is the primary function of the heart valves?
A. Permit forward blood flow while preventing backward flow
B. Separate the right and left sides of the heart
C. Control the speed at which blood enters and leaves the heart
D. Generate cardiac action potentials
E. Produce the normal ECG pattern
Correct answer: A
| 2026/27 Update | 100% Correct – WSU.
1. In smooth-muscle cells, calcium entering the cell first binds to ______; this
complex then activates myosin light-chain kinase to initiate contraction.
A. Troponin
B. Tropomyosin
C. Titin
D. Actin
E. Calmodulin
Correct answer: E. Calmodulin
Rationale: Smooth muscle does not use troponin to regulate contraction. Calcium binds
calmodulin, and the calcium-calmodulin complex activates myosin light-chain kinase, or MLCK.
2. Cross-bridge cycling between actin and myosin in smooth muscle requires:
A. Calcium binding to troponin to expose myosin-binding sites on actin
B. Troponin moving tropomyosin away from actin
C. Tropomyosin activating troponin
D. Phosphorylation of myosin heads by myosin light-chain kinase, allowing them to bind actin
E. Troponin removing phosphate from myosin
Correct answer: D
Rationale: Activated MLCK phosphorylates the regulatory light chains of smooth-muscle
myosin. This increases myosin ATPase activity and permits cross-bridge formation with actin.
3. Which option correctly identifies the calcium sources required for complete
cardiac-muscle contraction?
A. Extracellular fluid only
B. Sarcoplasmic reticulum only
C. Extracellular fluid and sarcoplasmic reticulum
D. Mitochondria
E. Nucleus
Correct answer: C
, Rationale: Calcium entering through L-type calcium channels triggers additional calcium
release from the sarcoplasmic reticulum. This is called calcium-induced calcium release.
4. What is the function of the sodium-calcium exchanger in cardiac-muscle cells?
A. Depolarize cardiac cells to produce contraction
B. Trigger calcium release from the sarcoplasmic reticulum
C. Move calcium from extracellular fluid into the cell to initiate contraction
D. Remove calcium from the cell by transporting it into extracellular fluid, promoting relaxation
E. Activate troponin directly
Correct answer: D
Rationale: The sodium-calcium exchanger generally uses the inward sodium gradient to move
calcium out of the cardiomyocyte, reducing cytosolic calcium and supporting relaxation.
5. Which event is not part of excitation-contraction coupling in cardiac muscle?
A. An action potential travels along the cardiac-cell membrane.
B. The action potential opens voltage-gated calcium channels, allowing calcium entry.
C. Entering calcium triggers calcium release from the sarcoplasmic reticulum.
D. SR calcium channels open and release calcium toward the myofilaments.
E. Calcium binds calmodulin to initiate contraction.
Correct answer: E
Rationale: In cardiac muscle, calcium binds troponin C—not calmodulin—to move tropomyosin
and initiate cross-bridge cycling.
6. What is the primary function of the heart valves?
A. Permit forward blood flow while preventing backward flow
B. Separate the right and left sides of the heart
C. Control the speed at which blood enters and leaves the heart
D. Generate cardiac action potentials
E. Produce the normal ECG pattern
Correct answer: A
