Cardiovascular System MCAT Practice Question
Researchers study the molecular basis of cardiac contraction by examining the interaction between actin and myosin filaments. During systole, myosin heads bind to actin in a calcium-dependent process initiated by troponin C binding calcium. The hydrolysis of ATP provides energy for the power stroke. Which of the following correctly describes the role of calcium in initiating cardiac contraction?
Answer choices
- ACalcium directly activates myosin ATPase activity
- BCalcium binds tropomyosin, moving it to expose myosin-binding sites on actin
- CCalcium binds troponin C, causing conformational change that moves tropomyosin and exposes actin-myosin binding sitesCorrect answer
- DCalcium phosphorylates regulatory light chains on myosin heads
- ECalcium activates calmodulin, which directly promotes actin polymerization
- FCalcium binds calsequestrin in the sarcoplasmic reticulum, releasing stored ATP needed for myosin head detachment
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