Question

which two structures block the active site on an actin filament?
○ fascicle & actin
○ actin & perimysium
○ troponin & tropomyosin
○ actin & myosin

a muscle fiber cannot contract until __________ rushes in to remove the blocking troponin and tropomyosin molecules and make the active site available to be engaged.
○ atp
○ calcium
○ myosin
○ sarcomeres

Explanation:

First Question (Which two structures block the active site on an actin filament?):

To determine the structures blocking the actin filament's active site, we recall muscle contraction physiology. Tropomyosin lies along actin filaments, and troponin is associated with it. Together, they block the active sites on actin, preventing myosin binding at rest. Other options (fascicle & actin, actin & perimysium, actin & myosin) are incorrect: fascicles are muscle bundles, perimysium is connective tissue, and myosin interacts with actin during contraction, not block it.

In muscle contraction, when a muscle is stimulated, calcium ions ($Ca^{2+}$) are released from the sarcoplasmic reticulum. Calcium binds to troponin, causing a conformational change that moves tropomyosin away from actin’s active sites, allowing myosin to bind. ATP provides energy for contraction but does not remove the blocking molecules; myosin is the motor protein, and sarcomeres are the contractile units. Thus, calcium is the correct answer.

Answer:

C. troponin & tropomyosin

Second Question (A muscle fiber cannot contract until ______ rushes in to remove the blocking troponin and tropomyosin molecules...):