Biology Support & Movement MDCAT 2019
PMDC Verified Question 69 of 75
The function of calcium ions in muscle contraction is to:
A
Bind to troponin molecule and cause them to move
B
Polarize visible light
C
Aid in the transmission of nerve impulse
D
Bind to tropomyosin molecule and cause them to form cross bridges
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option A: Bind to troponin molecule and cause them to move
Concept:

Role of Calcium in cross-bridge cycling.

Formula:

N/A (Conceptual)

Solution:

  • In a resting muscle, tropomyosin actively blocks the myosin-binding sites on actin.


  • When an action potential arrives, Ca\(^{2+}\) is released from the sarcoplasmic reticulum. The Ca\(^{2+}\) physically binds to specific subunits on the troponin complex.


  • This binding induces a massive conformational change in troponin, forcing it to drag tropomyosin out of the way, thereby exposing the active sites for cross-bridge formation.


Why other options are incorrect:

Calcium binds strictly to troponin (TnC), not directly to tropomyosin. While muscles have anisotropic properties under polarized light, Ca\(^{2+}\) is a chemical trigger, not a light polarizer.

Quality & Fidelity Assurance: Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.

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