In the sliding filament theory of
muscle contraction, what is the primary role of calcium ions?
To answer this question, we need to
learn some more information about the sliding filament theory of muscle contraction
and the role of calcium ions in this process.
When a muscle fiber is relaxed, a
protein filament called tropomyosin, shown here in blue, coils around a thin protein
filament called actin, shown here in orange. Tropomyosin blocks sections of the
actin filament that contain binding sites for the globular heads of another protein
filament called myosin, shown here in pink.
When the muscle fiber is stimulated
by an electrical impulse from a neuron, calcium ions are released from a specialized
organelle in the muscle fiber called the sarcoplasmic reticulum. These calcium ions bind to
tropomyosin, changing its shape. This moves tropomyosin away from
the myosin binding sites on the actin filament, exposing them. The movement of tropomyosin away
from the myosin binding sites allows the myosin heads to temporarily bind to the
actin filament, which will eventually result in contraction of the muscle fiber.
Now we know the correct answer to
this question. The primary role of calcium ions in
the sliding filament theory is to bind to tropomyosin, changing its shape and moving
it away from myosin binding sites.