Video Transcript
In the sliding filament theory of
muscle contraction, how does the myosin head resume its normal position? (A) Using energy from the
hydrolysis of ATP. (B) Using energy from the breakdown
of ADP. (C) Using the electrical
stimulation of the influx of calcium ions.
To answer this question, we need to
learn some more information about the sliding filament theory of muscle contraction
and the protein filaments that are involved 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 relaxed,
each myosin head is bound to a molecule of ADP and an inorganic phosphate. The release of calcium ions from a
specialized organelle in the muscle fiber called the sarcoplasmic reticulum triggers
tropomyosin to pull away from and expose these myosin binding sites on the actin
filament. This allows the myosin heads to
temporarily bind to the actin filament, forming crossbridges, which are otherwise
known as transverse links, between the molecules.
The formation of these crossbridges
releases the inorganic phosphate molecule from each myosin head. The myosin head then changes angle,
pulling the whole actin filament along in a process sometimes referred to as the
power stroke. The power stroke releases the
molecule of ADP from each myosin head. This allows ATP to bind to myosin
instead, leading to the breaking of the crossbridge between the actin and myosin
filaments and causing the myosin heads to detach. The myosin heads can then return to
their original position, further along the actin filament than they were before the
power stroke due to the actin filament being pulled along.
The ATP molecule bound to the
myosin head is hydrolyzed to produce ADP and an inorganic phosphate. The hydrolysis of ATP releases the
energy needed for the myosin head to be ready for another power stroke and bind to
another myosin binding site further along the actin filament.
Now we know that the correct answer
to this question is (A). The myosin head resumes its normal
position using energy from the hydrolysis of ATP.