Question Video: Recalling How a Myosin Head Resumes a Normal Position After Muscle Contraction | Nagwa Question Video: Recalling How a Myosin Head Resumes a Normal Position After Muscle Contraction | Nagwa

Question Video: Recalling How a Myosin Head Resumes a Normal Position After Muscle Contraction Biology • Third Year of Secondary School

In the sliding filament theory of muscle contraction, how does the myosin head resume its normal position?

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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.

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