Video Transcript
The diagram provided shows the
stages of the sliding filament theory. State the correct order.
To answer this question, we need to
learn some more information about the sliding filament theory of muscle
contraction.
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 sliding filament theory begins
with the release of calcium ions from a specialized organelle in the muscle fiber
called the sarcoplasmic reticulum, as described in statement (3).
Calcium ions bind to tropomyosin,
causing it to pull away from and expose these myosin binding sites on the actin
filament, as described in statement (2). This allows the myosin heads to
temporarily bind to the actin filament, forming cross bridges, which are otherwise
known as transverse links, between the molecules, as described in statement (5).
The formation of these cross
bridges releases the inorganic phosphate molecule from each myosin head. The myosin head then changes angle,
pulling the whole actin filament along. This process is sometimes called a
power stroke and is described in statement (7). The power stroke releases the
molecule of ADP from each myosin head. This is described in statement
(8).
This allows ATP to bind to myosin
instead, leading to the breaking of the cross bridge between the actin and myosin
filaments and causing the myosin heads to detach. This is described in statement
(6).
The myosin heads are now positioned
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. ATP hydrolysis requires an enzyme
called ATPase, as described in statement (4).
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. To prepare for this, the myosin
head returned to its original position, as described in statement (1).
Now we know the correct order of
the stages of the sliding filament theory and the correct answer to this question:
(3), (2), (5), (7), (8), (6), (4), (1).