Video Transcript
The sliding filament theory
outlines the process of muscle contraction. The diagram provided shows the
basic outline of a myosin and an actin filament before they have bound. Which structure is indicated by
X? (A) Sarcolemma, (B) tropomyosin,
(C) collagen, or (D) myosin-binding sites.
To answer this question, let’s take
a look at the filament shown in this diagram in a bit more detail. Muscle fibers generally contain
three types of protein filament: actin, myosin, and tropomyosin.
Actin is a thin filament consisting
of two strands twisted around each other. Myosin is a thicker filament with
globular heads that project outward. Tropomyosin coils around each actin
filament. The muscle fiber shown in the image
provided by the question is in a relaxed state. When a muscle fiber is relaxed,
tropomyosin blocks the binding sites for the myosin heads that are present on the
surface of each actin filament. This prevents the myosin heads from
binding to the actin filament.
During muscle contraction, calcium
ions pull away tropomyosin from these binding sites, allowing myosin to bind to the
actin filaments. This triggers a series of events
that pulls the actin filaments, overall resulting in a shortened, contracted muscle
fiber.
Using the information we have
learned, we can answer this question correctly. The structure in the diagram
indicated by X is (B) tropomyosin.
