Question

Which one of the following shortens when a muscle fiber contracts?
A) Thick filament
B) Sarcomere
C) Myosin molecule
D) Thin filament

Answer 1

Hint:
Sarcomere shortening is caused by the myosin filaments sliding past the actin filaments, with no change in the length of either type of filament. The thick and thin filaments of sarcomeres form a pattern of alternating light and dark bands.

Complete step by step answer:
Bipolar thick filaments walk toward the plus end of two sets of thin filaments of opposite orientations, driven by dozens of independent myosin heads that are positioned to interact with thin filaments. During muscle contraction the, I-bands shortens. The length of each sarcomere of a myofibril decreases when the muscle contracts. However thick and thin filaments do not shorten during muscle contraction.
The light bands are called I bands (isotropic) and contain only thin filaments. The dark bands of the sarcomere are called A-bands (anisotropic). Both thick and thin filaments constitute A-band. In the center of the A-band, there is an H-zone present that has thick filaments and no thin filaments. Proteins that hold the thick filament forms the M-line at the center of the H-zone so it is called midline as it is in the middle of the sarcomere. Actin is the component of thin filaments and myosin is the component of thick filaments.

Option ‘B’ is correct

Note:
Observing that thick and thin filaments do not shorten two different groups, Andrew Huxley and Ralph Niedergerke, and Hugh Huxley and jean Hanson concluded that thick filaments slide along the thin filament during muscle contraction, which is described as a sliding filament model.