Question Video: Describing How the Sarcomere Changes as Muscle Fibers Contract | Nagwa Question Video: Describing How the Sarcomere Changes as Muscle Fibers Contract | Nagwa

Question Video: Describing How the Sarcomere Changes as Muscle Fibers Contract Biology • Third Year of Secondary School

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The diagram provided shows how a sarcomere changes when a muscle fiber contracts. Which of the following best describes how the sarcomere changes? [A] The sarcomere shortens as the myosin pulls the actin filaments toward the M line. [B] The sarcomere extends as the actin filaments pull the myosin away from the M line.

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Video Transcript

The diagram provided shows how a sarcomere changes when a muscle fiber contracts. Which of the following best describes how the sarcomere changes? (A) The sarcomere shortens as the myosin pulls the actin filaments toward the M line. Or (B) the sarcomere extends as the actin filaments pull the myosin away from the M line.

To answer this question, let’s first look at what a sarcomere is. A sarcomere is the functional unit of a myofibril, which is a specialized organelle found in muscle fibers. Myofibrils are formed of many sarcomeres, each of which is defined by two Z lines which mark the sarcomere’s boundary.

In this diagram, we can see two of the protein filaments that are present in a sarcomere: thick myosin filaments and thinner actin filaments. The diagram shows a sarcomere in a relaxed muscle at the top and in a contracted muscle below. When the muscle contracts, the sarcomere shortens as the distance between the two successive Z lines shortens. This is because the H zone, a region in the center of the sarcomere that only contains thick myosin filaments, shortens when the muscle contracts.

The contraction occurs as myosin pulls the actin filaments toward the M line in the center of the sarcomere. When the muscle relaxes, the actin moves back to its original position, away from the M line, and the sarcomere extends again.

Using this information, we can deduce that the correct answer to this question asking about the changes to a sarcomere when a muscle fiber contracts is (A). When a muscle fiber contracts, the sarcomere shortens as the myosin pulls the actin filaments toward the M line.

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