Myofibrils are __.
a)the basic contractile units of skeletal muscle tissue
b)the contractile proteins located within a muscle cell
c)the boundaries of individual sarcomeres
d)specialized contractile organelles found in muscle cells
e)None of the listed responses is correct.
During contraction of a sarcomere, what happens to the A band?
a)It disappears.
b)It shortens, because the myosin myofibrils (thick filaments) shorten.
c)The pull of actin filaments causes it to lengthen.
d)The A bands overlap, due to the pull of the thin filaments on the thick filaments.
e) None of the listed responses is correct.
Would it be d or e? because nothing happens during contraction the A band sames the same size?
The correct answer and explanation is:
Question 1: Myofibrils are __________.
Correct Answer: d) specialized contractile organelles found in muscle cells.
Explanation:
Myofibrils are specialized organelles found in muscle cells (both skeletal and cardiac) that are responsible for the contractile properties of these cells. They are made up of repeating units called sarcomeres, which are the structural and functional units of contraction. Each sarcomere contains an arrangement of myofilaments, including actin (thin filaments) and myosin (thick filaments), which slide past each other during contraction.
The interaction between actin and myosin is what drives muscle contraction. Myofibrils are organized in a highly structured, striated pattern due to the alignment of sarcomeres, which gives skeletal and cardiac muscle their characteristic appearance under a microscope.
Myofibrils are densely packed within the muscle fiber, leaving little room for other organelles. They extend the entire length of the muscle cell, enabling the muscle to contract uniformly. In summary, myofibrils are not the basic contractile units themselves (that role belongs to sarcomeres), nor are they the individual proteins (like actin and myosin). Instead, they are the specialized organelles made up of these components, essential for muscle contraction.
Question 2: During contraction of a sarcomere, what happens to the A band?
Correct Answer: e) None of the listed responses is correct.
Explanation:
During sarcomere contraction, the A band remains the same length. The A band corresponds to the region of the sarcomere that contains the entire length of the thick (myosin) filaments. Since myosin filaments do not shorten during contraction, the A band does not change in size. Instead, the sarcomere shortens because the thin (actin) filaments slide inward toward the center of the sarcomere (the M line), reducing the width of the H zone and the I band.
Option d is incorrect because the A bands do not overlap during contraction; rather, the filaments slide past one another, maintaining the A band’s length.