Questions: Part A What changes occur in the sarcomere during muscle contraction? I band increases in length. Z discs move closer totether. A band decreases in length. The thin filament shortens.

During muscle contraction, several changes occur in the sarcomere, which is the basic functional unit of a muscle fiber. Let's evaluate each statement:

1. I band increases in length.
   This statement is incorrect. During muscle contraction, the I band, which is the region containing only thin filaments, actually decreases in length as the thin filaments slide over the thick filaments.

2. Z discs move closer together.
   This statement is correct. During contraction, the Z discs, which define the boundaries of a sarcomere, move closer together as the sarcomere shortens.

3. A band decreases in length.
   This statement is incorrect. The A band, which contains the entire length of the thick filaments, remains the same length during contraction. The thick filaments do not change in length.

4. The thin filament shortens.
   This statement is incorrect. The thin filaments do not shorten during contraction. Instead, they slide past the thick filaments, which causes the sarcomere to shorten.

In summary, during muscle contraction, the Z discs move closer together, the I band decreases in length, the A band remains the same length, and the thin filaments do not shorten but slide past the thick filaments.