Video Transcript
The diagram shows the basic structure of a sarcomere. Which letter indicates a Z line?
To answer this question, let’s start by addressing what a sarcomere is before we look
at its structure in more detail. A sarcomere is the functional unit of organelles that are found exclusively in muscle
cells called myofibrils. Myofibrils are long cylindrical organelles that are specialized for muscular
contraction. And they contain many repeating patterns of different protein filaments within each
sarcomere. Muscle contraction occurs when hundreds of thousands of sarcomeres shorten
simultaneously, causing the entire muscle to become shorter and wider and often
resulting in some form of movement.
Now, let’s take a look at each of the components of the sarcomere that have been
labeled in the diagram provided by the question. We mentioned that each sarcomere contains a few different protein filaments. One of these is a thin filament called actin, consisting of two strands twisted
around each other.
Another thicker filament called myosin is also present in each sarcomere. And its thickness makes it appear darker than the actin filaments. These are long rod-shaped filaments with globular heads that project outward. Myofibrils have alternating bands that appear lighter or darker in color due to the
composition of actin and myosin filaments within them in each sarcomere, which makes
them appear stripy, or striated.
The I bands, which have been labeled on this diagram with the letter Y are sometimes
called the light bands, as they do not contain myosin filaments, only the thinner
actin filaments. As a result, the I bands appear considerably lighter in color in micrograph images
than the rest of the sarcomere. The vertical lines within the I band, one of which has been labeled here with the
letter W, represent a region called the Z line. The Z lines usually appear slightly darker in micrographs as there are other protein
filaments present here in addition to actin. Two successive Z lines mark the boundary of each sarcomere.
The A bands, labeled here with the letter Z, are the regions of the sarcomere that do
contain the thicker, darker myosin filaments. And so they are sometimes called the dark bands. The outer edges of the A bands are darkest as they contain both actin and myosin
filaments overlapping. The inner edges of the A bands are not quite so dark, as they only contain myosin
filaments.
This region, which has been labeled here with the letter X, is called the H zone. The middle of the H zone is called the M line, or sometimes the M band, which can be
remembered as M stands for middle. On this diagram, the M line is labeled with the letter V. The H zone is what shortens during contraction as the actin filaments are pulled
closer to the M line by the myosin filaments. This causes the distance between the two Z lines to shorten and therefore the entire
sarcomere to shorten too.
After reviewing different components of the sarcomere, we can answer our question
correctly as we have just learned that the Z lines mark the boundaries of each
sarcomere within the I band. The letter that indicates a Z line on this diagram is therefore W.
