Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
reticulum. The membranes of the sarcoplasmic reticulum
have huge numbers of protein pumps that transport
calcium ions into the cisternae of the SR. The sarcoplasm
contains a large number of mitochondria, often packed
tightly between the myofibrils. These carry out aerobic
respiration, generating the ATP that is required for
muscle contraction.
The most striking thing about a muscle fibre is its
stripes, or striations. These are produced by a very regular
arrangement of many myofibrils in the sarcoplasm. Each
myofibril is striped in exactly the same way, and is lined
up precisely against the next one, so producing the pattern
you can see in Figures 15.25 and 15.26.
This is as much as we can see using a light microscope,
but with an electron microscope it is possible to see that
each myofibril is itself made up of yet smaller components,
called filaments. Parallel groups of thick filaments lie
between groups of thin ones. Both thick and thin filaments
are made up of protein. The thick filaments are made
mostly of myosin, whilst the thin ones are made mostly of
actin. Now we can understand what causes the stripes.
The darker parts of the stripes, the A bands, correspond
to the thick (myosin) filaments. The lighter parts, the
I bands, are where there are no thick filaments, only thin
(actin) filaments (Figure 15.27). The very darkest parts
of the A band are produced by the overlap of thick and
thin filaments, while the lighter area within the A band,
known as the H band, represents the parts where only the
thick filaments are present. A line known as the Z line
provides an attachment for the actin filaments, while the
M line does the same for the myosin filaments. The part
of a myofibril between two Z lines is called a sarcomere.
Myofibrils are cylindrical in shape, so the Z line is in fact
a disc separating one sarcomere from another and is also
called the Z disc.
thick filament made of myosin
thin filament made of actin
346
sarcomere M line Z line
thin (actin) filament
A band
H band
thick (myosin) filament
I band
M line
Figure 15.27 The structure of a myofibril.
QUESTION
15.7 a Use Figure 15.27 to make a simple diagram to show
the arrangement of the thick and thin filaments in a
sarcomere of a resting muscle. In your diagram, show
and label the following: one thick filament, four thin
filaments and two Z lines.
b Indicate and label the following on your diagram:
A band, I band and H band.
c The average length of a sarcomere in a resting
muscle is 2.25 μm. Use this figure to calculate the
magnification of your diagram.