Cambridge International A Level Biology Revision Guide

Chapter 15: Coordination
Structure of thick and thin filaments
Thick filaments are composed of many molecules of
myosin, which is a fibrous protein with a globular head.
The fibrous portion helps to anchor the molecule into the
thick filament. Within the thick filament, many myosin
molecules all lie together in a bundle with their globular
heads all pointing away from the M line.
The main component of thin filaments, actin, is a
globular protein. Many actin molecules are linked together
to form a chain. Two of these chains are twisted together
to form a thin filament. Also twisted around the actin
chains is a fibrous protein called tropomyosin. Another
protein, troponin, is attached to the actin chain at regular
intervals (Figure 15.28).
M line
How muscles contract
Muscles cause movement by contracting. The sarcomeres
in each myofibril get shorter as the Z discs are pulled closer
together. Figure 15.28 shows how this happens. It is known
as the sliding filament model of muscle contraction.
The energy for the movement comes from ATP
molecules that are attached to the myosin heads. Each
myosin head is an ATPase.
When a muscle contracts, calcium ions are released
from stores in the SR and bind to troponin. This stimulates
troponin molecules to change shape (Figure 15.28).
The troponin and tropomyosin proteins move to a
different position on the thin filaments, so exposing
parts of the actin molecules which act as binding sites for
1 When the muscle is relaxed,
tropomyosin and troponin
are sitting in a position
in the actin filament that
prevents myosin from
binding.
tropomyosin
actin
troponin
347
2 When muscle contraction
starts, the troponin and
tropomyosin change shape to
allow myosin heads to bind to
actin.
myosin head
3 Myosin heads tilt, pulling
the actin and causing the
muscle to contract by
about 10 nm.
4 ATP hydrolysis causes
the release of myosin
heads. They spring back
and repeat the binding
and tilting process.
Figure 15.28 The sliding filament model of muscle contraction.