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5. Proteins in structures Page 20
diagram of muscle contracting Figure 5
Muscle structure and muscle contraction
5.3 Muscle
In this section we shall concentrate on voluntary muscle, the kind you have in your arms and legs.

Scientists are interested in the structure of muscle for a variety of reasons. For example food scientists need to know details of muscle structure and chemistry so that they can advise food companies on how to treat meat and process meat-based products. Doctors, physiotherapists and sports scientists are interested in finding out more about muscle so that they can treat muscle injuries and diseases more effectively.

Muscle tissue contracts and relaxes when triggered by electrical stimuli from the brain, through the nerves. The electrical stimuli release calcium ions from a component in the muscle cell. The release of calcium ions initiates the muscle contraction. The contractions cause movement of the body. The forces involved can be enormous; all the effort that a weight lifter uses comes from muscle contraction. Where does the energy needed come from?

Special small molecules (ATP, adenosine triphosphate), produced during respiration, provide the store of energy that muscles use. When these small molecules break down they make energy available to the muscle. How can muscle turn this chemical energy into kinetic energy?

It is the proteins in the muscle that respond to nerve impulses by changing the packing of their molecules. To see how this works, though, we need to look at how the molecules are collected together and at their structures.

Hundreds of muscle fibres, each up to several centimetres long, are bundled together to make up a single muscle. Many small myofibrils make up each fibre (Figure 5). The myofibrils have a characteristic pattern of transverse lines, called striations, that are formed by the arrangements of protein molecules.

The protein molecules form filaments. There are two types of filament; thick and thin. Thick filaments contain myosin, thin filaments contain actin , troponin and tropomyosin. Scientists think that muscles contract by the two types of filament sliding over each other so that they overlap more (Figure 5).

diagram of muscle filaments
Figure 6
The sturcture of myosin in thick muscle filaments.
Myosin is made up of six polypeptide chains. Four are low molecular mass (light) chains, and two are high molecular mass (heavy) chains. The two heavy chains are twisted together for part of their length to form a coiled rod. Each heavy chain ends in a globular head region, which also contains two light chains. The rod is a two stranded a-helical coiled coil (Figure 6).

In thick filaments the myosin molecules are twisted in a bundle with the heads pointing out in a regular way from the main body of the filament (Figure 7).


Figure 7
A closer look at contraction. /FONT>

Unilever Education Advanced Series: Proteins
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