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Biology Unit 5
Topic 7: Run for your life
3. Explain the contraction of skeletal muscle in terms
of the sliding filament theory, including: the role of
actin, myosin, troponin, tropomyosin, calcium ions
(Ca²), ATP and ATPase.
The functional unit of a muscle fibre is the sarcomeres. When the muscle contracts, the thin actin
filaments move between the thick myosin filaments, shortening the length of the sarcomeres and
therefore shortening the length of the muscle.
The Sliding Filament Theory
1. Ca² diffuse out of the sarcoplasmic reticulum, when the muscles are stimulated at a
2. The Ca² binds to a protein called troponin, found on the actin filament.
3. This causes another protein tropomyosin, to change position, exposing the actins' binding
4. The myosin head forms cross bridges to bind to the actin filament.
5. ATP on the myosin head hydrolyses (in the presence of ATPase) to ADP, releasing energy
and changing the angle of the myosin head. This is the power stroke that moves the actin
towards to centre of the sarcomere.
6. More ATP is hydrolysed (in the presence of ATP and Ca²) again to ADP, providing energy to
return the myosin head to its original position.
7. The myosin head then attaches further down the actin fibre, as long as Ca² are still
produced. This will occur 50 -100 times per second until stimulation stops. At this point ATP
produces energy to return the Ca² to
the sarcoplasmic reticulum and the
Text Book: p. 148-149