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The sarcomere
A sarcomere is the smallest contractile
component of a muscle fibre. They are made
up of 2 types of protein filament (thin and
thick filaments) that work together in order to
produce the contractions in muscles.
These two types of protein are called actin and
myosin. Actin makes up the thin filaments, and
myosin makes up the thick filaments.…read more

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Actin filaments
These are made of two chains of G-actin,
coiled around each other like a double string
of beads. There is then a rod shaped molecule
called tropomyosin that winds round those to
reinforce it, with molecules of troponin that
are attached to every tropomyosin molecule.
The tropnin molecules binds to 3 things: the
actin molecules, the tropomyosin molecules
(which holds the tropomyosin and the actin
together) and calcium ions.…read more

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Myosin filaments
The thick filaments are made up of bundles of
myosin fibrils. Each one consists of a tail and
two heads. The heads stick out from opposite
ends of the filament.…read more

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The power stroke
This refers to the movement of the sarcomere.
The myosin filaments are held within the sarcomere by the
actin filaments, overlapping each other slightly . When the
muscles contract, calcium ions bind to the troponin
molecules, which uncovers them from the binding sites on
the actin filaments. This allows the myosin heads to bind
with the action binding sites on the filament, so a power
stroke can now occur. The myosin head then bends, making
the thin filament get pulled along, so they overlap further.
This is called a power stroke. ATP is used in this process, so
ADP + Pi are produced.
The crossbridge is broken as new ATP attaches to the myosin
head, which gives it the energy to move backwards and form
a crossbridge further along the thin filament, so it can bend
and push the thin filament along further.…read more

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When a power stroke occurs, it shortens the
whole length of the muscle, so the muscle
contracts.
When the nerves stop stimulating the muscles,
the Ca2+ ions are actively transported back into
the sarcoplasmic reticulum by carriers on its
membrane. This means the muscles won't
contract until there is more nervous
stimulation.
ATP is used in the bending of the myosin
heads- both when it is pulling the actin
filament, and when it is bending back to
rebind to another section of the filament
further along…read more

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