Muscle Contraction (AQA BIOL5 Notes)

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Muscle Contraction
Types of Muscle
Features of Skeletal Muscle Contraction
Structure of Skeletal Muscle
Arrangement
Filament Arrangement
Skeletal Muscle Proteins
Myosin
Actin
Skeletal Muscle Contraction
Sliding Filament Theory
The Role of ATP and Phosphocreatine
Slow & Fast Twitch Muscle Fibres
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Types of Muscle
There are three types of muscle:
1. Skeletal Muscle ­ This is muscle attached to bone and its primary function is
movement. It contracts rapidly and is attached to bones by inelastic tendons. When
the muscle contracts it pulls on the skeleton causing the bone to which it's attached
to move.
2. Smooth Muscle ­ This is found in the walls of tubular organs such as the arteries or
the gut. It contracts slowly.
3.…read more

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Features of Skeletal Muscle Contraction
Muscles contract (become shorter) and relax (return to their original length).
Actin filaments sliding over myosin ones cause the force of contraction.
The sliding filament mechanism generates force in one direction.
An opposing force is needed to pull the filaments apart and restore the muscle to its
original length. Muscles can only pull, not push.…read more

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Structure of Skeletal Muscle
Arrangement
Tendons at each end of the muscle connect it to the bone.
The muscle is made up of bundles of muscle fibres, bound together by connective tissue.
Each muscle fibre is a single muscle cell, surrounded by a cell surface membrane.
Inside the muscle fibre is the cytoplasm containing mitochondria and other organelles. There
are also numerous myofibrils, which are composed of repeated contractile units known as
sarcomeres.…read more

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Filament Arrangement
The above
picture is
from an
electron
microscope
and the
below one a
light
microscope.
Each myofibril is arranged into a number of sarcomeres, joined together end to end and
parallel to other sarcomeres in other myofibrils.
The vertical Z lines are discs holding the actin filaments in position, running across the
myofibril and denoting the beginning and end of the sarcomere.…read more

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Skeletal Muscle Proteins
Myosin
The tails of myosin molecules wrap around each other to form the thick filament whilst the
globular heads protrude in all directions to form the actinomyosin bridges with actin.…read more

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Each actin filament is made of two helical strands of globular actin molecules that twist
around each other to form the actin filament.
Actin filaments are associated with two accessory proteins:
Tropomyosin a rod shaped fibrous protein that link end to end to form two helical
strands wrapped around the actin filament. Its function is to switch on or off the
contractile mechanism.
Troponin ­ binds to Ca2+ and moves tropomyosin out of the way.…read more

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ABand The length remains the same as the length of myosin remains the same.
Becomes shorter as much of the actin has overlapped with myosin and
IBand
moved into the Aband. Thus there is a reduced area of only actin.
Becomes shorter or disappears as actin overlaps with myosin. There is a
HZone
reduced area of only myosin.…read more

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The myosin heads attached to ADP + Pi can now bind to the actin filament and form a cross
bridge, starting the cross bridge cycle:
Myosin heads combined with ADP and Pi bind to the actin filament.
Once attached the myosin heads change their angle, pulling the actin filament along
and releasing the ADP and Pi.
An ATP molecule now attaches to each myosin head causing detachment from the
actin filament.…read more

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The Role of ATP and Phosphocreatine
The energy required for muscle contraction is supplied by the hydrolysis of ATP:
ATP ADP + Pi
ATP is required for:
The myosin heads to change back to their original shape.
The reabsorption of Ca2+ into the sarcoplasmic reticulum by active transport.
ATP supplies energy for muscle contractions but during times of intense exercise ATP
reserves run out after three seconds. After this, ATP is rapidly resynthesised using
phosphate derived from phosphocreatine.…read more

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