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Organisms respond to changes in their internal and external
environments (AQA A2 Biology) PART 6 of 9 TOPICS
Skeletal muscles are stimulated to contract by nerves and act as
Muscles act as antagonistic pairs against an incompressible skeleton where if one muscle contracts
(agonist) it pulls the bone and the other muscle relaxes (antagonist). An example is biceps and triceps
in the arm. NB: Muscles do not push bones and only pull. Skeletal muscles are attached to bones by
tendons. Ligaments are attached from one bone to the other.
The gross structure is as follows: The muscle is made up of bundles which are packaged together.
These bundles contain fibres/fascicles which are packaged in a connective tissue and have a distinct
stripy look called striated tissue. These fibres/fascicles are known as muscle cells and have a
membrane called sarcolemma, a cytoplasm called the sarcoplasm and organelles called myofibrils.
These myofibrils are made up of 2 types of proteins: actin and myosin.
The microscopic structure should be known which is of the sarcomere. This is from one Z-line to
another Z-line and is one contractile unit of many in a myofibril which are long and cylindrical in shape
which shorten to create contraction. Between the two Z-lines in the middle is a line called the M-line.
Adjacent to the Z-lines inside are I-bands (light bands), one on each side, which are lighter in colour as
they only contain actin. The A-band (dark band) is from one I-band to the other and is darker in colour
as it contains actin and myosin. H-zone is in the middle of the A-band and contains only myosin.
The sliding filament theory is as follows: At a neuromuscular junction on a fibre, an action potential is
made and travels along the postsynaptic membrane and into the t-tubules which are indentations of
the muscle cell. Sarcoplasmic reticulum, located at the t-tubules, release Ca2+ ions into the
sarcoplasm when an action potential passes and is used to bind to tropomyosin which is wrapped
around actin covering myosin-head binding sites. When it does bind to tropomyosin, it changes shape
to expose the myosin-head binding sites so that the myosin heads can attach to form actinomyosin
bridges. NB: The myosin heads actually bind to a protein called troponin on tropomyosin but this
does not need to be known for AQA. The myosin heads swing backwards pulling the actin forwards
and then detaches by the hydrolysis of ATP by ATP hydrolase. NB: This enzyme is only active when
there is an influx of Ca2+ ions out of the sarcoplasmic reticulum. This process of attaching and
detaching continues in a prose called a power stroke and stops until the sarcomere has shortened so
much that the I-bands have gone smaller and the H-zone has gone smaller with the A-band and Z-lines
staying the same. The Ca2+ ions are actively transported into the sarcoplasmic reticulum using ATP.
The sarcomere returns to its original length when the antagonist muscle contracts.
The role of ATP in muscle contraction is to provide the energy as well as the many roles stated in the
above paragraph. Phosphocreatine/Creatine phosphate is used when ATP is short in supply even after
anaerobic respiration has taken place. It donates a phosphate group to ADP to make more ATP. NB:
Phosphocreatine may be abbreviated into PCr however this may get confusing as it may look like a
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Plus the full name may be given in the exam so learning
the full name will avoid confusion.
Structure Location General properties
Fast twitch muscle They have high stores of Mainly in the legs of Makes a large amount
fibres phosphocreatine/Creatin sprinters of ATP because of the
e phosphate, low levels huge amount of
of myoglobin so they are anaerobic respiration
white in colour, have but in low quantities.…read more