AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
The roles of actin, myosin, calcium ions and ATP in myofibril contraction.
Full specification wording
Skeletal muscles are stimulated to contract by nerves and act as: The roles of actin, myosin, calcium ions and ATP in myofibril contraction.
In the sliding filament mechanism, depolarisation travels down the transverse tubules and causes the sarcoplasmic reticulum to release calcium ions into the sarcoplasm, the cytoplasm of the muscle…
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 3 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
In the sliding filament mechanism, depolarisation travels down the transverse tubules and causes the sarcoplasmic reticulum to release calcium ions into the sarcoplasm, the cytoplasm of the muscle fibre. Calcium ions bind to troponin, changing its shape so that tropomyosin moves and exposes the myosin binding sites on actin. Myosin heads attach to these sites, forming actinomyosin bridges. The release of ADP and inorganic phosphate from the myosin head makes it bend, pulling the actin filament towards the centre of the sarcomere in the power stroke. A new ATP molecule then binds to the myosin head, causing it to detach from actin. ATP hydrolase hydrolyses this ATP to ADP and Pi, releasing energy to recock the head so it can bind further along. ATP is also used to actively transport calcium ions back into the sarcoplasmic reticulum, allowing relaxation.
What earns the marks
- Calcium ions diffuse out of the sarcoplasmic reticulum into the sarcoplasm, where they bind to troponin, causing tropomyosin to move and expose the myosin binding sites on actin.
- Myosin heads attach to the binding sites on the actin, forming actinomyosin bridges.
- The myosin head bends, pulling the actin filament towards the centre of the sarcomere (the power stroke), which is driven by the release of ADP and Pi.
- A new ATP molecule attaches to each myosin head, causing it to detach from the actin; hydrolysis of this ATP provides energy to recock the head.
- ATP is also used to actively transport calcium ions back into the sarcoplasmic reticulum, allowing the muscle to relax.
Common mistakes and how to put them right
Calling the actin binding site an active site.
Put it right: Use binding site.
Saying the filaments themselves contract or shorten, when they slide past each other.
Stating that ATP hydrolysis directly causes the power stroke.
Put it right: ATP hydrolysis recocks the myosin head, while the release of ADP and Pi drives the power stroke.
Saying calcium ions are released into the myofibril.
Put it right: They are released into the sarcoplasm, the cytoplasm of the muscle fibre.
Exam tips
- Name all four players in every answer: actin, myosin, calcium ions and ATP. Which are required depends on the wording of the question.
- Clearly distinguish the roles of ATP: binding causes detachment, while hydrolysis provides energy to recock the myosin head.
- When asked for the roles of ATP, remember the third use, active transport of calcium ions back into the sarcoplasmic reticulum.
Try the lesson
The start of the Skeletal muscles are stimulated to contract by nerves and act as: The roles of actin, myosin, calcium ions and ATP in myofibril contraction. lesson, made from checked teaching notes for this specification point.
Have a go first
Which protein forms the thick filaments of a myofibril?
Listen
In the sliding filament mechanism, depolarisation travels down the transverse tubules and causes the sarcoplasmic reticulum to release calcium ions into the sarcoplasm, the cytoplasm of the muscle fibre. Calcium ions bind to troponin, changing its shape so that tropomyosin moves and exposes the myosin binding sites on actin. Myosin heads attach to these sites, forming actinomyosin bridges. The release of ADP and inorganic phosphate from the myosin head makes it bend, pulling the actin filament towards the centre of the sarcomere in the power stroke. A new ATP molecule then binds to the myosin head, causing it to detach from actin. ATP hydrolase hydrolyses this ATP to ADP and Pi, releasing energy to recock the head so it can bind further along. ATP is also used to actively transport calcium ions back into the sarcoplasmic reticulum, allowing relaxation. A strong answer earns marks for these points. Calcium ions diffuse out of the sarcoplasmic reticulum into the sarcoplasm, where they bind to troponin, causing tropomyosin to move and expose the myosin binding sites on actin. Myosin heads attach to the binding sites on the actin, forming actinomyosin bridges. The myosin head bends, pulling the actin filament towards the centre of the sarcomere (the power stroke), which is driven by the release of ADP and Pi. A new ATP molecule attaches to each myosin head, causing it to detach from the actin; hydrolysis of this ATP provides energy to recock the head. A common slip: Stating that ATP hydrolysis directly causes the power stroke. Instead, ATP hydrolysis recocks the myosin head, while the release of ADP and Pi drives the power stroke. Exam tip: Name all four players in every answer: actin, myosin, calcium ions and ATP. Which are required depends on the wording of the question.
Key words
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actinomyosin bridges
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power stroke
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calcium ion release
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ATP hydrolysis on the myosin head
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See every definition
- actinomyosin bridges
- Structures formed when myosin heads attach to the myosin binding sites on actin.
- power stroke
- The bending of the myosin head that pulls the actin filament towards the centre of the sarcomere, driven by the release of ADP and Pi.
- calcium ion release
- Depolarisation travels down the transverse tubules and causes the sarcoplasmic reticulum to release calcium ions into the sarcoplasm.
- ATP hydrolysis on the myosin head
- ATP hydrolase hydrolyses ATP to ADP and Pi, releasing energy to recock the head so it can bind further along.
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