OCR · A-Level · Biology

    Course version: H420 · Version from Sept 2025

    (l) (i) the structure of mammalian muscle and the mechanism of muscular…

    Full specification wording

    Plant and animal responses: (l) (i) the structure of mammalian muscle and the mechanism of muscular contraction

    Mammalian skeletal muscle is made of bundles of fibres containing myofibrils.

    Checked against the OCR H420 specification (opens in a new tab) on .

    • Point 11 of 12
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Mammalian skeletal muscle is made of bundles of fibres containing myofibrils. Each myofibril shows repeating sarcomeres bounded by Z-lines, with thick myosin filaments and thin actin filaments. The sliding filament model explains contraction: a nerve impulse releases calcium ions from the sarcoplasmic reticulum; Ca²⁺ binds troponin, moving tropomyosin to expose actin binding sites; myosin heads bind actin, form cross-bridges and pivot, pulling actin filaments towards the centre of the sarcomere. ATP binding detaches myosin heads and ATP hydrolysis re-cocks them for the next cycle. As actin slides over myosin, the sarcomere shortens and the muscle contracts, while the filaments themselves do not change length.

    What earns the marks

    • Skeletal muscle fibres contain myofibrils with repeating sarcomeres bounded by Z-lines, containing actin and myosin filaments.
    • A nerve impulse triggers release of Ca²⁺ from the sarcoplasmic reticulum.
    • Ca²⁺ binds troponin, moving tropomyosin to expose actin binding sites.
    • Myosin heads bind actin to form cross-bridges and pivot, sliding actin filaments and shortening the sarcomere.
    • ATP is required to detach myosin heads and to re-cock them for further cycles.

    Common mistakes and how to put them right

    • Thinking the filaments themselves shorten.

      Put it right: The filaments slide past each other and the sarcomere shortens, not the actin or myosin.

    • Omitting the role of Ca²⁺ and troponin.

      Put it right: Without Ca²⁺ binding troponin, tropomyosin blocks the binding sites and cross-bridges cannot form.

    • Forgetting ATP is needed for detachment as well as contraction.

      Put it right: Without ATP, myosin heads cannot release actin.

    Exam tips

    • Sequence the events in order: impulse, Ca²⁺ release, troponin/tropomyosin, cross-bridge, power stroke, ATP detachment.
    • Name the structures precisely (sarcomere, Z-line, actin, myosin, sarcoplasmic reticulum) to gain credit.
    • State clearly that the sarcomere shortens because filaments slide, not because filaments contract.

    Try the lesson

    The start of the Plant and animal responses: (l) (i) the structure of mammalian muscle and the mechanism of muscular contraction lesson, made from checked teaching notes for this specification point.

    Have a go first

    What is the functional unit of contraction in a skeletal muscle fibre called?

    Listen

    Mammalian skeletal muscle is made of bundles of fibres containing myofibrils. Each myofibril shows repeating sarcomeres bounded by Z-lines, with thick myosin filaments and thin actin filaments. The sliding filament model explains contraction: a nerve impulse releases calcium ions from the sarcoplasmic reticulum; Ca²⁺ binds troponin, moving tropomyosin to expose actin binding sites; myosin heads bind actin, form cross-bridges and pivot, pulling actin filaments towards the centre of the sarcomere. ATP binding detaches myosin heads and ATP hydrolysis re-cocks them for the next cycle. As actin slides over myosin, the sarcomere shortens and the muscle contracts, while the filaments themselves do not change length. A strong answer earns marks for these points. Skeletal muscle fibres contain myofibrils with repeating sarcomeres bounded by Z-lines, containing actin and myosin filaments. A nerve impulse triggers release of Ca²⁺ from the sarcoplasmic reticulum. Ca²⁺ binds troponin, moving tropomyosin to expose actin binding sites. Myosin heads bind actin to form cross-bridges and pivot, sliding actin filaments and shortening the sarcomere. Exam tip: Sequence the events in order: impulse, Ca²⁺ release, troponin/tropomyosin, cross-bridge, power stroke, ATP detachment.

    See it

    A vertical flowchart with nine nodes. A nerve impulse leads to Ca²⁺ released from the sarcoplasmic reticulum, which leads to Ca²⁺ binding troponin, which leads to tropomyosin moving to expose actin binding sites, which leads to myosin heads binding actin and forming cross-bridges, which leads to myosin heads pivoting and pulling actin filaments, which leads to actin sliding over myosin and the sarcomere shortening, which leads to ATP binding and detaching myosin heads, which leads to ATP hydrolysis re-cocking the myosin heads. An arrow returns from the final re-cocking step back to the cross-bridge formation step, showing the cycle repeats.
    The sequence of events in the sliding filament model, from nerve impulse and calcium release through cross-bridge formation and the power stroke to ATP-driven detachment and re-cocking.

    Key words

    Say what each word means before you turn the card.

    Sarcomere structure

    Try answering aloud or in your head before turning the card.

    Sliding filament model

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    Role of Ca²⁺ and troponin

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    ATP in the cross-bridge cycle

    Try answering aloud or in your head before turning the card.

    See every definition
    Sarcomere structure
    Repeating units of a myofibril bounded by Z-lines, containing thick myosin filaments and thin actin filaments.
    Sliding filament model
    Actin filaments slide over myosin as myosin heads pivot, so the sarcomere shortens while the filaments themselves do not change length.
    Role of Ca²⁺ and troponin
    Calcium ions released from the sarcoplasmic reticulum bind troponin, moving tropomyosin to expose actin binding sites.
    ATP in the cross-bridge cycle
    ATP binding detaches myosin heads from actin and ATP hydrolysis re-cocks them for the next cycle.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A vertical flowchart with nine nodes. A nerve impulse leads to Ca²⁺ released from the sarcoplasmic reticulum, which leads to Ca²⁺ binding troponin, which leads to tropomyosin moving to expose actin binding sites, which leads to myosin heads binding actin and forming cross-bridges, which leads to myosin heads pivoting and pulling actin filaments, which leads to actin sliding over myosin and the sarcomere shortening, which leads to ATP binding and detaching myosin heads, which leads to ATP hydrolysis re-cocking the myosin heads. An arrow returns from the final re-cocking step back to the cross-bridge formation step, showing the cycle repeats.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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