The Sliding Filament Model of Muscle Contraction · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Ropes That Slide, Not Shrink

Science · The Human Body · ages 17-18
Name ______________________   Date ____________

  1. Which description correctly identifies actin and myosin filaments?

    • Actin forms thick filaments with pulling heads; myosin forms thin filaments.
    • Both proteins form thin filaments with pulling heads; neither forms thick filaments.
    • Actin forms thin filaments; myosin forms thick filaments with pulling heads.

  2. As a sarcomere shortens, how do its bands change?

    • The I bands and H zone narrow; the A band keeps its width.
    • The I bands keep their width; the A band and H zone both become narrower.
    • The I bands and H zone become wider; the A band becomes progressively narrower.

  3. How does calcium expose myosin-binding sites on actin?

    • It binds actin, dissolving thin filaments and exposing the thick myosin filaments.
    • It binds troponin, changing its shape and moving tropomyosin aside.
    • It binds myosin, permanently joining thick filaments to the surrounding thin filaments.

  4. Which statement correctly describes ATP's roles in contraction and relaxation?

    • ATP binding resets myosin; ATP splitting detaches it; calcium pumps work without ATP.
    • ATP binding detaches myosin; ATP splitting shortens actin; calcium pumps work without ATP.
    • ATP binding detaches myosin; ATP splitting resets it; calcium pumps also use ATP.

  5. Start with a reset myosin head attaching to actin. Which sequence follows?

    • ATP binds and detaches the head; power stroke; ATP splits and resets the head.
    • Power stroke; ATP binds and detaches the head; ATP splits and resets the head.
    • ATP splits and resets the head; ATP binds and detaches the head; power stroke.

  6. Why does the A band's width stay constant during sarcomere shortening?

    • It spans the thick myosin filaments, whose length does not change.
    • It spans the gap between the Z lines, which keep the same distance apart.
    • It spans the thin actin filaments, which become progressively shorter as they slide.

  7. In a muscle preparation, calcium is lowered to stop new cross-bridges from forming. No ATP is available. Myosin heads already attached to actin will therefore detach normally.

    Circle one:   True   False

  8. A student says calcium travels from the motor neuron through T tubules to troponin. Which correction fits skeletal muscle signalling?

    • T tubules carry acetylcholine into the fibre, where it binds troponin and exposes sites on actin.
    • T tubules carry a muscle action potential that triggers calcium release from the fibre's internal store.
    • T tubules carry calcium from the neuron, without a muscle action potential or release from an internal calcium store.
LightMySky · lightmysky.comW1-mt_BsB4s4HfD8-s1

Answer key

For grown-ups. Fold this page away before handing over the rest.

Ropes That Slide, Not Shrink W1-mt_BsB4s4HfD8-s1

  1. Actin forms thin filaments; myosin forms thick filaments with pulling heads. · Actin forms the thin filaments. Heads on thick myosin filaments bind and pull actin.
  2. The I bands and H zone narrow; the A band keeps its width. · More overlap reduces the thin-only I bands and thick-only H zone. The A band still spans the unchanged thick-filament length.
  3. It binds troponin, changing its shape and moving tropomyosin aside. · Calcium binds troponin. The resulting shape change moves tropomyosin away from binding sites on actin.
  4. ATP binding detaches myosin; ATP splitting resets it; calcium pumps also use ATP. · ATP binding releases actin. Hydrolysis supplies energy to reset myosin. Separate calcium pumps use ATP to return calcium to storage.
  5. Power stroke; ATP binds and detaches the head; ATP splits and resets the head. · After attachment comes the power stroke. ATP binding releases actin, and hydrolysis of that ATP resets the head.
  6. It spans the thick myosin filaments, whose length does not change. · The A band spans the full length of the thick myosin filaments. Sliding changes overlap, not that length.
  7. False · Preventing new cross-bridges does not release heads already attached to actin. Normal detachment requires ATP binding, so lowering calcium alone cannot produce normal relaxation when ATP is unavailable.
  8. T tubules carry a muscle action potential that triggers calcium release from the fibre's internal store. · The muscle action potential travels through T tubules. Calcium is released from the muscle fibre's sarcoplasmic reticulum and binds troponin.
Worksheet · LightMySky