Sliding Filament Model/Power Stroke

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1. Outline the structure of actin

  • Found at the centre of the myosin filaments, no myosin heads are in this region of myosin filaments
  • The actin filament is formed from a helix of actin sub-units, each contains a binding site for the myosin heads, two other proteins, tropomyosin and troponin are attached to the actin fibre
  • ADP and Pi, this entire process repeats, causing the sarcomere to contract fully
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2. Calcium ions allow muscles to contract, without them, the binding sites for the myosin head group on the actin fibre are covered by tropomyosin subunits, this means that

  • A myosin head group can't attatch to any such binding site, cross bridges can't form and muscle contraction can't occur
  • Changes the shape of the troponin, which moves the tropomyosin away from the binding sites on the actin - this means cross bridges can form allowing the power stroke and muscle contraction to occur

3. When nervous stimulation stops at the neuromuscular junction, calcium ions are ________ back into sarcoplasmic reticulum by caarrier proteins on the membrane, this leads to muscle relaxtion

  • Wound around the actin, reinforcing it
  • Actively transported
  • Changes the shape of the troponin, which moves the tropomyosin away from the binding sites on the actin - this means cross bridges can form allowing the power stroke and muscle contraction to occur

4. There is only sufficient ATP available in a muscle fibre for around

  • 1-2 seconds worth of contraction
  • The phosphate group from creatine phosphate can be transfered to ADP to form ATP quickly by the enzyme creatine phosphotransferase - this enables muscle contraction for a further 2 - 4 seconds

5. When an action potential arrives via a neurone, at nmj, calcium ions are released from the sarcoplasmic reticulum - these calcium ions diffuse through the sarcoplasm and bind to troponin molecules (bead structure on tropomyosin) - this binding

  • Changes the shape of the troponin, which moves the tropomyosin away from the binding sites on the actin - this means cross bridges can form allowing the power stroke and muscle contraction to occur
  • A myosin head group can't attatch to any such binding site, cross bridges can't form and muscle contraction can't occur

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