AQA BIOL5 Muscle contraction

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  • Muscle Contraction
    • Structure of a skeletal muscle
      • Actin - thinner with tropomyosin covering the binding sites
      • Myosin - thicker with long rod-shaped fibres and bulbous heads
      • Light bands = Isotropic (I bands) just actin
      • Z-lines. Distance between each Z-line is a sarcomere
      • Medium colour region = H zone Only myosin
      • Dark bands = Anisotropic (A bands) Actin and Myosin overlap
    • Slow-twitch muscle fibres
      • Contract more slowly
      • Provide less powerful contractions over longer period
      • Adapted for aerobic respiration
      • Supply of glycogen
      • Large supply of blood vessels
      • For endurance
    • Fast-twitch muscle fibres
      • Contract more rapidly
      • Powerful contractions over a short period
      • Adapted for intense exercise
      • Thicker and more numerous myosin filaments
      • Anaerobic respiration enzymes
      • Lots of phospho creatine
    • The Sliding filament mechanism of muscle contraction
      • The action potential travels down T tubules to the ER
      • The action potential opens calcium channels on the ER releasing calcium into the cytoplasm
      • The calcium ions cause the tropomyosin to move so are not blocking the actin binding sites
      • An ADP molecule attaches to myosin head and myosin heads attaches to binding site forming a cross-bridge
      • Once attached the myosin filaments cocks its head creating a power stoke thus pulling actin along and relasing the ADP molecule
      • An ATP molecule attaches to the myosin head to detach it and calcium ions activate ATPase to hydrolyse the ATP providing energy for the mysoin head to return to its original angle to bind again
      • Where nervous stimulation ceases calcium ions active transported back into ER using hydrolysis energy
      • Tropomyosin blocks the actin binding sites again so myosin heads cannot bind
    • Energy supply during muscle contraction
      • Needed for the movement of myosin heads
      • Needed for the re absorption of calcium ions
      • In an active muscle where there is no time for blood supply to replenish ATP need a means of generating ATP anaerobically
      • Phospho creatine regenerates ATP
      • Phospho creatine is stored in muscle as a reserve for phosphate to combine with ADP
      • Store replenished using phosphate from ATP when muscle is relaxed
  • Evidence for sliding filament mechanism
    • More overlap of actin and myosin in a contracted muscle
    • The I band becomes narrower
    • The Z=lines move closer together
    • The H zone becomes narrower
    • The A band stays the same width
    • Muscle Contraction
      • Structure of a skeletal muscle
        • Actin - thinner with tropomyosin covering the binding sites
        • Myosin - thicker with long rod-shaped fibres and bulbous heads
        • Light bands = Isotropic (I bands) just actin
        • Z-lines. Distance between each Z-line is a sarcomere
        • Medium colour region = H zone Only myosin
        • Dark bands = Anisotropic (A bands) Actin and Myosin overlap
      • Slow-twitch muscle fibres
        • Contract more slowly
        • Provide less powerful contractions over longer period
        • Adapted for aerobic respiration
        • Supply of glycogen
        • Large supply of blood vessels
        • For endurance
      • Fast-twitch muscle fibres
        • Contract more rapidly
        • Powerful contractions over a short period
        • Adapted for intense exercise
        • Thicker and more numerous myosin filaments
        • Anaerobic respiration enzymes
        • Lots of phospho creatine
      • The Sliding filament mechanism of muscle contraction
        • The action potential travels down T tubules to the ER
        • The action potential opens calcium channels on the ER releasing calcium into the cytoplasm
        • The calcium ions cause the tropomyosin to move so are not blocking the actin binding sites
        • An ADP molecule attaches to myosin head and myosin heads attaches to binding site forming a cross-bridge
        • Once attached the myosin filaments cocks its head creating a power stoke thus pulling actin along and relasing the ADP molecule
        • An ATP molecule attaches to the myosin head to detach it and calcium ions activate ATPase to hydrolyse the ATP providing energy for the mysoin head to return to its original angle to bind again
        • Where nervous stimulation ceases calcium ions active transported back into ER using hydrolysis energy
        • Tropomyosin blocks the actin binding sites again so myosin heads cannot bind
      • Energy supply during muscle contraction
        • Needed for the movement of myosin heads
        • Needed for the re absorption of calcium ions
        • In an active muscle where there is no time for blood supply to replenish ATP need a means of generating ATP anaerobically
        • Phospho creatine regenerates ATP
        • Phospho creatine is stored in muscle as a reserve for phosphate to combine with ADP
        • Store replenished using phosphate from ATP when muscle is relaxed

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