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Role of ATP and Phosphocreatine

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BIOL 201
Elaine Power

Role of ATP and Phosphocreatine in Providing the Energy Supply During Muscle Contraction Stimulation Of Muscle Fibres By The Nervous System  CONTRACTION → myosin heads attach to actin binding sites / form temporary cross bridges / bridges rapidly break and reform / new cross bridges form further along actin filament / causing shortening of each sarcomere  WHEN STIMULATION STOPS → Ca2+actively taken up by sarcoplasmic reticulum / myosin head detaches from actin / cross bridges reform / muscle relaxes  NO ATP AVAILABLE → cross bridges cannot detach / muscle becomes stiff / unable to relax / extreme form: rigor mortis / occurs after death Cycle Of Events During Contraction Of A Myofibril  Ca2+ ions enter sarcoplasm during wave of depolarisation  Bind to troponin / changes shape of protein / removes block of tropomyosin / exposes actin binding sites  ATP binds to myosin / stimulates ATPase / RELEASES ENERGY  Allows myosin heads to form cross bridges with actin  Allows POWER STROKE: myosin head changes angle / pulls on actin filaments o Width of I band, H zone decrease → filaments overlap increases o Z lines move closer together → length of sarcomere decreases o No change to A band → lengths of filaments stay constant  Allows Ca2+ ions to be pumped back in by active transport  New ATP binds to myosin / allows detachment from actin  Myosin head changes to original position (cross bridges reform)  Next attachment to actin filament and power stroke can occur o Ca2+ and ATP required for cycle to continue Energy In Active Muscle Cells  Breakdown of phosphocreatine / releases PI + energy / attach to ADP / forms ATP
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