BIOL 2020U Lecture Notes - Capillary, Citric Acid Cycle, Glycogen

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Published on 29 Jan 2013
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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
o PHOSPHOCREATINE + ADP → CREATINE + ATP
o ATP is used faster than it can be supplied by respiration
o Phosphocreatine allows regeneration of ATP without respiration
Thus, Muscle cells continue exercise until slower pathways synthesis ATP
o Breakdown of glycogen in muscle cells / aerobic respiration of glucose
o Aerobic respiration of glucose, fatty acids from bloodstream / fatty acids last
longer
Prolonged exercise / not enough O2 for aerobic respiration
o Anaerobic respiration continues
o Lactate may cause cramps
Table 16-9-1: Structure, location and general properties of slow and fast skeletal muscle fibres
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