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A level Had a look Nearly there Nailed it! 5.1.5l; 1.1.2a Muscular contraction Muscles contract through the interaction of two key proteins, actin and myosin, using ATP as an energy source. For contraction to occur, part of the myosin molecule (head group) attaches to a binding site on the actin filament; a cross-bridge is formed. The head group bends, pulling the actin filament along, and ADP and Pi are released – the power stroke. The attachment of a new ATP molecule to the myosin head breaks the cross-bridge. The head group moves back to its original confirmation as ATP is hydrolysed to ADP and Pi, so another cross-bridge can be formed. ADP Contraction cycle ADP continues if ATP is available and ATP Pi Ca2+ level in the sarcoplasm is high Motor neurone 1 Impulses arriving at the neuromuscular junction cause vesicles to fuse with the pre-synaptic membrane and release acetylcholine into the gap Vesicles Muscle fibre membrane − 6 Acetylcholinesterase in the gap rapidly breaks down acetylcholine so that contraction only occurs when impulses arrive continuously + − − − 3 Depolarisation + wave travels + 2 Acetylcholine binds to down tubules + + receptors on the muscle (T system) + fibre membrane − − (sarcolemma) causing depolarisation − − − − + + + + + + Ca2+ Ca2+ Ca2+ Ca2+ 5 Ca2+ binds to proteins in the muscle, which leads to contraction ADP ADP The neuromuscular junction Myelin sheath The power stroke Pi Mechanism of muscular contraction 4 T system depolarisation leads to Ca2+ release from stores in sarcoplasmic reticulum (specialised endoplasmic reticulum) The binding of Ca2+ to troponin causes the myosin binding sites on the actin filament (hidden by tropomyosin) to be revealed so the ‘power stroke’ can proceed. ATP is unstable and only small amounts of it exist in cells at any one time. Contraction of muscles uses lots of ATP and the ATP immediately available is used up after 1−2 seconds. Suggest how more ATP can be supplied to contracting muscles. (3 marks) Aerobic respiration in muscle cell mitochondria produces ATP. Blood brings oxygen to muscle cells so glucose can be respired to generate ATP from the Krebs cycle and oxidative phosphorylation. Monitoring muscle contraction Investigations using the calf muscle removed from a Single stimulus frog’s leg with the nerve still attached, show how an increase in stimulus frequency reduces time between contractions. Repeated large stimuli, close together, give a sustained and powerful contraction known as Time tetanus. Single stimulus Time Repeated stimuli 118 Contraction recorded Repeated stimuli Increasing level of stimulation Time Contraction recorded Multiple stimuli lead to tetanus; eventually muscle fatigue reduces the level of contraction ow would a lack H of Ca2+ affect muscle contraction? (2 marks) Pi