Describe the effect of calcium ions on tropomyosin and actin in skeletal muscle.

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Within the muscle fibre tropomyosin is wrapped around actin filaments and it blocks the binding sites for the myosin-heads.

When calcium ion concentration increases, calcium ions bind to sites on the tropomyosin (troponin-c sites) and this causes a conformational change in shape.

This change in tropomyosin’s shape exposes the myosin-head binding sites on the actin filaments and allows cross-bridge binding to occur.

This then allows the actin and myosin fibres to slide over each other and shorten (contract) the muscle fibre.  This is known as Sliding Filament Theory.  

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