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

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.