International A Level Biology / Muscles, movement and the heart

The sliding filament theory

Edexcel International A Level Biology (YBI11), chapter 21: Muscles, movement and the heart, specification point 7.11. Free lesson.

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The sliding filament theory explains how actin and myosin slide past each other to shorten a sarcomere. Calcium ions expose actin’s binding sites, allowing myosin heads to form cross-bridges and pull the actin.

What actin and myosin are made of

The thin filament is mostly actin, arranged as two twisted strands. The thick filament is a bundle of myosin molecules. Their heads stick out towards the actin and can attach to it. Each actin has a myosin binding site where a myosin head can attach. Tropomyosin is a protein strand wound along the actin. Troponin is attached to it and binds calcium ions.

How calcium ions expose actin’s binding sites

At rest, tropomyosin covers the binding sites, so myosin heads cannot attach. When an impulse arrives, calcium ions are released from the sarcoplasmic reticulum. They bind to troponin, changing its shape, so troponin pulls tropomyosin aside. The exposed sites allow myosin heads to bind to actin, forming cross-bridges. The impulse travels along the sarcolemma and down the T tubules. When impulses stop, calcium ions are actively pumped back into the sarcoplasmic reticulum using ATP. Tropomyosin covers the sites again, and the muscle relaxes.

How the cross-bridge cycle pulls actin

The cycle repeats while calcium ions keep the binding sites exposed and ATP is available. The power stroke happens while the myosin head is attached to actin. It tilts and pulls actin towards the centre of the sarcomere, releasing ADP and phosphate. ATP binds to the myosin head, making it detach from actin and breaking the cross-bridge. The enzyme ATPase in the head hydrolyses ATP to ADP and phosphate. The released energy re-cocks the detached head, ready to bind further along the actin.

Which parts of a sarcomere change length

As actin slides towards the centre, the Z lines move closer and the sarcomere shortens. The filaments themselves do not shorten. Instead, their overlap changes. The I band shortens because more actin overlaps the myosin. The H zone shortens and may disappear because actin moves into the middle. The A band stays the same length because it represents the length of the myosin filaments.

Using sarcomere lengths in a calculation

The I band is the part of the sarcomere outside the A band. Since the A band stays the same length, subtract it from the sarcomere length to find the I band. For a ratio of relaxed to contracted length, divide the relaxed length by the contracted length and write the result as a ratio with the relaxed value first.

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