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A muscle shortens because filaments slide past one another, not because actin and myosin filaments become shorter. Connect calcium regulation, ATP cycling and whole-muscle action.
What you'll be able to do
Skeletal muscle contains bundles of muscle fibres, each containing many myofibrils. Repeating sarcomeres contain thin actin and thick myosin filaments arranged between Z lines. During contraction, overlap increases and Z lines move closer together; filament lengths remain essentially unchanged.
The A band corresponds to the thick-filament length and stays approximately constant, while the I band and H zone narrow in a contracting sarcomere. Muscles pull rather than actively push, so antagonistic muscle pairs act around joints against a supporting skeleton. Explain which muscle contracts and which relaxes in a specified movement.
Muscle electrical activity causes calcium release from internal stores. Calcium-dependent changes move tropomyosin away from myosin-binding sites on actin, allowing cross-bridges to form. AQA requires the roles of calcium and tropomyosin; detailed recall of troponin is not required for this specification.
An energised myosin head binds actin and undergoes a power stroke. ATP binding permits detachment from actin; ATP hydrolysis re-energises the head for another cycle. Calcium removal helps restore the blocking position and relaxation. ATP is also needed for ion handling, so lack of ATP cannot be described simply as a normal relaxed state.
Phosphocreatine can rapidly donate phosphate to ADP to regenerate ATP during short periods of high demand. Its store is finite and must be replenished, so it is not a permanent replacement for respiratory ATP production.
Slow fibres generally have many mitochondria, good capillary supply and substantial myoglobin, supporting prolonged aerobic activity and fatigue resistance. Fast fibres can generate rapid, powerful contractions and often rely more on rapid glycolytic ATP supply, with greater fatigue in the simplified comparison. Fibre properties vary, so use information supplied about a particular subtype rather than treating every fast fibre as identical.
Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.
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