How is the structure of skeletal muscle related to the way it contracts, and what is the role of ATP and calcium ions?
Muscles and movement: the ultrastructure of skeletal muscle and the sarcomere (actin, myosin, the A band, I band and H zone), the sliding filament theory of contraction, the roles of calcium ions, troponin, tropomyosin and ATP in the cross-bridge cycle, the neuromuscular junction, and the supply of ATP for contraction by aerobic respiration, anaerobic respiration and creatine phosphate.
A CCEA A-Level Biology answer on muscles and movement. Covers skeletal muscle ultrastructure and the sarcomere, the sliding filament theory, the roles of calcium ions, troponin, tropomyosin and ATP in the cross-bridge cycle, the neuromuscular junction, and how ATP is supplied for contraction.
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What this dot point is asking
CCEA wants you to describe the ultrastructure of skeletal muscle and the sarcomere, explain the sliding filament theory of contraction and the roles of calcium ions, troponin, tropomyosin and ATP, describe the neuromuscular junction, and explain how ATP is supplied for contraction by aerobic respiration, anaerobic respiration and creatine phosphate.
Skeletal muscle ultrastructure
Within a sarcomere the bands are named by what filaments are present. The A band is the full length of the myosin filaments (where myosin is present, with or without overlapping actin). The I band is the region with only actin (either side of a Z line). The H zone is the central region with only myosin. Muscle fibres also contain many mitochondria (to supply ATP) and a network called the sarcoplasmic reticulum that stores and releases calcium ions.
The sliding filament theory
The contraction is driven by the cross-bridge cycle, which needs calcium ions and ATP:
- An impulse causes calcium ions to be released from the sarcoplasmic reticulum into the cytoplasm.
- Calcium ions bind to troponin, changing its shape so that tropomyosin moves away from the actin, exposing the myosin binding sites.
- Myosin heads attach to the actin binding sites, forming cross-bridges.
- The myosin heads bend (the power stroke), pulling the actin towards the centre of the sarcomere; this shortens the sarcomere.
- ATP binds to the myosin head, causing it to detach from the actin.
- The myosin head hydrolyses the ATP (it acts as an ATPase), using the energy to return to its upright position ready to bind further along the actin. The cycle repeats while calcium ions and ATP remain.
When stimulation stops, calcium ions are pumped back into the sarcoplasmic reticulum (using ATP), tropomyosin covers the binding sites again, and the muscle relaxes.
The neuromuscular junction
The neuromuscular junction is a specialised synapse where a motor neurone meets a muscle fibre. An action potential arriving at the neurone triggers the release of the neurotransmitter acetylcholine, which diffuses across the cleft and binds to receptors on the muscle fibre membrane. This depolarises the membrane and the impulse spreads into the fibre, triggering the release of calcium ions and the cross-bridge cycle described above.
Supplying ATP for contraction
Contraction continually uses ATP (for the power stroke recovery, for detaching the heads, and for pumping calcium back). A muscle fibre supplies this ATP in three ways:
- Aerobic respiration in the many mitochondria is the main source during normal activity, giving a high yield of ATP from glucose and oxygen.
- Anaerobic respiration provides extra ATP quickly during intense exercise when oxygen runs short, but produces lactate and gives a low yield.
- Creatine phosphate stored in the fibre rapidly transfers a phosphate to ADP to regenerate ATP for the first few seconds of effort, before respiration catches up.
Examples in context
Example 1. Rigor mortis. After death, ATP production stops. Without ATP the myosin heads cannot detach from the actin, so the cross-bridges stay locked and the muscles become rigid. This everyday observation confirms that ATP is essential for the myosin head to release from actin, not just for the power stroke.
Example 2. A sprint versus a marathon. A 100 m sprinter relies heavily on creatine phosphate and anaerobic respiration for rapid ATP, building up lactate and an oxygen debt. A marathon runner works at a pace that aerobic respiration can sustain, using the many mitochondria in slow muscle fibres, which is why pacing matches the ATP supply route to the demand.
Try this
Q1. Name the ion that binds to troponin to start contraction. [1 mark]
- Cue. Calcium ions, released from the sarcoplasmic reticulum.
Q2. Explain why the A band does not change width during contraction. [2 marks]
- Cue. The A band is the length of the myosin filaments, which do not change length; only the actin slides, so the A band stays the same while the I band and H zone narrow.
Q3. State two ways a muscle fibre can regenerate ATP quickly during intense exercise. [2 marks]
- Cue. Anaerobic respiration (producing lactate) and transfer of phosphate from creatine phosphate to ADP.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20196 marksDescribe the sliding filament theory of muscle contraction, including the roles of calcium ions and ATP.Show worked answer →
A 6-mark describe answer needs the trigger, the cross-bridge cycle and the role of ATP.
When an impulse arrives, calcium ions are released from the sarcoplasmic reticulum. The calcium ions bind to troponin, changing its shape, which moves the tropomyosin away from the binding sites on the actin filament. This exposes the myosin binding sites.
Myosin heads attach to the actin binding sites, forming cross-bridges. The myosin heads then bend (the power stroke), pulling the actin filaments past the myosin towards the centre of the sarcomere, so the sarcomere shortens. ATP then binds to the myosin head, causing it to detach from the actin.
ATP is hydrolysed by the myosin head (which acts as an ATPase), releasing energy that returns the head to its upright position, ready to bind further along the actin. The cycle repeats many times as long as calcium ions and ATP are present, so the filaments slide further and the muscle contracts.
Markers reward (1) calcium release and binding to troponin, (2) tropomyosin moving to expose binding sites, (3) cross-bridge formation, (4) the power stroke pulling actin, (5) ATP causing detachment, and (6) ATP hydrolysis re-cocking the head.
CCEA 20214 marksDescribe what happens to the appearance of a sarcomere, including the A band, I band and H zone, when a muscle contracts.Show worked answer →
The filaments do not get shorter; they slide past one another, so the bands change in a predictable way.
During contraction the actin (thin) filaments slide between the myosin (thick) filaments towards the centre of the sarcomere, so the sarcomere as a whole shortens and the two Z lines move closer together.
The A band, which corresponds to the length of the myosin filaments, stays the same width because the myosin filaments do not change length. The I band (where only actin is present) gets narrower as the actin slides in. The H zone (the central region with only myosin) also gets narrower, because the actin filaments overlap further into the centre.
Markers reward the sarcomere shortening with Z lines closer, the A band unchanged, and the I band and H zone narrowing.
CCEA 20184 marksDescribe how a muscle fibre obtains the ATP needed for repeated contractions during intense exercise.Show worked answer →
A full answer covers the three sources and when each is used.
Most ATP is normally supplied by aerobic respiration in the many mitochondria of the muscle fibre, using glucose and oxygen. During intense exercise, when oxygen cannot be delivered fast enough, anaerobic respiration provides extra ATP quickly by glycolysis, producing lactate, although the yield per glucose is much lower.
A muscle fibre also stores creatine phosphate, which can rapidly transfer a phosphate group to ADP to regenerate ATP for the first few seconds of intense effort, before respiration catches up. After exercise, the oxygen debt is repaid to remove the lactate.
Markers reward aerobic respiration as the main source, anaerobic respiration producing lactate when oxygen is limiting, and creatine phosphate regenerating ATP rapidly at the start.
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Sources & how we know this
- CCEA GCE Biology specification — CCEA (2016)