SPEC 3.6.3 - RESPONSES
Muscle Contraction
Sarcomere structure, sliding filament theory, the roles of calcium ions and ATP, phosphocreatine, and slow vs fast muscle fibres
Structure of Skeletal Muscle
Skeletal muscle is highly organised from organ level down to molecular level — the examiner expects you to know each level of organisation.
LevelStructureDescription
OrganSkeletal muscleAttached to bone via tendons; acts in antagonistic pairs against an incompressible skeleton
CellMuscle fibreGiant multinucleate cell; enclosed by the sarcolemma; contains many myofibrils
OrganelleMyofibrilCylindrical unit inside a muscle fibre; composed of repeating sarcomeres
MolecularSarcomereFunctional unit of a myofibril; region between two Z-lines
Antagonistic Pairs
Muscles can only pull — never push. They work in antagonistic pairs against an incompressible skeleton. When one muscle contracts (the agonist), the other relaxes (the antagonist). Example: biceps and triceps act on the elbow joint.
Sarcomere Ultrastructure
The sarcomere is the unit you must be able to label and explain in detail:
StructureContainsChange during contraction
A-bandThick myosin filaments (+ overlapping actin)Width stays the same
I-bandThin actin filaments onlyNarrows (shortens)
H-zoneMyosin only (central A-band)Narrows / disappears
Z-lineAnchor for actin filamentsMove closer together
Key Molecules
Actin
A thin filament protein arranged in two helical chains. Contains myosin-binding sites. At rest, these sites are blocked by tropomyosin.
Myosin
A thick filament protein with globular heads. Heads bind to actin and hydrolyse ATP. The head acts as an ATPase.
Tropomyosin
A regulatory protein that, at rest, physically blocks the myosin-binding sites on actin. When Ca²⁺ is present, tropomyosin moves to expose the binding sites, allowing cross-bridge formation.
Troponin — Spec Note
Troponin is the protein that Ca²⁺ actually binds to, which then causes tropomyosin to move. However, the role of troponin is NOT required by AQA spec 3.6.3. You only need to know: Ca²⁺ → tropomyosin moves → binding sites exposed. Do not mention troponin unless the question provides it as context.
Sliding Filament Mechanism
Neither actin nor myosin changes length — the sarcomere shortens because actin slides over myosin:
1
AP at NMJ → ACh released → sarcolemma depolarises → AP travels along T-tubules
2
AP reaches sarcoplasmic reticulum → Ca²⁺ released into sarcoplasm
3
Ca²⁺ causes tropomyosin to move → myosin-binding sites on actin exposed
4
Myosin head binds to actin → actinomyosin cross-bridge formed
5
Power stroke: myosin head pivots → actin pulled towards M-line → sarcomere shortens
6
ATP binds to myosin head → cross-bridge detaches from actin
7
ATP hydrolysed by myosin ATPase → myosin head re-cocked (returns to upright position)
8
If Ca²⁺ still present → cycle repeats. When stimulation stops → Ca²⁺ pumped back into SR → tropomyosin covers sites → relaxation
The Three Roles of ATP in Muscle Contraction
ATP is required at three distinct points — the examiner will award marks for each separately:
Role 1: Cross-bridge detachment
ATP binds to the myosin head after the power stroke → the cross-bridge between myosin and actin breaks. Without ATP, the myosin head remains locked onto actin (as in rigor mortis).
Role 2: Re-cocking the myosin head
ATP is hydrolysed to ADP + Pᵢ by myosin ATPase → the energy released returns the myosin head to its upright (cocked) position, ready for the next power stroke.
Role 3: Active transport of Ca²⁺
When stimulation ceases, ATP powers the active transport of Ca²⁺ back into the sarcoplasmic reticulum. This causes tropomyosin to re-cover the binding sites → muscle relaxes.
Exam Technique
Many students only state one or two roles of ATP. The examiner wants all three. A common mistake is saying "ATP provides energy for contraction" — this is too vague. Name each specific step.
Rigor Mortis — ATP in Context
Rigor mortis (stiffening of muscles after death) is a classic AQA application of the ATP cross-bridge detachment role:
Why muscles stiffen after death
After death, ATP production ceases. Without ATP, myosin heads cannot detach from actin after the power stroke — cross-bridges remain permanently formed. Muscles become rigid and fixed at whatever length they were at death.
Why rigor mortis eventually resolves
After ~24–48 hours, proteolytic enzymes (released from lysosomes as cells autolyse) break down the actin and myosin proteins themselves → muscles become flaccid again.
Exam Technique
If asked why muscles stiffen: "ATP is required for myosin heads to detach from actin. After death, ATP is no longer produced, so myosin heads remain permanently bound to actin — cross-bridges cannot be broken."
Phosphocreatine (PC)
Phosphocreatine
An energy store in muscle cells that rapidly donates a phosphate group to ADP to regenerate ATP. Provides a very fast, anaerobic source of ATP for the first few seconds of intense muscle activity.
The Reaction
ADP + Phosphocreatine → ATP + Creatine
Enzyme: creatine kinase. The reaction is rapid and reversible. During recovery from exercise, ATP is used to re-phosphorylate creatine back to phosphocreatine.
Advantages of PC
No oxygen required
No lactate produced
Reaction is almost instantaneous
Provides ATP faster than respiration
Limitations of PC
Limited store — lasts only ~10 seconds
Must be replenished by aerobic respiration during recovery
Slow vs Fast Skeletal Muscle Fibres
AQA requires you to compare the structure, location and properties of slow (Type I) and fast (Type II) fibres:
FeatureSlow twitch (Type I)Fast twitch (Type II)
ATP productionAerobic (oxidative phosphorylation)Anaerobic (glycolysis)
Myoglobin contentHigh — stores O₂Low
ColourRedWhite/pale
MitochondriaManyFew
Fatigue rateSlow to fatigueFatigues rapidly
Contraction speedSlowFast
Typical useEndurance / posture (e.g. back muscles)Short bursts of power (e.g. sprinting)
Memory Aid
Slow = Sustained (aerobic, many mitochondria, high myoglobin, red, resistant to fatigue). Fast = Furious but Fleeting (anaerobic, few mitochondria, low myoglobin, white, rapid fatigue).
What Changes — and What Doesn't
This is a high-frequency examiner favourite — students must explain why each band changes or stays the same:
StructureDuring contractionWhy
SarcomereShortensZ-lines move closer together as actin slides over myosin
A-bandWidth unchangedMyosin filaments do not change length or move — they stay fixed
I-bandNarrowsActin slides into the A-band, reducing the actin-only region
H-zoneNarrows / disappearsActin slides in to overlap with myosin in the central region
Actin filamentLength unchangedActin slides but does not shorten
Myosin filamentLength unchangedMyosin heads pivot but the filament itself does not shorten
Examiner Alert
The most common error: students say "the A-band shortens" during contraction. The A-band width is constant — it equals the length of the myosin filament, which does not change. Only the I-band and H-zone shorten.
DEEP Chain: Muscle Contraction
D
Define Muscle contraction occurs by the sliding filament mechanism: actin filaments slide over myosin filaments, shortening the sarcomere. Neither filament type changes in length.
E
Explain the Mechanism An AP at the NMJ → ACh → sarcolemma depolarises → AP along T-tubules → Ca²⁺ released from sarcoplasmic reticulum → Ca²⁺ causes tropomyosin to move → myosin-binding sites on actin exposed → myosin heads bind actin (cross-bridge) → power stroke: heads pivot, actin pulled toward M-line → ATP binds myosin → cross-bridge detaches → ATP hydrolysed by ATPase → myosin head re-cocked → cycle repeats. Stimulation ceases → Ca²⁺ pumped back into SR (ATP required) → tropomyosin re-covers sites → relaxation.
E
Evidence / Example A-band width is unchanged (myosin constant); I-band and H-zone narrow as actin slides in. ATP has three roles: cross-bridge detachment, re-cocking the myosin head, and active transport of Ca²⁺ back into the SR. Rigor mortis demonstrates that ATP is essential for detachment — without ATP, cross-bridges remain permanently formed.
P
Practical / Quantitative Phosphocreatine (PC) provides rapid ATP regeneration: ADP + PC → ATP + creatine (creatine kinase). This lasts only ~10 seconds before aerobic/anaerobic respiration must take over. Slow twitch fibres (aerobic, many mitochondria, high myoglobin) sustain contraction; fast twitch fibres (anaerobic, few mitochondria) provide rapid but brief bursts.
Examiner Alerts
Error 1: A-band shortens
The A-band does NOT shorten during contraction. Its width equals the length of the myosin filament, which is fixed. Only the I-band and H-zone narrow.
Error 2: Only one role of ATP
ATP has three distinct roles: (1) cross-bridge detachment, (2) re-cocking the myosin head, (3) active transport of Ca²⁺ back into the SR. Stating only "energy for contraction" earns no marks.
Error 3: Naming troponin
Troponin is not required by AQA spec 3.6.3. The expected answer is: Ca²⁺ → tropomyosin moves → binding sites on actin exposed. Do not add troponin unless explicitly prompted.
Error 4: Filaments shorten
Neither actin nor myosin filaments change length during contraction. The sarcomere shortens entirely because actin slides over myosin — the sliding filament mechanism.
Synoptic Links
🔗 Synoptic Link
Synaptic Transmission (6.05): The NMJ links this lesson directly to Lesson 6.05 — the ACh → sarcolemma → T-tubule → Ca²⁺ sequence spans both lessons. Paper 3 questions frequently ask you to trace from nerve impulse through to sarcomere shortening in a single extended response.
🔗 Synoptic Link
ATP & Respiration (5.02–5.03): The three ATP sources for muscle contraction (phosphocreatine, anaerobic glycolysis, aerobic respiration) map directly onto Module 5. Slow twitch fibres use oxidative phosphorylation; fast twitch fibres rely on glycolysis — this connects to the full respiratory pathway.
🔗 Synoptic Link
Protein Structure (1.04): Actin, myosin, and tropomyosin are all proteins whose function depends on tertiary structure. The complementary shape between the myosin head and actin binding site is directly analogous to enzyme–substrate specificity.
🔗 Paper 3 Essay Trigger
Muscle contraction is a prime target for the synoptic essay on "The role of ATP in biological processes" — covering all three ATP roles in contraction plus phosphocreatine regeneration.
Practice Question
A student examines electron micrographs of a sarcomere at rest and during maximum contraction.
Describe the changes to the A-band, I-band and H-zone during contraction, and explain, in terms of the sliding filament mechanism, why each change occurs. [6 marks]
D:
The sliding filament mechanism involves actin filaments sliding over myosin filaments — neither filament type changes in length.
E:
A-band: width unchanged. The A-band spans the full length of the myosin filaments. Since myosin filaments do not change in length or move, the A-band width remains constant throughout contraction.
E:
I-band: narrows. The I-band contains only actin filaments. As actin slides toward the M-line during the power stroke, the actin-only region decreases — so the I-band shortens. H-zone: narrows / disappears. The H-zone is the central region of the A-band containing only myosin. As actin slides inward and overlaps with myosin in this central region, the H-zone becomes smaller and may disappear entirely at maximum contraction.
P:
The mechanism: Ca²⁺ released from the SR causes tropomyosin to move, exposing myosin-binding sites on actin. Cross-bridges form; the power stroke of the myosin head pulls actin toward the M-line. ATP is required to break cross-bridges and re-cock the myosin head for the next cycle. The sarcomere as a whole shortens as Z-lines are pulled closer together.
Summary
Sliding Filament — 8 Steps
StepEventKey molecule
1–2AP → T-tubules → Ca²⁺ released from SRCa²⁺
3Ca²⁺ → tropomyosin moves → sites exposedTropomyosin
4Myosin head binds actin → cross-bridgeActin, myosin
5Power stroke → actin pulled to M-line—
6ATP binds myosin → cross-bridge detachesATP
7ATP hydrolysed → myosin head re-cockedATPase, ADP+Pᵢ
8Stimulation stops → Ca²⁺ pumped to SR → relaxationATP (active transport)
3 Roles of ATP
1. Detach cross-bridge 2. Re-cock myosin head 3. Pump Ca²⁺ back into SR
Phosphocreatine
ADP + PC → ATP + creatine (creatine kinase). Rapid, anaerobic, ~10 seconds only.
Exam Tip
The A-band never changes width. I-band and H-zone both narrow. ATP is needed in three places — detachment, re-cocking, Ca²⁺ return. Troponin is not required — say Ca²⁺ causes tropomyosin to move.
Next: Structure of Skeletal Muscle