AQA A-level Biology: Muscle Contraction (7402) – Revision Notes

These notes condense section 3.6.3 of the AQA AS and A-level Biology specification for first teaching 2015 (7402), which is A-level only. The specification does not require the role of troponin, so it is not included. For full explanations and worked examples, use the study guide, then try the practice questions. Course pages: AQA A-level Biology hub and the printable checklist. Not sure where your gaps are? Take the free diagnostics.

Key terms

Term	Meaning
Antagonistic pair	Two muscles whose actions oppose: one contracts while the other relaxes
Muscle fibre	A long, multinucleate cell of skeletal muscle
Sarcolemma	Cell surface membrane of a muscle fibre
Sarcoplasmic reticulum	Endoplasmic reticulum of muscle that stores calcium ions
Myofibril	Bundle of actin and myosin filaments inside a fibre, made of sarcomeres
Sarcomere	Distance from one Z line to the next
Actinomyosin bridge	The link formed when a myosin head binds to actin
Phosphocreatine	Phosphate store that regenerates ATP from ADP without oxygen

Structure at a glance

Muscle, then bundles of fibres, then one fibre, then myofibrils, then sarcomeres, then filaments.

Region	Appearance	Contains	Change in contraction
A band	Dark	Thick (myosin) filaments, whole length	None
I band	Light	Thin (actin) filaments only	Shorter
H zone	Centre of A band	Thick filaments only	Narrower, may vanish
Z line	Dark line	Anchors thin filaments	Lines move closer
M line	Centre of A band	Links thick filaments	Stays in the middle
Sarcomere	Z to Z	One repeating unit	Shorter

Under the optical microscope, skeletal muscle in longitudinal section shows parallel fibres, striations and several nuclei per fibre near the edge.

Method: the cross-bridge cycle in order

1. Nerve impulse, depolarisation of the sarcolemma, release of calcium ions from the sarcoplasmic reticulum.
2. Calcium ions move tropomyosin, exposing the binding sites on actin.
3. Myosin head (with ADP) binds to actin: actinomyosin bridge.
4. Head tilts, pulling actin towards the centre of the sarcomere. ADP is released.
5. New ATP binds, so the head detaches.
6. ATPase on the head hydrolyses ATP to ADP and phosphate, and the head returns to its starting position.
7. The cycle repeats while calcium ions are present.
8. Stimulation stops: calcium ions are actively transported back into the sarcoplasmic reticulum (needs ATP), tropomyosin blocks the sites, the muscle relaxes.

Roles to learn word for word

Molecule	Role in contraction
Actin	Thin filament; has binding sites for myosin heads; is pulled towards the centre of the sarcomere
Myosin	Thick filament with heads that bind actin, tilt and act as an ATPase
Tropomyosin	Covers the binding sites on actin at rest; moved by calcium ions
Calcium ions	Released by the sarcoplasmic reticulum; cause tropomyosin to move
ATP	Detaches the head from actin; hydrolysed to reset the head; powers calcium ion pumps
Phosphocreatine	Phosphorylates ADP to ATP quickly, with no oxygen or lactate

ATP supply

Source	Speed	Duration
Phosphocreatine	Fastest	Seconds
Anaerobic glycolysis	Fast	Short; lactate builds up
Aerobic respiration	Slower	Long periods

Phosphocreatine + ADP → creatine + ATP. It is rebuilt during recovery with ATP from aerobic respiration.

Slow and fast twitch fibres

Feature	Slow twitch	Fast twitch
Speed and force	Slower, weaker	Faster, stronger
Fatigue	Resistant	Quick
ATP supply	Aerobic	Anaerobic and phosphocreatine
Mitochondria, capillaries	Many	Fewer
Myoglobin	A lot (red)	Little
Glycogen and phosphocreatine	Less	More
Myosin filaments	Thinner	Thicker
Typical location	Postural muscles, such as the back and calf	Muscles for rapid powerful action, such as eye muscles and limb muscles in sprinting

Must-know distinctions

• Filaments slide; filaments do not shorten. The A band is constant.
• Calcium ions affect tropomyosin, not the head directly.
• ATP detaches the head; the tilt happens after binding.
• Phosphocreatine is not a respiratory substrate. It gives its phosphate to ADP, so it regenerates ATP.
• Flexor and extensor both exist at one joint; whichever contracts, the other relaxes.

Method: a micrograph calculation

1. Measure the image length in mm.
2. Actual length = image length ÷ magnification.
3. Convert mm to µm by multiplying by 1000.
4. I band in a sarcomere = sarcomere − A band.
5. H zone = sarcomere − 2 × thin filament length.

Antagonistic pairs and the skeleton

• Muscles pull only when they contract; they cannot push or lengthen themselves.
• The skeleton is incompressible, so it acts as a rigid lever.
• Elbow: biceps (flexor) contracts and triceps (extensor) relaxes to bend; the reverse straightens the arm.
• Tendons join muscle to bone.
• Model answer shape: name the contracting muscle, name the relaxing muscle, say what moves.

Worked reminders

Phosphocreatine store. Data supplied: 3 mmol ATP and 12 mmol phosphocreatine per kg, ATP used at 1.5 mmol per kg per second.

1. ATP alone: 3 ÷ 1.5 = 2 s.
2. With phosphocreatine: (3 + 12) ÷ 1.5 = 10 s.
3. Extra time provided: 10 − 2 = 8 s.

Fibre comparison. Fibre P reaches fatigue after 35 s of continuous stimulation and fibre Q after 840 s.

1. Q is the slow twitch fibre, as it resists fatigue.
2. 840 ÷ 35 = 24, so Q lasts 24 times longer.
3. Link to structure: more mitochondria and myoglobin allow aerobic respiration to carry on supplying ATP.

Fatigue

• Repeated contraction needs more ATP than aerobic respiration alone can supply.
• Anaerobic glycolysis makes lactate, so pH falls and enzyme activity drops.
• Investigation: count grip squeezes in equal time blocks; control load, time and rest; take informed consent from volunteers.
• Describe the trend with figures from the table, for example a percentage decrease between the first and last blocks.

Rigor mortis

After death, ATP is no longer made. Calcium ions stay in the sarcoplasm and cross-bridges stay attached because no ATP binds to detach the heads. The muscle stays stiff until the tissue breaks down.

Answer frames for longer questions

Explain how the sarcomere shortens. Calcium ions released, tropomyosin moves, binding sites exposed, myosin heads bind to actin, heads tilt and pull actin inwards, ATP detaches the heads, ATP hydrolysis resets them, cycle repeats, Z lines come closer.

Explain what happens when stimulation stops. Calcium ions pumped back by active transport, tropomyosin covers the sites again, no bridges, muscle relaxes, antagonist stretches it.

Explain why a sprinter relies on phosphocreatine. It regenerates ATP in one step, faster than glycolysis or aerobic respiration, with no oxygen needed and no lactate made; the store is small, so it supports only a few seconds of effort.

Quick self-test

1. A sarcomere measures 55 mm on an image at ×25 000. Calculate its actual length in µm.
2. It contracts to 1.8 µm. Calculate the percentage decrease in length, to 1 decimal place.
3. Which band does not change length during contraction, and why?
4. A sarcomere is 42 mm on an image at ×20 000 and each thin filament is 0.8 µm long. Calculate the H zone length.
5. Muscle holds 4 mmol of ATP and 11 mmol of phosphocreatine per kg and uses ATP at 2.5 mmol per kg per second. How long does ATP alone last, and ATP plus phosphocreatine?
6. State what tropomyosin does in a relaxed muscle.
7. Give two roles of ATP in contraction.
8. Fibre sample: 90 of 200 fibres are fast twitch. Give the percentage of slow twitch fibres.
9. Name two features of slow twitch fibres that suit them to endurance.
10. Why does a muscle need an antagonist?

Answers

1. 55 ÷ 25 000 = 0.0022 mm = 2.2 µm.
2. (2.2 − 1.8) ÷ 2.2 × 100 = 18.2%.
3. The A band, because the thick myosin filaments stay the same length as the thin filaments slide.
4. 42 ÷ 20 000 = 0.0021 mm = 2.1 µm; 2.1 − (2 × 0.8) = 0.5 µm.
5. ATP alone: 4 ÷ 2.5 = 1.6 s. With phosphocreatine: (4 + 11) ÷ 2.5 = 6.0 s.
6. It covers the myosin binding sites on actin, so no bridges form.
7. Any two: detaches the head from actin; hydrolysed by ATPase to reset the head; powers the active transport of calcium ions back into the sarcoplasmic reticulum.
8. 200 − 90 = 110; 110 ÷ 200 × 100 = 55% slow twitch.
9. Any two: many mitochondria, much myoglobin, rich capillary supply, resistance to fatigue.
10. A muscle can only pull when it contracts, so a second muscle must pull the bone back the other way.

Where marks are usually lost

• Writing that the A band, or the filaments, get shorter.
• Saying calcium ions bind to myosin or actin.
• Leaving out that a fresh ATP is needed to detach the head.
• Forgetting the ATP used to pump calcium ions back.
• Describing phosphocreatine as a respiratory substrate instead of a phosphate donor.
• Not converting mm to µm in micrograph questions.
• Naming a pair of muscles without saying which one relaxes.
• Calling fast fibres red or slow fibres pale.

Official syllabus

AQA AS and A-level Biology specification (7401, 7402), for first teaching 2015, AQA. Section 3.6.3, Skeletal muscles are stimulated to contract by nerves and act as effectors (A-level only).