OxfordAQA International AS and A-level Biology: Skeletal muscle and sliding filaments (9610) – Revision Notes

Condensed recall for section 3.4.4, skeletal muscles as effectors, in OxfordAQA International AS and A-level Biology (9610, Version 5.1). For full explanations, use the study guide. Test yourself with the practice set, tick off the printable checklist, return to the course hub, and find weak areas with the free diagnostics.

3.4.4.1 Structure

Gross: muscle = bundles of fibres in connective tissue, with blood supply and motor neurones; tendons join muscle to bone.

Fibre (one multinucleate cell):

Part	Job
Sarcolemma	Cell surface membrane
Sarcoplasm	Cytoplasm: mitochondria, glycogen, myoglobin
Sarcoplasmic reticulum	Stores calcium ions, releases them on stimulation
Myofibril	Bundle of filaments; chain of sarcomeres

Sarcomere (Z line to Z line):

Z |---- I ----|=========== A band ===========|---- I ----| Z
  thin only        thick + thin overlap  H zone   thin only
                                      (thick only)

• Thin filament: actin, with tropomyosin along it, attached to Z lines.
• Thick filament: myosin, with globular heads that bind actin.
• A band = length of thick filaments.
• H zone = thick filaments only.
• I band = thin filaments only.

The sliding filament theory

Component	Role
Calcium ions	Released from sarcoplasmic reticulum; make tropomyosin move off the binding sites
Tropomyosin	Blocks myosin-binding sites on actin at rest
Actin	Thin filament; carries binding sites; is pulled
Myosin	Head binds actin (actomyosin bridge), tilts, pulls; head has ATPase activity
ATP	Binds head, causing detachment; hydrolysis resets the head; powers calcium pumps

Method in steps: the cycle

1. Calcium ions present, binding sites exposed.
2. Myosin head (with ADP and phosphate) binds actin: bridge.
3. Head tilts, thin filament pulled towards the sarcomere centre; ADP and phosphate released.
4. ATP binds head: head detaches.
5. ATP hydrolysed by ATPase: head returns to original position.
6. Repeat while calcium ions stay high.

Relaxation: no more stimulation; ATP-driven pumps return calcium ions to the sarcoplasmic reticulum; tropomyosin covers the sites again; bridges cannot form; the muscle lengthens passively.

Stiffness without ATP: with no ATP the heads cannot detach, so bridges stay in place and the muscle stays rigid.

What changes on contraction

Feature	Contraction	Relaxation
Sarcomere	Shorter	Longer
I band	Shorter	Longer
H zone	Shorter (may vanish)	Longer
A band	Same	Same
Z lines	Closer	Further apart
Filament lengths	Same	Same

Reading images: formulas

Quantity	Rule
Actual length	image length ÷ magnification
Magnification	image length ÷ actual length
I band in one sarcomere	sarcomere − A band
H zone	sarcomere − (2 × thin filament length), if positive
Percentage change	change ÷ original × 100

Convert mm to µm: multiply by 1000.

Worked reminder. At ×16 000, a sarcomere of 41.6 mm is 41.6 ÷ 16 000 = 0.0026 mm = 2.6 µm. If the thin filaments are 0.95 µm, the H zone is 2.6 − 1.9 = 0.7 µm. If it then shortens to 2.1 µm, the decrease is (2.6 − 2.1) ÷ 2.6 × 100 = 19.2%.

Method in steps: ordering myofibril images

1. Find Z lines and measure each sarcomere (or compare spacing).
2. Longer sarcomere = more relaxed.
3. Check that the A band is the same in every image.
4. Compare I band and H zone widths.
5. State the order and give the evidence.

3.4.4.2 Antagonistic pairs and the skeleton

• Muscles can only pull, not push.
• In a pair, one contracts while the other relaxes; each reverses the other’s action.
• The skeleton is incompressible: bones act as levers and transmit the pull through tendons.
• Examples: biceps and triceps at the elbow; gastrocnemius and tibialis anterior at the ankle.

Energy supply

Source	Features
ATP store	Small; lasts a moment
Phosphocreatine	Phosphate passes to ADP to make ATP and creatine; very fast; no oxygen; no lactate; runs out in a few seconds
Anaerobic glycolysis	Quick extra ATP; lactate formed
Aerobic respiration	Mitochondria; sustained supply; also rebuilds phosphocreatine at rest

Worked reminder. Stores of 4.0 mmol ATP and 12.0 mmol phosphocreatine per kg, used at 2.0 mmol per kg each second: ATP alone 2.0 s; both together (4.0 + 12.0) ÷ 2.0 = 8.0 s.

Slow and fast fibres

	Slow	Fast
Speed and force	Slow, sustained	Rapid, forceful
Fatigue	Resistant	Quick
ATP source	Aerobic	Anaerobic glycolysis and phosphocreatine
Colour	Red (myoglobin)	Pale
Mitochondria and capillaries	Many	Fewer
Location	Postural muscles, for example in the back and calf	Muscles that move quickly, for example around the eye and in the hand

Model answers to recall

“Explain how a muscle fibre shortens” (a suggested structure). Calcium ions are released from the sarcoplasmic reticulum. They cause tropomyosin to move, exposing binding sites on actin. Myosin heads bind to actin, forming actomyosin bridges. The heads change shape, dragging the thin filaments past the thick filaments towards the middle of the sarcomere. ATP binds, so the heads detach. ATP hydrolysis resets the heads, and the cycle repeats while calcium ions remain. The Z lines approach one another, so the sarcomere is shorter.

“Explain why the A band does not change length.” The A band marks the length of the thick filaments, which stay constant. Thin filaments move between them, so the I band and H zone change instead.

“Explain why muscles work in pairs.” A contracting muscle can only pull. Its partner must contract to pull the bone back the other way, while the first relaxes. The incompressible skeleton acts as a lever and transmits the pull.

“Suggest why phosphocreatine is useful at the start of a sprint.” It makes ATP in one fast step, needs no oxygen, and the supply of ATP from respiration has not yet risen to match the demand.

Quick comparison of what is seen at each stage

State	Sarcomere	I band	H zone	A band
Stretched	Longest	Widest	Widest	Constant
Resting	Long	Wide	Present	Constant
Partly contracted	Shorter	Narrower	Narrower	Constant
Fully contracted	Shortest	Narrowest	Gone	Constant

When you answer an image question, name the band, say whether it is longer or shorter than in the comparison image, and then link it to the filaments sliding.

Fibre data reminder

Percentage of slow fibres = slow fibres ÷ total fibres × 100. In 200 fibres, 130 slow gives 65%. A higher percentage of slow fibres suggests better endurance. A table showing more mitochondria and more capillaries per fibre points to a slow fibre; a shorter time to peak force and a faster fall in force points to a fast fibre.

Must-know distinctions

• Filaments slide; they do not shorten. A band constant; I band and H zone shorten.
• Detach versus reset. ATP binding detaches the head; ATP hydrolysis resets it.
• Calcium ions expose; tropomyosin blocks. Both are needed for the story.
• Antagonist versus agonist. The contracting muscle is the prime mover; its partner is relaxed.
• Phosphocreatine versus ATP. Phosphocreatine regenerates ATP; it does not power the head directly.

Quick self-test

1. Which band stays the same length when a sarcomere shortens?
2. What stops myosin heads binding to actin in a relaxed muscle?
3. What happens to the H zone when the sarcomere shortens?
4. State two roles of ATP in contraction.
5. A sarcomere measures 46 mm on an image at ×20 000. Calculate its actual length in µm.
6. A sarcomere is 2.7 µm long and the A band is 1.6 µm. Calculate the I band length.
7. Why can a single muscle not move a bone in both directions?
8. A muscle holds 5.0 mmol ATP and 10.0 mmol phosphocreatine per kg and uses 2.5 mmol per kg each second. How long do the two stores last together?
9. Give two features of slow fibres that suit sustained activity.
10. Where is phosphocreatine rebuilt, and with what energy supply?

Answers

1. The A band.
2. Tropomyosin covers the binding sites on actin.
3. It gets shorter (and may disappear), because the thin filaments slide towards the centre.
4. Binds to the myosin head to detach it from actin; hydrolysis resets the head. Also: powers the pumps returning calcium ions to the sarcoplasmic reticulum.
5. 46 ÷ 20 000 = 0.0023 mm = 2.3 µm.
6. 2.7 − 1.6 = 1.1 µm.
7. Muscles can only pull. A second muscle is needed to pull the bone back.
8. (5.0 + 10.0) ÷ 2.5 = 6.0 s.
9. Any two of: many mitochondria, myoglobin, rich blood supply, aerobic respiration, resistance to fatigue.
10. In the sarcoplasm of the muscle fibre, during rest, using ATP made by aerobic respiration.

Where marks are usually lost

• Saying that the A band, the filaments or the myosin get shorter.
• Explaining the cycle without saying that calcium ions must expose the binding sites first.
• Stating that ATP “gives energy for contraction” without linking it to detachment or resetting of the head.
• Leaving out the pumping of calcium ions back when asked about relaxation.
• Mixing up I band and H zone, or A band and H zone, on a diagram.
• Forgetting to convert mm to µm before dividing by magnification.
• Calling phosphocreatine a long-term fuel.
• Describing antagonistic muscles as one pulling and one pushing.
• Giving fibre properties without saying which type you mean.

Official syllabus

Source: OxfordAQA International AS and A-level Biology (9610), specification Version 5.1, for International AS exams May/June 2017 onwards and International A-level exams May/June 2018 onwards, published by OxfordAQA. Section 3.4.4, Skeletal muscles as effectors.