Key Idea: Topic B3.3 is the HL-only story of muscle and motility — how a chemical signal is turned into mechanical movement. Movement comes from a musculoskeletal system: muscles pull on bones, and the bones act as levers that pivot at joints. A muscle can only pull (shorten), never push, so muscles are arranged in antagonistic pairs. Inside a muscle, the contractile unit is the sarcomere. Its filaments — actin (thin) and myosin (thick), held in place by titin — slide past each other so the sarcomere shortens: the sliding filament model. The trigger is Ca²⁺, which moves troponin and tropomyosin off actin so myosin can grab it, and the power is supplied by ATP. Expect this on Paper 1 (label-the-sarcomere and 'which band shortens?' MCQs) and Paper 2 (the 'explain the sliding filament model' and 'describe the role of Ca²⁺ / ATP' extended responses).
🦴 Adaptations for movement & the musculoskeletal system (2.8.1)
Animals move because muscles pull on a skeleton. A bone is a rigid lever, a joint is the pivot it turns about, and the contracting muscle supplies the effort. Tendons anchor muscle to bone (so the pull is transmitted), while ligaments join bone to bone and stabilise the joint. The key limitation to remember: a muscle can only shorten and pull — it cannot actively push. That single fact is why bones need a second muscle to move them back, and why this topic keeps returning to antagonistic pairs.
| Part of the system | What it is | Its role in movement |
|---|---|---|
| Bones | Rigid levers of the skeleton | Transmit force; a joint acts as the pivot (fulcrum) the lever turns about |
| Skeletal muscle | Bundles of fibres that shorten when stimulated | The effort — pulls on a bone to move it; can only PULL, never push |
| Tendon | Tough collagen cord | Anchors muscle to bone so the pull is transmitted to the lever |
| Ligament | Tough collagen band | Joins bone to bone across a joint and stabilises it |
| Synovial joint | Lubricated movable joint (e.g. elbow, knee) | Lets the lever rotate with low friction; cartilage cushions the bone ends |
tendon ties muscle to bone; ligament links bone to bone. Muscles PULL only — so they come in pairs.
🧬 Sarcomere structure: actin, myosin & titin (2.8.2)
A muscle fibre is packed with myofibrils, and each myofibril is a chain of repeating units called sarcomeres. One sarcomere runs from one Z-disc to the next. Two kinds of filament fill it. Thin actin filaments are anchored to the Z-discs and reach inward; thick myosin filaments sit in the centre around the M-line and carry the heads that do the pulling. A third, elastic protein — titin — runs from the Z-disc to the myosin, keeping myosin centred and giving the sarcomere its springiness. The thin filament also carries tropomyosin and troponin, the switch that controls when contraction can happen.
One sarcomere runs from Z-disc to Z-disc. Thin ACTIN filaments anchor to the Z-discs; thick MYOSIN filaments sit in the centre around the M-line. The I band is actin-only, the A band is the full myosin length, and the H zone is myosin-only. Compare the RELAXED row with the CONTRACTED row: the filaments do NOT shorten — they slide and overlap MORE, so the Z-discs are pulled closer and the I band and H zone shrink. That is the sliding filament model (2.8.2 + 2.8.3).
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
| Filament / protein | Thick or thin? | What it does |
|---|---|---|
| Myosin | THICK filament | Has heads that form cross-bridges with actin and pull it inward; the heads are the 'motor' |
| Actin | THIN filament | Anchored to the Z-discs; myosin heads grab it and slide it toward the centre |
| Titin | Elastic protein | Springs from the Z-disc to the myosin; keeps myosin centred and gives the sarcomere passive recoil after stretch |
| Tropomyosin | On the thin filament | A long strand that lies over actin's myosin-binding sites and BLOCKS them when the muscle is at rest |
| Troponin | On the thin filament | Binds Ca²⁺; when it does, it shifts tropomyosin OFF the binding sites so contraction can start |
I band = actin only (no overlap). A band = the full myosin length. H zone = myosin only (the gap in the middle of the A band). The Z-disc bounds the sarcomere; the M-line marks its centre. A common slip: the A band never changes length, because the myosin filament itself does not shorten.
🔁 The sliding filament model of contraction (2.8.3)
The central idea: during contraction the filaments do NOT get shorter — the actin slides over the myosin, so they overlap more. The myosin heads form cross-bridges with actin and ratchet it toward the centre, pulling the Z-discs closer together. So the whole sarcomere shortens, the I band shrinks (more overlap) and the H zone shrinks (actin slides into it) — but the A band stays the same because the myosin length is unchanged. Read those three band changes straight off the relaxed-vs-contracted rows of the diagram above.
| Region of the sarcomere | What lies there | What happens during contraction |
|---|---|---|
| Sarcomere (Z to Z) | The whole repeating unit | Gets SHORTER (Z-discs pulled together) |
| I band | Actin only (no myosin overlap) | Gets SHORTER (more overlap) |
| H zone | Myosin only (no actin overlap) | Gets SHORTER (actin slides in) |
| A band | Full length of the myosin filament | STAYS THE SAME (myosin length is unchanged) |
Shortens: the sarcomere, the I band, the H zone. Unchanged: the A band (and the filaments themselves — they slide, they don't shrink). Memory hook: the H zone and I band say 'HI, we shrink'; the A band says 'A for Always the same'.
🧪 Calcium, troponin/tropomyosin & ATP in contraction (2.8.4)
At rest, tropomyosin lies across actin and blocks the myosin-binding sites, so no cross-bridges can form. Contraction needs a chemical switch and a chemical fuel. The switch is Ca²⁺: when a nerve impulse releases calcium ions from the sarcoplasmic reticulum, Ca²⁺ binds troponin, which drags tropomyosin off the binding sites and exposes them. The fuel is ATP: it lets each myosin head detach from actin and then, when hydrolysed, re-cocks the head ready for the next stroke. No ATP means heads stay stuck to actin (this is why muscles stiffen in rigor mortis).
The contraction cycle when a muscle is stimulated
- A nerve impulse arrives and Ca²⁺ is released from the sarcoplasmic reticulum into the muscle fibre.
- Ca²⁺ binds to troponin, which pulls tropomyosin off the myosin-binding sites on actin — the sites are now exposed.
- Myosin heads bind to actin, forming cross-bridges.
- The heads pivot (the power stroke), pulling actin toward the centre — the sarcomere shortens. ADP + Pᵢ are released.
- A new ATP binds each myosin head, making it detach from actin.
- ATP is hydrolysed, re-cocking the head so it can bind again further along — the cycle repeats while Ca²⁺ and ATP are present.
Key Idea: ATP is used twice per cross-bridge cycle, and both jobs are about letting go, not pulling: 1. Detachment — ATP binding to the myosin head makes it release actin. 2. Re-cocking — hydrolysing that ATP (to ADP + Pᵢ) re-energises / re-positions the head so it can bind further along. The actual power stroke itself releases the stored energy (ADP + Pᵢ leave) — it does not burn a fresh ATP at that instant.
Ca²⁺ = the key that unlocks the door (moves tropomyosin off actin). ATP = the engine that lets the head let go and re-cock. Take away the key → no binding; take away the engine → heads jam (rigor).
💪 Antagonistic muscles, joints & diversity of movement (2.8.5)
Because a muscle can only pull, bones are moved by antagonistic pairs: while one muscle (the agonist) contracts to move the lever one way, its partner (the antagonist) relaxes; to reverse the movement, the roles swap. The classic example is the elbow: the biceps contracts to bend (flex) the arm while the triceps relaxes; the triceps contracts to straighten (extend) it while the biceps relaxes. Different joint types allow a diversity of movement — a hinge joint (elbow, knee) bends in one plane, while a ball-and-socket joint (shoulder, hip) rotates in many directions.
| At the elbow joint | Bends the arm (flexion) | Straightens the arm (extension) |
|---|---|---|
| Muscle that contracts (agonist) | Biceps | Triceps |
| Muscle that relaxes (antagonist) | Triceps | Biceps |
| Why it must work in a pair | Muscle can only PULL, so a second muscle is needed to pull the bone back | The same — the pair moves the lever in opposite directions |
The agonist is the muscle doing the movement (contracting); the antagonist is its partner that relaxes to allow it. For 'how do you bend the arm?': biceps contracts (agonist), triceps relaxes (antagonist). To straighten it, swap them. Always state both muscles and what each is doing for full marks.
Biceps bends; triceps straightens. One pulls, one lets go — then they trade jobs. Hinge = one direction (a door), ball-and-socket = all directions (a joystick).
✍️ Worked examples
IB-style question — what changes in the sarcomere
A myofibril is examined when relaxed and again when fully contracted. State what happens to the length of the sarcomere, the I band, the H zone and the A band, and explain why the A band differs from the others. [4]
Model answer:
Sarcomere. It shortens — the Z-discs are pulled closer together as the filaments overlap more.
I band and H zone. Both shorten — the actin slides further over the myosin, reducing the actin-only (I band) and myosin-only (H zone) regions.
A band. It stays the same length.
Why the A band differs. The A band is the length of the myosin filament, and the filaments do not shorten — they slide (the sliding filament model). So everything that depends on overlap shrinks, but the myosin length is fixed. (1 mark each: sarcomere shorter / I & H shorter / A band unchanged / because filaments slide, myosin length fixed.)
Sarcomere, I band and H zone all shorten; the A band is unchanged because it equals the myosin filament length and the filaments slide rather than shorten (sliding filament model).
IB-style question — the role of calcium and ATP
Explain the roles of calcium ions and of ATP in skeletal muscle contraction. [5]
Model answer:
Calcium is released. A nerve impulse triggers Ca²⁺ release from the sarcoplasmic reticulum into the muscle fibre.
Calcium exposes the binding sites. Ca²⁺ binds troponin, which moves tropomyosin off the myosin-binding sites on actin, so the sites are exposed and myosin heads can attach.
Cross-bridge and power stroke. Myosin heads bind actin and pivot (power stroke), sliding actin inward; ADP + Pᵢ are released.
ATP detaches the head. A new ATP binds each myosin head, causing it to detach from actin.
ATP re-cocks the head. Hydrolysis of that ATP re-energises and re-positions the head so it can bind again — the cycle repeats while Ca²⁺ and ATP are present. (1 mark each: Ca²⁺ released / Ca²⁺→troponin→tropomyosin moves, sites exposed / cross-bridge + power stroke / ATP binding detaches head / ATP hydrolysis re-cocks head.)
Ca²⁺ from the sarcoplasmic reticulum binds troponin, shifting tropomyosin off actin's binding sites so myosin can attach and pull (power stroke); ATP then binds to detach each myosin head and, on hydrolysis, re-cocks it for the next cycle.
IB-style question — bending the arm
Using the elbow as an example, explain why skeletal muscles must work in antagonistic pairs. [3]
Model answer:
Muscles only pull. A muscle can only contract and pull a bone — it cannot push it back. So one muscle alone could move the lever only one way.
The pair pulls in opposite directions. A second muscle is needed on the other side of the joint to pull the bone back; the two are an antagonistic pair.
Apply to the elbow. To bend the arm the biceps contracts (agonist) and the triceps relaxes (antagonist); to straighten it the triceps contracts and the biceps relaxes. (1 mark: muscles only pull; 1 mark: need a partner to reverse the movement; 1 mark: correct biceps/triceps roles for the elbow.)
Because a muscle can only pull, not push, a bone needs a second muscle to move it back; at the elbow the biceps contracts (and triceps relaxes) to bend the arm, and the triceps contracts (and biceps relaxes) to straighten it — an antagonistic pair.
✅ Quick self-check
Tap each card to check yourself.
What makes up the musculoskeletal system, and what's the rule about muscles? Bones act as levers, joints as pivots, muscles supply the effort; tendons tie muscle to bone and ligaments tie bone to bone. The key rule: a muscle can only PULL (shorten), never push.
Name the filaments and proteins of a sarcomere and what each does. Thick myosin (heads form cross-bridges and pull), thin actin (anchored to Z-discs, gets pulled), elastic titin (centres myosin and gives recoil), tropomyosin (blocks binding sites at rest) and troponin (binds Ca²⁺ to unblock them).
What is the sliding filament model — and which regions shorten? Actin slides over myosin so they overlap more; the filaments do not shorten. The sarcomere, I band and H zone shorten; the A band (myosin length) stays the same.
What are the roles of Ca²⁺ and ATP in contraction? Ca²⁺ binds troponin, shifting tropomyosin off actin so myosin can bind (the switch). ATP binds to detach each myosin head and, on hydrolysis, re-cocks it for the next power stroke (the fuel).
Why must muscles work in antagonistic pairs? Because a muscle only pulls, a partner is needed to reverse the movement. At the elbow the biceps contracts (triceps relaxes) to bend the arm, and the triceps contracts (biceps relaxes) to straighten it.
Exam Tips
- Master idea: muscles pull on bones that act as levers; the contractile unit is the sarcomere, and contraction = the sliding filament model.
- Sarcomere parts: actin (thin, on Z-discs), myosin (thick, central, has heads), titin (elastic, centres myosin). Tropomyosin + troponin are the actin-filament switch.
- Band changes during contraction: sarcomere, I band and H zone all SHORTEN; the A band stays the same because the myosin filament length is fixed — the filaments slide, they don't shrink.
- Calcium is the switch: Ca²⁺ (from the sarcoplasmic reticulum) binds troponin → tropomyosin moves off actin → binding sites exposed. No Ca²⁺, no contraction.
- ATP is the fuel and is used twice per cycle, both for letting go: binding ATP detaches the myosin head, hydrolysing it re-cocks the head. No ATP → heads jam (rigor).
- Muscles only PULL, so they act in antagonistic pairs. At the elbow: biceps bends (agonist), triceps straightens. Always name BOTH muscles and what each is doing.
- Joint types give diverse movement: hinge (elbow, knee) bends in one plane; ball-and-socket (shoulder, hip) rotates in many directions.
- On Paper 1, be ready to label Z-disc, I band, A band, H zone, M line on a sarcomere and to say which shorten on contraction.