B3.3 Muscle and Motility (HL)

IB DP BIOLOGY Theme B: Form and Function B3.3 Muscle and Motility

Revision Notes · Higher Level only Fahad H. Ahmad

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Original notes prepared for the IB Diploma Programme Biology course (2025 syllabus)

What the syllabus requires

B3.3 Muscle and Motility is an HL-only sub-topic. Everything in this booklet is examinable at Higher Level and only at Higher Level. Use the checklist below as a final revision sweep.

Understanding You should be able to...

Movement in animals Explain how muscles, the skeleton and joints act together to produce movement, and why muscles work in antagonistic pairs.

Skeletal muscle structure Describe the hierarchy muscle → fibre → myofibril → sarcomere and label the banding pattern.

Myofilaments Identify thick (myosin) and thin (actin, tropomyosin, troponin) filaments and their roles.

Sliding filament model Explain sarcomere shortening in terms of filaments sliding, and predict changes to the I band, H zone and A band.

Cross-bridge cycle Sequence attachment, power stroke, detachment and re-cocking, and state the roles of ATP.

Role of calcium Trace the pathway from action potential to Ca2+ release, troponin binding and exposure of binding sites.

Motor units and fibre types Explain recruitment, summation and the difference between fast and slow twitch fibres.

Other motility Outline microtubule sliding in cilia and flagella and amoeboid movement.

Exam note: This is a data-rich topic. Expect labelled electron-micrograph diagrams of a sarcomere, sequencing questions on the cross-bridge cycle, and extended-response questions linking Ca2+, ATP and the sliding filament model.

1. Movement in animals: muscles, bones and joints

Animals move by converting the chemical energy of ATP into mechanical work. Three components act together as a lever system: muscles supply the force, bones act as rigid levers, and joints act as the pivots about which the bones turn. Muscle can only actively generate force by shortening (contracting) and pulling; it cannot actively push or lengthen itself.

Antagonistic muscle pairs

Because a muscle can only pull, a single muscle can move a bone in one direction only. To return the bone, a second muscle that pulls in the opposite direction is needed. Two muscles that produce opposite movements at a joint form an antagonistic pair: as one (the agonist) contracts, the other (the antagonist) relaxes.

• At the elbow, the biceps (a flexor) contracts to bend the arm while the triceps relaxes.
• To straighten the arm, the triceps (an extensor) contracts while the biceps relaxes.
• The biceps and triceps attach to bones by tendons; the two bones are joined at the elbow by ligaments.

Synovial joint structure

Freely movable joints such as the elbow, knee and hip are synovial joints. Their structure reduces friction and allows a wide range of movement while remaining stable.

Structure Function

Cartilage (articular) Smooth layer covering the ends of the bones; reduces friction and absorbs shock/compression.

Synovial fluid Lubricates the joint and reduces friction between the cartilage surfaces; also supplies nutrients to the cartilage.

Synovial membrane Lines the joint capsule and secretes the synovial fluid.

Joint capsule Tough fibrous sleeve that encloses and seals the joint, holding the synovial fluid in.

Ligaments Connect bone to bone across the joint, providing stability and limiting the range of movement to prevent dislocation.

Tendons Connect muscle to bone so that muscle contraction is transmitted to the bone as movement.

Key distinction: Tendons join muscle to bone; ligaments join bone to bone. Mixing these up is one of the most common errors in this topic.

2. Structure of skeletal (striated) muscle

Skeletal muscle is also called striated muscle because under the microscope it shows regular light and dark cross-bands (striations). It is organised as a set of structures nested inside one another, each level smaller than the last.

Level Description

Muscle A whole organ (e.g. the biceps), made of many bundles of parallel muscle fibres.

Muscle fibre (muscle cell) A single, very long, multinucleate cell formed by the fusion of many cells during development; contains many myofibrils. Surrounded by the sarcolemma (cell membrane); the cytoplasm is the sarcoplasm.

Myofibril A cylindrical bundle of myofilaments running the length of the fibre; the contractile machinery.

Sarcomere The repeating functional (contractile) unit of a myofibril, running from one Z line to the next.

Why are muscle fibres multinucleate?

Each fibre is formed by the fusion of many precursor cells, so it retains many nuclei. A single fibre is far too large and too metabolically demanding to be serviced by one nucleus, so multiple nuclei allow rapid synthesis of the large amounts of contractile protein required.

The banding pattern of the sarcomere

The striations arise from the ordered, overlapping arrangement of thick and thin filaments within each sarcomere. Learn each band precisely.

Feature What it is

Z line (Z disc) The boundary at each end of a sarcomere; thin filaments are anchored here. One sarcomere = Z line to Z line.

I band A light band containing only thin (actin) filaments, no overlap with thick filaments. Straddles a Z line, so it is shared between two adjacent sarcomeres.

A band A dark band, the full length of the thick (myosin) filaments; includes the region where thick and thin filaments overlap.

H zone A lighter region in the middle of the A band containing only thick filaments (no thin-filament overlap).

M line A line down the centre of the H zone that anchors the thick filaments and holds them in register.

Memory aid: The lIght band is the I band (actin only); the dArk band is the A band (myosin, including overlap). The Hole in the middle is the H zone (myosin only).

Figure 1. Structure of a sarcomere (Z line to Z line). Thick (myosin) filaments make up the dark A band; thin (actin) filaments make up the light I band; the myosin-only centre is the H zone (M line at its middle). Drawn to scale for a relaxed sarcomere of 2.6 µm: A band 1.6 µm, I band 1.0 µm, H zone 0.6 µm.

3. The myofilaments

Each sarcomere contains two kinds of protein filament arranged in a highly ordered, overlapping lattice.

Filament Composition and features

Thick filament Made of many myosin molecules. Each myosin has a long tail and a globular head that projects outward. The heads form the cross-bridges: they can bind actin, hydrolyse ATP and swivel.

Filament Composition and features

Thin filament A double helix of actin monomers, each of which has a myosin-binding site. Wound along the actin is the protein tropomyosin, which blocks the binding sites at rest. Attached to tropomyosin at intervals is troponin, a protein that binds Ca2+.

Roles of the four proteins

• Myosin → thick filament; its heads generate force in the power stroke and act as ATPase enzymes.
• Actin → thin filament; provides the binding sites that the myosin heads pull on.
• Tropomyosin → blocks the myosin-binding sites on actin when the muscle is relaxed.
• Troponin → binds Ca2+; when it does, it pulls tropomyosin aside to expose the binding sites.

4. The sliding filament model

The sliding filament model explains how a sarcomere shortens. The filaments themselves do not change length. Instead, the thin (actin) filaments are pulled inward, sliding past the thick (myosin) filaments toward the centre of the sarcomere. Because thin filaments at both ends slide inward, the two Z lines are drawn closer together and the whole sarcomere shortens.

What changes and what stays the same during contraction

Feature During contraction Why

Sarcomere length Shortens Z lines pulled closer together.

I band Shortens Overlap increases, so the actin-only region shrinks.

H zone Shortens Thin filaments move inward into the H zone, reducing the myosin-only region.

A band Stays the SAME Its length equals the length of the thick filaments, which do not change.

Zone of overlap Increases Actin and myosin overlap more as filaments slide together.

Examiner favourite: If asked what happens to the A band during contraction, the answer is: it stays constant. The A band length equals the thick-filament length, and filaments slide rather than shorten. Many students wrongly say all bands shorten.

When a muscle is stretched, the same logic runs in reverse: the I band and H zone widen, the overlap decreases, but the A band still stays constant.

Figure 2. Sliding-filament model. On contraction the sarcomere shortens 2.6 → 2.2 µm (a 15.4% decrease). Thin filaments slide inward, so the I band narrows (1.0 → 0.6 µm, -40%) and the H zone narrows (0.6 → 0.2 µm, -66.7%), while the A band stays constant at 1.6 µm.

Figure 3. Length-tension relationship. Active tension tracks the thick-thin filament overlap: it is maximal over the optimum plateau (2.0-2.2 µm) and falls to zero at 3.6 µm where overlap is lost. The relaxed length (2.6 µm) sits on the descending limb at 71%; shortening toward 2.2 µm raises tension to 100%.

5. The cross-bridge cycle

Sliding is driven by the myosin heads repeatedly attaching to actin, pulling, releasing and re-attaching further along. One complete round is the cross-bridge cycle; each cycle moves the thin filament a small step, and millions of cycles occurring asynchronously produce a smooth, sustained contraction. Start from a myosin head already 'cocked' and carrying the products of ATP hydrolysis (ADP + Pi).

Step Event

1 Attachment The energised myosin head binds to an exposed binding site on actin, forming a cross-bridge.

2 Power stroke The head releases ADP and Pi and swivels/tilts, pulling the thin filament toward the centre of the sarcomere. This is the force-generating step.

3 Detachment A new ATP molecule binds to the myosin head. This binding lowers the head's affinity for actin, so the cross-bridge breaks and the head detaches.

4 Re-cocking (hydrolysis)

The head hydrolyses ATP → ADP + Pi; the energy released re-cocks the head back to its high-energy position, ready to bind again further along the actin.

The role of ATP

ATP is needed at two distinct points, and in muscle physiology it has three essential roles overall. Being explicit about all of these scores full marks.

• ATP binding to the myosin head causes it to detach from actin (step 3).
• ATP hydrolysis provides the energy to re-cock the head for the next power stroke (step 4).
• ATP also powers the calcium pump that returns Ca2+ to the sarcoplasmic reticulum during relaxation (section 6).

Rigor mortis link: After death ATP production stops. Without ATP the myosin heads cannot detach from actin, so the cross-bridges lock and the muscle becomes rigid − this is rigor mortis. It is direct evidence that ATP is needed for detachment, not for the power stroke.

6. Calcium ions and the sarcoplasmic reticulum

The cross-bridge cycle can only run when the actin binding sites are exposed, and this is controlled by calcium ions. Ca2+ is stored in the sarcoplasmic reticulum (SR) − a specialised endoplasmic reticulum that forms a network around each myofibril. Excitation of the muscle releases Ca2+ and switches contraction on.

From nerve impulse to contraction

• A motor neuron releases neurotransmitter at the neuromuscular junction, triggering an action potential that spreads across the sarcolemma and down the T-tubules into the fibre.
• The action potential causes the SR to release stored Ca2+ into the sarcoplasm, so the Ca2+ concentration around the myofibrils rises sharply.
• Ca2+ binds to troponin, changing its shape.
• The troponin change pulls tropomyosin away from the actin, exposing the myosin-binding sites.
• Myosin heads can now bind actin, and the cross-bridge cycle runs − the muscle contracts (as long as ATP and Ca2+ are present).

Relaxation

When the nerve stimulation stops, Ca2+ is actively pumped back into the SR by calcium pumps in the SR membrane, using ATP. As the sarcoplasmic Ca2+ concentration falls, Ca2+ leaves troponin, tropomyosin moves back to cover the binding sites, cross-bridges can no longer form, and the muscle relaxes. Elastic recoil and the contraction of the antagonistic muscle then return the sarcomeres to their resting length.

Two jobs for ATP: Contraction needs ATP for the cross-bridge cycle (detachment + re-cocking); relaxation also needs ATP to pump Ca2+ back into the SR. Both processes are active and energy-consuming.

7. Motor units, control of force and fibre types

A single contraction of a whole muscle can be weak or powerful. The nervous system grades the force using motor units.

Motor units

A motor unit is a single motor neuron together with all the muscle fibres it stimulates. When the neuron fires, every fibre in its unit contracts together. Small motor units (few fibres) allow fine control (e.g. eye muscles); large motor units (many fibres) give powerful but coarse contractions (e.g. thigh muscles).

Grading the force of contraction

• Recruitment − to increase force, the nervous system activates a greater number of motor units at the same time.
• Summation (frequency) − to increase force, motor neurons fire action potentials more frequently, so successive twitches add together before the fibre can fully relax, giving a larger, sustained (tetanic) contraction.

Fast-twitch and slow-twitch fibres

Feature Slow-twitch (Type I) Fast-twitch (Type II)

Contraction speed Slow Fast

Main ATP source Aerobic respiration Anaerobic respiration (glycolysis)

Mitochondria Many Few

Myoglobin / capillaries High (red in colour) Low (paler)

Fatigue resistance High − resists fatigue Low − fatigues quickly

Best suited to Endurance / posture (e.g. marathon) Short, powerful bursts (e.g. sprinting)

Application: Most muscles contain a mix of both fibre types; the proportion varies between people and muscles. Endurance athletes tend to have a higher proportion of slow-twitch fibres, sprinters more fast-twitch.

8. Other forms of motility

Muscle is not the only way cells and organisms move. The syllabus requires a brief outline of two further mechanisms, both driven by motor proteins and cytoskeletal filaments.

Cilia and flagella

Eukaryotic cilia and flagella move by the sliding of microtubules. Their core (the axoneme) has the classic 9 + 2 arrangement: nine pairs of microtubules around two central ones. The motor protein dynein uses ATP to make adjacent microtubule doublets slide against one another; because the doublets are anchored, this sliding is converted into bending, producing the beating movement that propels a cell (e.g. a sperm flagellum) or moves fluid past it (e.g. ciliated cells of the airways).

Amoeboid movement

Cells such as Amoeba and human white blood cells crawl by amoeboid movement. The cell extends a projection called a pseudopodium ('false foot') by the controlled assembly (polymerisation) of actin filaments at the leading edge, coupled with sol-gel transformations of the cytoplasm. The cell anchors the pseudopodium and flows its contents forward. This ATP-dependent, actin-based crawling underlies phagocytosis and the movement of white blood cells toward sites of infection.

Common thread: All three systems − skeletal muscle, cilia/flagella and amoeboid movement − use motor proteins (myosin or dynein) walking along cytoskeletal filaments (actin or microtubules), powered by ATP.

9. Worked examples and exam skills

Worked example 1 − labelling a sarcomere

A micrograph shows one repeating unit of a myofibril. Identify: (a) the boundary marking the ends of the unit, (b) the light band with actin only, (c) the dark band, (d) the lighter central region of the dark band.

• The Z line − a sarcomere runs from one Z line to the next.
• The I band − light, thin (actin) filaments only, straddling the Z line.
• The A band − dark, the full length of the thick (myosin) filaments including the overlap region.
• The H zone − thick filaments only, with the M line down its centre.

Worked example 2 − bands during contraction

Explain what happens to the width of the I band, the H zone and the A band as a sarcomere contracts, and why.

The thin filaments slide inward past the thick filaments, so the overlap increases. The I band shortens (actin-only region shrinks) and the H zone shortens (thin filaments intrude into the myosin-only region).

The A band stays the same width, because it equals the length of the thick filaments, and the filaments slide rather than change length.

Worked example 3 − sequence the cross-bridge cycle

Put the following into the correct order, starting from an energised myosin head: power stroke; ATP hydrolysis re-cocks the head; myosin binds actin; ATP binds and head detaches.

• Myosin head binds actin (cross-bridge forms).
• Power stroke − head swivels, releasing ADP + Pi and pulling the thin filament inward.
• ATP binds the head, which then detaches from actin.
• ATP hydrolysis re-cocks the head, ready to bind again further along.

Worked example 4 − the role of calcium

Explain how a rise in Ca2+ concentration switches on contraction.

An action potential causes the sarcoplasmic reticulum to release Ca2+. The Ca2+ binds to troponin, which changes shape and pulls tropomyosin off the actin. This exposes the myosin-binding sites, allowing cross-bridges to form and the cycle to run. When Ca2+ is pumped back into the SR (using ATP), tropomyosin re-covers the sites and the muscle relaxes.

Common pitfalls

• Saying the filaments shorten. They do not − they slide past each other; the sarcomere shortens.
• Claiming the A band shortens. It stays constant during both contraction and stretching.
• Thinking ATP is only for the power stroke. ATP is needed for detachment (binding) and for re-cocking (hydrolysis), and again to pump Ca2+ back into the SR.
• Confusing tropomyosin (blocks the sites) with troponin (binds Ca2+).
• Confusing tendons (muscle to bone) with ligaments (bone to bone).

10. Quick reference

Item Key point

Antagonistic pair Biceps (flexor) and triceps (extensor); one contracts while the other relaxes.

Tendon vs ligament Tendon: muscle → bone. Ligament: bone → bone.

Muscle hierarchy Muscle → fibre (multinucleate) → myofibril → sarcomere.

Banding I band = actin only (light); A band = full thick filament (dark); H zone = myosin only; M line centres it; Z line bounds the sarcomere.

Sliding filament Actin slides past myosin; I band and H zone shorten; A band constant.

Cross-bridge cycle Bind → power stroke → ATP binds, detach → ATP hydrolysed, re-cock.

Roles of ATP Detachment; re-cocking; pumping Ca2+ back into the SR.

Calcium switch AP → SR releases Ca2+ → troponin → tropomyosin moves → sites exposed.

Item Key point

Force control Recruitment (more motor units) and summation (higher firing frequency).

Fibre types Slow twitch: aerobic, fatigue-resistant, endurance. Fast twitch: anaerobic, powerful, fatigues fast.

Other motility Cilia/flagella: microtubules slide via dynein. Amoeboid: actin polymerises to form pseudopodia.

11. Test yourself

Attempt these without notes; full answers follow.

• Explain why skeletal muscles must operate in antagonistic pairs, using the biceps and triceps as an example.
• State one function each of synovial fluid, cartilage and ligaments in a synovial joint.
• Name the repeating contractile unit of a myofibril and state which structures mark its boundaries.
• Describe what happens to the I band, H zone and A band when a sarcomere contracts, and explain why the A band does not change.
• Outline the cross-bridge cycle in four steps, starting from an energised (cocked) myosin head.
• Explain the two distinct roles of ATP in the cross-bridge cycle, and state a third role of ATP in muscle relaxation.
• Trace the sequence of events from an action potential arriving at a muscle fibre to the exposure of the myosin-binding sites.
• Distinguish between recruitment and summation as ways of increasing the force of a muscle contraction.

Answers

• A muscle can only generate force by contracting (shortening) and pulling; it cannot push. So one muscle can move a bone in only one direction. The biceps contracts to flex (bend) the arm; to extend (straighten) it, the triceps must contract while the biceps relaxes. The two produce opposite movements, so they are antagonistic.
• Synovial fluid: lubricates the joint / reduces friction. Cartilage: reduces friction and absorbs shock at the bone ends. Ligaments: join bone to bone, stabilising the joint and limiting movement.
• The sarcomere; it runs from one Z line to the next Z line.
• The I band shortens and the H zone shortens (thin filaments slide inward, increasing overlap). The A band stays constant because its width equals the length of the thick (myosin) filaments, and the filaments slide past each other rather than shortening.
• (i) The myosin head binds an exposed site on actin. (ii) Power stroke: the head swivels, releasing ADP + Pi and pulling the thin filament toward the sarcomere centre. (iii) ATP binds the head, which detaches from actin. (iv) ATP is hydrolysed to ADP + Pi, re-cocking the head for the next cycle.
• In the cross-bridge cycle, ATP binding to the myosin head causes it to detach from actin, and ATP hydrolysis provides energy to re-cock the head. A third role: ATP powers the pumps that return Ca2+ to the sarcoplasmic reticulum during relaxation.
• Action potential spreads across the sarcolemma and down the T-tubules → the sarcoplasmic reticulum releases Ca2+ → Ca2+ binds troponin → troponin changes shape and moves tropomyosin → the myosin-binding sites on actin are exposed.
• Recruitment: increasing force by activating a greater number of motor units simultaneously. Summation: increasing force by stimulating the fibres more frequently, so twitches overlap and add together before relaxation, producing a larger sustained contraction.