B3.3 Muscle and motility [HL only]

Muscle and motility connect sarcomere contraction, antagonistic muscles, skeletons, synovial joints and locomotor adaptations to movement across biological scales efficiently.

Learning objectives

B3.3.1(HL)—Movement adaptations• Movement is universal, but only some organisms show locomotion• Motile organisms move for food, mates, migration, or escape• Sessile organisms such as coral or plants still move body parts or grow toward stimuli B3.3.2(HL)—Sliding filament model• Sarcomeres contain actin thin filaments and myosin thick filaments between Z lines• Calcium binds troponin, moving tropomyosin away from actin binding sites• Myosin heads use ATP in a cross-bridge cycle to slide actin and shorten sarcomeres B3.3.3(HL)—Titin and antagonistic muscles• Titin acts as a molecular spring in sarcomeres and centres myosin• Titin recoils after stretching and prevents overstretching• Antagonistic muscles are needed because muscles actively contract but do not actively extend B3.3.4(HL)—Motor units• Motor neurons connect to skeletal muscle fibres at neuromuscular junctions• Acetylcholine release at motor end plates triggers sarcolemma excitation• Excitation releases calcium from sarcoplasmic reticulum to start contraction B3.3.5(HL)—Skeletons as anchorage and levers• Skeletons provide support, protection, muscle anchorage, and lever systems• Vertebrate endoskeletons attach muscles by tendons• Arthropod exoskeletons use chitin, joints, and internal muscle attachments B3.3.6(HL)—Synovial joint movement• Synovial joints contain lubricating synovial fluid between cartilage-covered bones• Tendons attach muscles to bones; ligaments connect bones and stabilize joints• The hip is a ball-and-socket joint between femur and pelvis B3.3.7(HL)—Range of motion• Range of motion depends on joint type, bone surfaces, ligaments, and muscles• Ball-and-socket joints such as shoulder and hip allow circumduction• Hinge joints such as elbow and knee mainly allow flexion and extension B3.3.8(HL)—Intercostal muscles• External and internal intercostal muscles act antagonistically during ventilation• External intercostals lift ribs up and out for inspiration• Internal intercostals pull ribs down and in during forced expiration B3.3.9(HL)—Reasons for locomotion• Locomotion allows animals to forage, avoid predators, find mates, and migrate• Examples include blackbirds feeding, hares escaping predators, orangutans finding mates, and whales migrating• Locomotion can improve survival and reproductive success B3.3.10(HL)—Swimming adaptations in marine mammals• Marine mammals have streamlined bodies, reduced drag, flippers, and up-down tail flukes• Reduced pelvic bones and absent hind limbs reduce resistance in water• Cetacean blowholes allow rapid surface breathing and close during dives

Movement changes position or shape; locomotion moves the whole organism from one place to another. A sessile organism remains attached yet may move body parts or grow directionally.

A side-by-side visual showing a motile animal moving its whole body and a sessile organism still showing local movement or growth toward a stimulus.

Locomotion can improve access to food, escape from predators, mate finding and migration. The benefit must outweigh its metabolic cost and exposure to risk.

A sarcomere runs from Z line to Z line. During contraction, myosin pulls thin actin filaments toward the centre, increasing overlap and bringing Z lines closer; neither filament shortens.

An electron micrograph is paired with an interpretive drawing labeling thick myosin filaments and thin actin filaments in a single sarcomere.

I band and H zone narrow → overlap increases
A band stays constant → myosin length is unchanged
Z lines approach → sarcomere shortens

Ca²⁺ permits cross-bridge formation, while ATP controls detachment and re-cocking. Repeated myosin power strokes slide actin toward the sarcomere centre.

Show ordered contraction states so the learner can sequence Contraction.

1 Ca²⁺ binds troponin; tropomyosin moves
2 energized myosin–ADP–Pi binds actin
3 Pi/ADP release drives the power stroke
4 ATP binding detaches myosin
5 ATP hydrolysis re-cocks the head

Skeletal muscles generate active force by shortening; they cannot actively push themselves longer. Titin and antagonistic muscle pairs provide two different routes back toward the starting state.

titin spans from thick filament toward the Z line, centres myosin and resists overstretching
stretched titin stores elastic energy and recoils as the sarcomere relaxes
an agonist contracts while its antagonist relaxes; reversing the movement swaps their roles

Titin restores sarcomere alignment and passive length; it does not replace the opposing muscle force needed to rotate a joint in the reverse direction.

A motor unit is one motor neuron and all muscle fibres it controls. At each neuromuscular junction, neuronal excitation is converted into sarcolemma excitation and then Ca²⁺ release from sarcoplasmic reticulum.

motor-neuron action potential → terminal Ca²⁺ entry → ACh exocytosis → receptor activation → muscle action potential → T-tubules → SR Ca²⁺ release → cross-bridge cycling

Muscles pull; they do not push. A tendon transmits tension to a rigid skeletal element, a joint acts as fulcrum, and an antagonistic muscle produces the reverse movement.

A diagram shows the biceps flexing the forearm as a lever, with the hand carrying the load, the muscle providing effort, and the elbow joint acting as the fulcrum, alongside a labeled lever schematic.

At the elbow: joint = fulcrum, biceps force = effort, forearm/hand load = resistance. Attachment close to the joint sacrifices force advantage for speed and range.

A synovial joint combines smooth cartilage, lubricating synovial fluid, stabilizing ligaments and force-transmitting tendons. Articulating surface shape constrains its directions of movement.

ball-and-socket hip/shoulder → movement in three planes, including circumduction
hinge knee/elbow → mainly flexion and extension
goniometer → angular change between starting and safe maximum positions

External and internal intercostals have opposing fibre directions. Their contraction moves ribs differently, changing thoracic volume and therefore lung pressure.

A front rib-cage diagram showing external intercostals raising the ribs up and out and internal intercostals pulling the ribs down and in.

external intercostals contract → ribs up/out → volume rises → pressure falls → inspiration
internal intercostals contract in forced expiration → ribs down/in → volume falls → pressure rises → air exits

Water is dense, so marine mammals combine a streamlined body with controlled thrust and rapid surface ventilation. They remain air-breathing mammals.

An annotated dolphin or whale showing a streamlined outline, flippers, horizontal tail flukes, reduced pelvic region, and blowhole at the top of the head.

Feature	Mechanical or respiratory effect
tapered body and reduced projections	reduces drag in dense water
forelimb flippers	steer and stabilize the body
horizontal tail flukes driven by axial muscles	generate up-down thrust
dorsal blowhole	permits rapid ventilation with little emergence

motor signal → ACh → muscle action potential → SR Ca²⁺ → exposed actin sites → cross-bridge cycling → sarcomere shortening → tendon tension → joint rotation

ATP detaches and re-cocks myosin; Ca²⁺ reuptake stops new bridges; titin restores passive sarcomere length, while antagonistic muscle action reverses joint movement. Whole-body form then determines whether force becomes efficient locomotion.

2 marks

Microscopic eukaryotes include Euglena and Paramecium. Outline the range of cellular structures used for locomotion in these organisms.

8 marks

Actin and myosin are two proteins found in muscles. Explain how skeletal muscle contracts, including the interaction of these proteins.

2 marks

Explain the role of the protein titin in muscle relaxation.

1 mark

What happens when an action potential reaches motor end plates?

1 mark

Outline how the muscle attachment of insects differs from humans.

2 marks

State the function of structures I and II.

I:

II:

2 marks

Compare the movements of the hip joint and the knee joint.

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—B4.1 Adaptation to environmentRead

B3.3 Muscle and motility [HL only]

Learning objectives

Movement Does Not Always Mean Locomotion

Sliding Filaments Shorten the Sarcomere

Calcium Opens the Site; ATP Resets the Myosin Head

Titin Recoils; Antagonistic Muscles Reverse Movement

A Motor-Neuron Signal Releases Calcium around Myofibrils

A Skeleton Converts Muscle Tension into Rotation

Joint Shape Sets the Available Range of Motion

Antagonistic Intercostals Change Thoracic Volume

Marine Mammals Reduce Drag and Surface Efficiently

Summary: From Molecular Pull to Locomotion

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