B3.3 Muscle and motility [HL only]

Movement is a universal feature of living organisms, but locomotion means movement of a motile organism from one location to another. Sessile organisms are attached to a surface and cannot move independently; plants, fungi and sponges are examples. Motile organisms use their own metabolic energy to move, often with structures such as legs, wings, fins or flagella. Locomotion can improve survival by finding food, escaping danger, finding a mate or migrating, although it increases energy and nutritional demands.

• Movement can involve growth or movement of parts; locomotion moves the whole organism.
• Sessile organisms remain attached but may move parts or grow toward stimuli.
• Motile organisms move independently using metabolic energy.
• Reasons for locomotion include foraging, escape, mating and migration.

Muscle contraction occurs when myosin heads pull actin filaments toward the center of a sarcomere; the filaments slide rather than shrink.

ATP energizes myosin, and calcium exposes binding sites on actin. Repeated cross-bridge cycles draw actin inward, shortening the sarcomere while filament lengths remain constant.

Trace: calcium exposes sites; myosin binds; ATP-driven head movement pulls actin; new ATP detaches and resets the head.

During contraction, Z-lines move closer and the sarcomere shortens, but actin and myosin molecules keep their lengths.

‘Sliding filament’ does not mean actin dissolves or myosin shortens; overlap changes.

The sliding filament cycle explains how actin moves. Calcium binds troponin, moving tropomyosin away from actin binding sites. Myosin heads form cross-bridges with actin. ATP binding breaks the cross-bridge; ATP hydrolysis re-cocks the myosin head; the head binds again and releases ADP and phosphate during the power stroke, pulling actin toward the centre of the sarcomere.

• Calcium exposes actin binding sites by acting through troponin and tropomyosin.
• ATP is needed both to detach myosin and to re-cock the myosin head.
• Repeated cross-bridge cycling slides actin inward and shortens the sarcomere.

A motor neuron triggers skeletal muscle contraction at the neuromuscular junction. An action potential reaches the motor end plate, acetylcholine is released and the sarcolemma is depolarized; the signal causes the sarcoplasmic reticulum to release Ca²⁺. Ca²⁺ exposes actin binding sites and cross-bridge cycling shortens the sarcomere. Acetylcholinesterase removes acetylcholine and Ca²⁺ pumps return calcium to the sarcoplasmic reticulum, so tropomyosin covers the binding sites and the fibre relaxes. A motor unit is one motor neuron plus all the fibres it controls. Titin provides elastic recoil, centres myosin and limits overstretching; antagonistic muscles are needed because muscle contraction produces force in one direction, not active extension.

• One motor neuron and its muscle fibres form a motor unit.
• Acetylcholine and Ca²⁺ initiate contraction.
• Acetylcholinesterase and Ca²⁺ reuptake allow relaxation.
• Titin recoils the sarcomere; antagonistic pairs reverse movement.

Bones provide rigid anchorage and joints act as levers, converting muscle tension into movement and distributing forces.

Muscles attach across joints, so their line of pull creates a turning moment. The position of the joint, load and attachment changes the trade-off between force and movement speed or range.

Analyze a lever by identifying: fulcrum; muscle force; load; direction and distance of each force.

A forearm lifts a weight when the biceps pulls on the radius around the elbow, even though the muscle force and load act at different distances.

A skeleton does not amplify every force; lever geometry can favor speed or range at the cost of mechanical advantage.

A synovial joint permits movement while limiting friction and instability. At the hip, the head of the femur fits into the pelvis; articular cartilage covers the bone ends, a synovial membrane produces lubricating fluid, ligaments stabilize the joint, and tendons attach muscles to bones. A ball-and-socket joint such as the hip allows movement in several planes, including circumduction. A hinge joint such as the knee mainly allows flexion and extension. Range of motion can be measured as a joint angle with a goniometer or image analysis.

• Cartilage reduces friction; synovial fluid lubricates.
• Ligaments connect and stabilize bones; tendons connect muscle to bone.
• Hip = ball-and-socket with wide multi-plane movement.
• Knee = hinge joint, mainly flexion and extension.
• Range of motion is recorded as an angle in degrees.

Intercostal muscles alter rib position and thoracic volume, helping create pressure differences that move air during breathing.

When inspiratory muscles contract, ribs move to increase thoracic volume and lower pressure. During quiet expiration, relaxation and elastic recoil reduce volume and raise pressure.

Trace breathing as: muscle action; rib movement; thoracic volume; pressure; air flow.

External intercostal contraction lifts the ribs, volume rises and air flows into the lungs because internal pressure falls below atmospheric pressure.

Muscles do not pull air directly into alveoli; they create pressure differences by changing thoracic volume.

Marine mammals reduce drag and control buoyancy with streamlined bodies, flippers, insulating tissues and powerful muscles adapted to swimming.

A tapered body lowers turbulence, flippers steer and propel, and blubber insulates while contributing to buoyancy. Efficient swimming conserves oxygen during dives.

Connect an adaptation to water movement: shape reduces drag; flippers change direction; insulation limits heat loss.

A streamlined seal can travel farther per stroke than a blunt body because less energy is lost to drag.

No single feature explains swimming success; streamlining may trade maneuverability or heat dissipation against efficiency.