Filaments slide past filaments. ATP burns. Bodies move. Cells, sarcomeres, joints.
Every sub-topic below feeds at least one of these questions.
How do muscles contract and cause movement?
What are the benefits to animals of having muscle tissue?
An extra 10 sub-topics for HL — same syllabus, deeper mechanism.
Adaptations for movement as a universal feature of living organisms
Sliding filament model of muscle contraction
Role of the protein titin and antagonistic muscles in muscle relaxation
Structure and function of motor units in skeletal muscle
Roles of skeletons as anchorage for muscles and as levers
Movement at a synovial joint
Range of motion of a joint
Internal and external intercostal muscles as an example of antagonistic muscle action to facilitate internal body movements
Reasons for locomotion
Adaptations for swimming in marine mammals
From bacteria following a sugar gradient to humans running marathons, movement is a universal feature of living things. Even "sessile" organisms move parts of themselves.
Bacteria use flagella to swim toward food.
Amoeba moves by reorganising its cytoskeleton, extending pseudopodia.
Paramecium swims using thousands of cilia.
Mammals use muscles attached to bones.
Adult barnacles are attached to rocks but extend feathery cirri to filter food from water.
Plants show tropisms — directional growth toward stimuli like light (phototropism).
Muscle contraction is the sliding of actin and myosin filaments past each other within sarcomeres — not the shortening of the filaments themselves.
Muscle tissue only exerts force by contracting. Returning to its relaxed state needs something else — either a molecular spring (titin) or an opposing muscle (antagonist).
Titin spans from the Z line to the M line through each sarcomere. It is highly elastic — helps the sarcomere recoil after stretching, and prevents overstretching during force production.
Muscles work in opposing pairs. When biceps contracts (flexing elbow), triceps relaxes and stretches. To extend the elbow, triceps contracts and biceps relaxes.
A motor unit is one motor neuron plus all the muscle fibres it innervates. The neuromuscular junction is where the signal jumps from nerve to muscle.
Motor neurons can be over 1 m long in humans (and over 5 m in giraffes). Their structure:
At the neuromuscular junction, acetylcholine released by the motor neuron binds to receptors on the sarcolemma, triggering an action potential in the muscle fibre.
Skeletons give muscles something to pull against, and translate muscle contraction into useful body movement.
Internal skeleton. Bones are inside; muscles attach to the outside of bones via tendons. Bones act as levers — contraction of muscles produces movement around joints.
External skeleton made of chitin. Muscles attach to the inside of the exoskeleton. Same lever principle — the exoskeleton transfers force into movement.
A synovial joint is a freely movable joint with a fluid-filled capsule. The IB-named example is the hip — femur fitting into the acetabulum of the pelvis.
| Structure | Function |
|---|---|
| Bones | Act as levers. Femur + pelvis in the hip joint. |
| Cartilage | Covers bone ends — reduces friction, absorbs shock. |
| Synovial fluid | Lubricates cartilage; supplies nutrients and O₂ to cartilage. |
| Joint capsule | Seals the synovial fluid in; limits joint movement. |
| Tendons | Anchor muscles to bones. |
| Ligaments | Connect bones to bones; stabilise the joint. |
Two joint types with different movement capabilities. Range of motion can be quantified with a goniometer.
Allows movement in only one direction (like a door hinge). Examples: elbow, knee, finger joints. Flexion (bending) and extension (straightening).
Spherical bone head fits into cup-shaped socket. Allows flexion, extension, abduction, adduction, and rotation. Examples: hip, shoulder.
Goniometry measures joint angles with a goniometer. Used in physiotherapy to assess injuries, recovery, and to compare healthy ranges of motion.
Internal and external intercostal muscles are oriented in opposite diagonal directions — when one contracts it stretches the other, storing energy in titin.
Their opposite orientations mean each layer stretches the other when it contracts — a textbook example of antagonistic muscle action in internal body movement.
Four major drivers of locomotion in animals — each with named examples.
Moving to find food. From grazing herbivores to predators tracking prey, foraging is the most universal driver.
Moving away from threats. Pronghorns have evolved extreme speed to outrun (now-extinct) predators.
Moving to encounter or attract mates. Male birds of paradise perform elaborate dance routines to attract females.
Seasonal long-distance movement for food, breeding or climate. Wildebeest follow rainfall across the Serengeti.
Whales, dolphins and seals evolved from terrestrial ancestors. Their adaptations to swimming illustrate evolutionary plasticity.
If you can't define one of these in a sentence, that's where to revise next.
“What are the advantages and disadvantages of dispersal of offspring from their parents?”
“In what ways does locomotion contribute to evolution within living organisms?”