Unit 9 Support and locomotion
Key unit competence: Explain and demonstrate modes of locomotion in protists, insects, fish, amphibians, birds and mammals.
Key ideas
- Movement is a change in position of part of an organism (all living things, including plants); locomotion is the movement of the whole organism from place to place. Animals move to find food, water, mates and shelter, to escape predators and unfavourable conditions, and to disperse to new areas.
- Locomotion in most animals depends on muscles (made of actin and myosin filaments that slide to produce contraction) pulling on a skeleton, which supports the body, protects organs and acts as a system of levers.
- Hydrostatic skeleton: fluid under pressure in a body cavity, against which muscles contract (earthworm, Hydra, roundworms). In the earthworm, circular and longitudinal muscles contract alternately in waves (peristalsis); chaetae grip the soil. Exoskeleton: chitin cuticle outside the muscles (arthropods); calcareous shells in molluscs; must be moulted to grow. Endoskeleton: living bone and cartilage inside the body (vertebrates), grows with the body.
- Non-muscular movement in protists: Amoeba moves by pseudopodia (cytoplasm flows forward: amoeboid movement, by changes between gel and sol states of cytoplasm). Paramecium moves by beating cilia in coordinated waves. Euglena and sperm move by flagella.
- Fish: streamlined body; myotomes (blocks of muscle) on each side contract alternately, throwing the body and tail into S-shaped waves that push water backwards (forward thrust from the caudal fin). Dorsal and ventral fins prevent rolling and yawing; paired pectoral and pelvic fins control pitching, steering and braking. The swim bladder adjusts buoyancy so the fish stays at a depth without effort. Scales overlap backwards and mucus reduces friction.
- Terrestrial vertebrates (amphibians, reptiles, mammals) use limbs as levers; the pentadactyl limb is attached to the body by the pectoral and pelvic girdles. Muscles work in antagonistic pairs (e.g. biceps/triceps; flexors and extensors). Toads and frogs jump using long, powerful hind limbs and swim with webbed feet.
- Bird flight: forelimbs modified into wings with feathers (large, light surface); hollow bones with struts, fused bones and no teeth (light, rigid body); large keel on the sternum for attachment of huge pectoralis major (downstroke) and pectoralis minor/supracoracoideus (upstroke) muscles; air sacs and efficient lungs supply oxygen; streamlined body; excellent eyesight and balance. Modes: flapping, gliding, soaring (on rising warm air), hovering. Ducks also swim with webbed feet.
- Insects: the exoskeleton acts as levers moved by muscles attached inside; legs are jointed. Grasshoppers jump with long hind legs (the femur contains large extensor muscles). Flight: direct flight muscles attached to the wings, or indirect muscles that change the shape of the thorax so that the wings flap very fast.
- Movement in plants: tropisms and nastic responses; support in plants comes from turgor pressure, lignified xylem, collenchyma and sclerenchyma.
Common misconceptions
- "Movement and locomotion mean the same." No: locomotion is moving the whole organism; plants show movement but not locomotion.
- "Muscles can push bones." No: muscles can only pull (contract); a second, antagonistic muscle returns the bone.
- "Fish stay up in water by swimming all the time." No: the swim bladder gives neutral buoyancy, so bony fish can hover without effort.
- "Birds' bones are heavy for strength." No: they are hollow with internal struts: light but strong.
Worked examples
Example 1. Explain how an earthworm moves through soil using its hydrostatic skeleton.
Solution
In a segment, circular muscles contract and longitudinal muscles relax: the segment becomes long and thin and pushes forward. Then longitudinal muscles contract and circular muscles relax: the segment becomes short and fat. Chaetae anchor the fat segments while the front extends, so waves of contraction move the worm forward. The fluid in the coelom, which cannot be compressed, transmits the force.
Example 2. Explain the role of the fins and swim bladder in a tilapia.
Solution
The caudal (tail) fin gives forward thrust; dorsal and anal fins stop rolling and yawing; pectoral and pelvic fins control pitching, steering and braking. The swim bladder is filled with gas; changing its volume changes the fish's density so it can stay at any depth without using energy.
Practice questions
Distinguish between movement and locomotion and give three reasons why animals need locomotion.
Show answer
Movement is a change in position of part of an organism; locomotion is movement of the whole organism from place to place. Reasons: to find food and water, escape predators, find mates, find shelter, escape harsh conditions, disperse.
Common mistake: Saying plants show locomotion.
Compare the three types of skeleton with examples.
Show answer
Hydrostatic: fluid under pressure (earthworm, Hydra). Exoskeleton: hard chitin or shell outside, moulted (insects, crabs, snails). Endoskeleton: living bone/cartilage inside, grows with the animal (fish, mammals).
Common mistake: Saying snails have an endoskeleton.
Explain how Amoeba moves.
Show answer
Cytoplasm flows forward forming a pseudopodium; the inner fluid endoplasm becomes gel-like ectoplasm at the front while the back converts back to fluid; the body flows into the pseudopodium.
Common mistake: Saying Amoeba uses cilia.
Describe how a fish swims forward.
Show answer
Myotomes on each side contract alternately, bending the body and tail from side to side in S-shaped waves; the tail and caudal fin push water backwards and sideways, giving forward thrust.
Common mistake: Saying fins alone push the fish.
State five adaptations of birds for flight.
Show answer
Wings with feathers; hollow light bones; keel for large flight muscles; streamlined body; air sacs and efficient lungs; no teeth (light beak); fused rigid skeleton; good eyesight.
Common mistake: Listing features unrelated to flight.
Explain how antagonistic muscles move the forelimb of a mammal.
Show answer
The flexor (biceps) contracts and the extensor (triceps) relaxes to bend the limb; the extensor contracts and the flexor relaxes to straighten it; muscles pull on bones by tendons across joints.
Common mistake: Saying one muscle pushes.
Compare the jumping of a grasshopper and a frog.
Show answer
Both use long, powerful hind limbs acting as levers. Grasshopper: muscles inside an exoskeleton of the femur; frog: muscles outside bones of an endoskeleton, with a strong pelvic girdle; both fold and suddenly extend the legs.
Common mistake: Saying the grasshopper has bones.
Explain how plants are supported without a skeleton.
Show answer
Turgid cells press against cell walls (turgor); lignified xylem and sclerenchyma give strength; collenchyma gives flexible support; wood (secondary xylem) supports trees.
Common mistake: Saying plants have bones.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Flight in which a bird uses rising warm air without flapping is: a. buoyancy 2. Paramecium moves by beating its: b. hydrostatic skeleton 3. The swim bladder of a bony fish controls: c. soaring 4. An earthworm is supported by a: d. cilia Show answer
1 → c (soaring) · 2 → d (cilia) · 3 → a (buoyancy) · 4 → b (hydrostatic skeleton)
- Eagles and vultures soar.
- Cilia beat in coordinated waves.
- It allows the fish to hover at a depth.
- Fluid in the coelom acts as the skeleton.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
pseudopodia · chaetae · girdles · ribs- Amoeba moves by .
- The bone-and-muscle structures attaching limbs to the vertebral column are the .
- The bristles that grip the soil during earthworm locomotion are .
Show answer
1: pseudopodia · 2: girdles · 3: chaetae
- Cytoplasm flows forward into a false foot.
- Pectoral and pelvic girdles.
- They anchor segments.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. The movement of the whole organism from place to place is: a. inside of the exoskeleton 2. The fin that gives the main forward thrust in a fish is the: b. caudal fin 3. An exoskeleton made of calcium carbonate is found in: c. locomotion 4. In insects, muscles are attached to the: d. snails Show answer
1 → c (locomotion) · 2 → b (caudal fin) · 3 → d (snails) · 4 → a (inside of the exoskeleton)
- Plants show movement but not locomotion.
- It is the tail fin.
- It is a calcareous shell.
- The exoskeleton acts as levers.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
mouthparts · antagonistic pairs · hind legs · pectoralis major muscle- The downstroke of a bird's wing is powered by the .
- Muscles that produce opposite movements at a joint are .
- Grasshoppers jump using their long .
Show answer
1: pectoralis major muscle · 2: antagonistic pairs · 3: hind legs
- It is the largest flight muscle.
- One contracts while the other relaxes.
- The femur contains strong muscles.