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B3.3.9 (HL)—Reasons for locomotion

Locomotion helps animals obtain food, escape predators, find mates, migrate between habitats and improve survival or reproductive success overall directly.

Syllabus
First assessment 2025
Objective
B3.3.9
Level
HL

Sessile, motile and locomotion

HL only
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.

Movement is universal in living organisms, but locomotion is movement of the whole organism from one place to another. Motile organisms locomote; sessile organisms remain attached yet can still move parts or grow directionally.

A motile animal can change location using metabolic energy and structures such as legs, wings or fins. A sessile coral can move tentacles, and a rooted plant can grow or bend toward a stimulus without locomoting.

Locomotion can improve survival and reproduction: a blackbird forages for food, a hare escapes a predator, an orangutan searches for a mate, and a whale migrates between feeding and breeding areas.

A plant shoot curving toward light demonstrates movement through differential growth, whereas a bird flying to a feeding site demonstrates locomotion because its whole body changes location.

Sessile does not mean incapable of movement, and locomotion is not cost-free: it requires energy and often increases nutritional demand and exposure to risk.

Muscle And Motility

HL only

A strong answer links movement benefit, muscle contraction mechanism, force transfer, joint range, and locomotion adaptations. For contraction, use calcium-troponin-tropomyosin and ATP-driven myosin cross-bridge cycling. For movement, use antagonistic muscles, tendons, ligaments, skeletons as levers, and joint type. For locomotion, link body form to survival or swimming advantage. Locomotion can improve survival and reproductive success.

  • Contraction answers need calcium, actin binding sites, myosin cross-bridges, ATP, and sarcomere shortening.
  • Movement answers need antagonistic pairs because muscles contract but do not actively extend.
  • Joint and skeleton answers need tendon versus ligament, lever action, synovial fluid, and range of motion.
  • Locomotion answers should link examples to food, escape, mate finding, or migration: blackbirds feeding, hares escaping predators, orangutans finding mates, and whales migrating.

Concept essentials

  • Locomotion can help animals find food and water.
  • Movement away from predators can improve survival.
  • Locomotion can help individuals find mates or breeding sites.
  • Migration can move animals between seasonal habitats.