Movement and reasons for locomotionSpec B3.3.1, B3.3.9
In short
Movement is a universal feature of living organisms: even sessile organisms, which stay fixed in one place, move parts of their bodies, while motile organisms move from place to place. Locomotion is movement of the whole organism, and animals use it for foraging for food, escaping from danger, searching for a mate and migration.
All living things move in some way, from cytoplasm streaming inside cells to whole animals travelling thousands of kilometres. Organisms are either motile (able to move from place to place) or sessile (fixed in one place).
| Species | Type | Adaptations for movement |
|---|---|---|
| Atlantic salmon (Salmo salar) | Motile | Streamlined body; blocks of muscle along each side contract alternately to bend the body and tail from side to side; fins give stability and steering. Adults migrate from the sea up rivers to spawn. |
| Venus flytrap (Dionaea muscipula) | Sessile | Rooted in the soil, so it cannot move from place to place. The two lobes of each modified leaf snap shut in about a tenth of a second when trigger hairs are touched twice in quick succession, trapping insects. Plants have no muscle: the movement comes from rapid changes in turgor that make the curved lobes flip shape. |
Locomotion is movement of the whole organism from one place to another.
| Reason | Example |
|---|---|
| Foraging for food | Honeybee workers fly from the hive to flowers to collect nectar and pollen. |
| Escaping from danger | Squid escape predators by jet propulsion, forcing water out of the mantle cavity. |
| Searching for a mate | Male silk moths fly towards females by following the pheromone the females release. |
| Migration | Arctic terns fly between Arctic breeding grounds and the Antarctic each year. |
Linking question: in what ways does locomotion contribute to evolution? Moving to find mates, food or new habitats affects which individuals survive and reproduce, and so which alleles are passed on.
Written and checked against the IB Biology HL specification · Updated October 2026