Demonstrate understanding of the responses of plants and animals to their external environment
What this domain tests
This domain assesses how plants and animals respond to external stimuli. You need to explain receptors, effectors and responses, and compare plant tropisms and nastic responses with animal taxes, kinesis, migration and biological rhythms.
Core rules and concepts
- A stimulus is detected by a receptor; a response is carried out by an effector.
- Plant tropisms are directional growth responses: phototropism (light), geotropism (gravity), hydrotropism (water), thigmotropism (touch).
- Nastic responses are non-directional, such as the closing of mimosa leaflets or Venus flytrap closure.
- Plant hormones: auxin promotes cell elongation and is redistributed in tropisms; gibberellin promotes stem elongation and seed germination; abscisic acid closes stomata and promotes dormancy; ethylene ripens fruit and promotes abscission.
- Animal responses include taxes (directional movement, e.g., positive phototaxis), kinesis (non-directional change in activity rate), migration, homing, and circadian rhythms.
- Communication includes pheromones, sound and visual signals.
Worked example
A bean seedling is lit from one side. Auxin is produced at the shoot tip and moves to the shaded side. Higher auxin concentration causes greater cell elongation on the shaded side, so the shoot bends towards the light. This is positive phototropism. The response increases light capture for photosynthesis. If the tip is removed, bending does not occur, showing the tip is the site of auxin production.
Traps that cost marks
Calling a tropism a nastic response; confusing taxis with kinesis; saying plants move without explaining growth; forgetting that auxin is redistributed, not just produced.