1. Unit 5.9 Response to stimuli
By Sarah 6UC

2. 9.1 Sensory reception
Stimulus
A detectable change in the internal/external environment of an organism
E.g. harmful: predators/beneficial: a source of food
Receptor
Cells or organs which detect stimuli
Transforms the energy into some form of energy that can be processed by the organism
Coordinator
Allows communication between receptors and effectors
E.g. chemical: hormones (plants + animals)/electrical: nerves (ONLY animals)
Effector
A range of different cells, tissues, organs and systems
Carries out the response
Response
Increases an organism’s chances of survival and of raising offspring, passing on their alleles to the next generation
There is a selection pressure favouring organisms with more appropriate responses

3. Taxis (plural: taxes)
The direction of the response is determined by the direction of the stimulus (a motile organism moving its whole body towards a favourable stimulus or away from an unfavourable one)
Classified as:
Positive: movement towards the stimulus/Negative: movement away from the stimulus
Nature of the stimulus
Examples:
Single-celled algae moving towards light (positive phototaxis)
Earthworms moving away from light (negative phototaxis)
Bacteria moving towards a region of high glucose concentration (positive chemotaxis)

4. Kinesis (plural: kineses)
Important when a stimulus is less directional (e.g. temperature/humidity)
The organism does NOT move towards or away from a stimulus
Moves and changes direction rapidly in response to an unpleasant stimulus (increases the chance of moving back into favourable conditions)
Examples:
Woodlice – lose water from their bodies in dry conditions:
In a dry area: movement is more rapid and changes direction more rapidly
In a moist area: slows down and change direction less often

5. Tropisms
Growth movement of part of a plant in response to a directional stimulus
Classified as:
Positive or negative
Named after the stimulus
Examples:
Plant shoots grow towards light (positive phototropism)
Plant roots grow away from light (negative phototropism) and towards gravity (positive geotropism)
Plant roots grow towards water (positive hydrotropism)