Unit 5.9 Response to stimuli By Sarah 6UC

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

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)

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

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)