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Animal Behaviour and Plant Responses. Year 13 Biology Detecting a stimulus • A stimulus is a change in the environment (external or internal) that causes a response in an organism. • A receptor is any cell or group of cells that can detect this change • A effector is a cell or group of cells that can respond to the change. Orientation responses of plants • Plants respond to light, gravity, water, chemicals and touch. They do this by a growth curve towards or away from a stimulus. • If the growth is towards the stimulus we say it is positive; if it is away from the stimulus we say it is negative. • The prefixes you learnt before tell us the type of stimulus e.g. photo = light. Tropisms • A tropism is a GROWTH response towards or away from an environmental stimulus coming from one direction. • For example – If the shoot of a plant grows towards the light, we say it is positively phototropic – If the root of a seedling grows down, then we say it is positively geotropic Nastic Responses • The response of plants to diffuse stimuli that do not come from any particular direction, such as the temperature, humidity and light that surround a plant, are called nastic responses. • For example – the opening and closing of flowers in response to different light intensities. Nastic Responses • Nastic movements are classified according to the nature of the stimulus • For example – Photonasty is the response to alterations in the light intensity – Thermonasty is the response to changes in the temperature Taxis • This is the movement of the whole animal, towards or away from a stimulus which is coming from one side only. • As with tropisms, movement towards a stimulus is positive and away from a stimulus is negative. • The stimulus is also denoted by the same prefixes. Taxis • Examples – Flatworms moving towards a pieces of raw meat are showing positive chemotaxis – Moths flying to a light are positively phototactic – Trout will line themselves up in an upstream direction, so they are positively rheotactic Taxis • Taxes often involve moving the head (which carries the sensory receptors) from side to side. • If there are two sensory organs then the animal can move directly towards or away from the stimulus as it can constantly check the position of the stimulus. • If there is only one sensory organ, the animal must move around to get information about the stimulus. E.g. a maggot must move its head from side to side to keep in position. It performs a zigzag ‘direct’ line. Kinesis • This is a non-directional response to a stimulus. • It is the change in activity rate in response to a change in the intensity of the stimulus. • Example – If woodlice are placed in a wet/dry choice chamber, the animals in the dry side increases their random movements and rate of turning compared with those on the wet side. Kinesis • Orthokinesis – the speed of the movement is related to the intensity of the stimulation • Klinokinesis – the amount of random turning is related to the intensity of the stimulation. • See diagrams for summary. Pheromones • A pheromone is a chemical produced by an animal and released into the external environment where it has an effect on the physiology or behaviour of members of the same species.