1. Animal Behaviour and Plant Responses.
Year 13 Biology

2. 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.

3. 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.

4. 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

5. 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.

6. 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

7. 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.

8. 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

9. 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.

10. 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.

11. 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.

12. 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.