Sensory Reception

A stimulus is a detectable change in the environment of an organism that produces a response.  
The ability to respond to stimuli increases the chances of survival for an organism. Organisms that survive have a greater chance of raising offspring and of passing their alleles to the next generation.  Therefore there is always a selection pressure, favouring organisms with more appropriate responses.  

Stimuli are detected by calls known as receptors which transform the energy of a stimulus into some form of energy that can be rocess by the organism that leads to a response.  The response is carried out by one or more of a range of different cells, tissues, organs and systems. These are known as effectors.  

One type of communication between receptors and effectors is hormones, this is slow and found in plants and animals.  
Animals have a rapid means of communication. - Each receptor and effector is linked to a central coordinator of some type. - nervous system.

Stimulus --> receptor --> coordinator --> effector --> response 

A taxis is  simple response whose direction is detemined by the direction of the stimulus.  As a result a motole organism responds directly to envinonmental changes by moving an unfavourable one. Taxes are classified according  to whether the movement is towards the stimulus or away from the stimulus and also by the nature of the stimulus.  

Examples:

• Single celled algae will move towards the light(positive phototaxis). This increases their chanves of survival since, being photosynthetic, they need light to manufacture food. 
• Earthworms will move away from light (negative phototaxis). This increase their chances of survival because it takes them into the soil, where they are better able to conserve water, find food and avoid predators. 
• Some species of bacteria will move towards a region where glucose is more highly concentrated (positive chemotaxis). This increases their chances of survival because they use glucose as a source of food.  

• Kinesis is a form of response which the organism does not move towards or away from a stimulus. Instead the more unpleasant the stimulus, the more rapidly it moves and the more rapidly it changes direction. This type of response is meant to bring the organism back to favourable conditions.  

A Tropism is growth movement of part of a plant in response to a directional stumulus. In almost all cases the plant part grows towards (positive stimulus) or away(negative stimulus) from the stimulus.  

Examples:

• Plant roots grow towards light (positive phototropism) so that their leaves are in the mose favourable position to capture light for photo synthesis.
• Plant roots grow away from the light (negative phototropism) and towards gravity (positive geotropism). In both cases the response increases the probablilty that roots will grow into the soil, where they are better able to absorb water and mineral ions. 
• Plant roots grow towards water (positive hydrotropism) so that, within the soil, root systems will develop where there is most water.