branch ANIMALS

=TAXES= A Taxis is a directional response to a directional stimulus '''Taxes usually involve movement of whole organism (motility) rather than growth''' '''Taxes are Directional and can be can''' • '''''Positive''''' -towards the stimulus • '''''Negative''''' -away from the stimulus [image:] '''Positive Chemotaxis''' The bacteria shown (left) are attracted to a chemical stimulus on the left hand side of the image. They have moved towards it into an area where it is most concentrated. [image:] '''Positive Phototaxis''' Many insects display a positive ''phototaxis'' and will fly towards a light source. Taxes can be: In order for an animal to exhibit a directional response toward a directional stimulus, it must have some method of recognising the direction of the stimulus (e.g. to fly towards the light the bugs must have some way of knowing from which direction the light is coming). The direction can be determined by two different methods: ===Tropotaxis=== '''Two or more receptors sample and compare the intensity of the stimulus on either side of the organism giving it an idea of the direction of the stimulus'''. When we hear a noise we almost instinctively know where it came from. We actually determine the direction of the noise by which ear receives the sound first and loudest. ===Klinotaxis=== '''A single receptor samples the intensity of the stimulus as the organism moves in order to determine the direction of the stimulus'''. Think of swimming in a pool which has a ‘hot’ end and a ‘cold’ end. If you were in the middle of the pool and did not know which end was which, you probably couldn’t tell the difference by just sitting there. You would have to start moving through the water to determine the direction of the temperature gradient. Sort of a “you’re getting warmer, warmer…”