Taxes and kineses as simple responses that can maintain a mobile organism…

The idea

Taxis and kinesis maintain mobile organisms in favourable environments. A taxis is a directional response: the organism moves its whole body towards or away from a directional stimulus (e.g., woodlice showing negative phototaxis move away from light). A kinesis is a non-directional response to a non-directional stimulus like humidity. On meeting unfavourable conditions, the organism speeds up and turns more frequently, doubling back to re-enter the favourable area. In favourable conditions, it slows and turns less, remaining in the optimal environment.

What earns the marks

• Taxis and kinesis maintain mobile organisms in favourable environments to reduce predation or water loss.
• Taxis is a directional response involving movement towards or away from a stimulus.
• Kinesis involves changing the rate of movement or turning in response to a non-directional stimulus.
• Organisms turn more frequently or move faster in unfavourable conditions to return to favourable areas.

Common mistakes and how to put them right

• Confusing a tropism (plant growth) with a taxis (movement in mobile organisms, including algae).

• Describing a kinesis as movement away from unfavourable conditions, which incorrectly implies direction.

• Naming a taxis without specifying the direction (e.g., omitting 'positive' or 'negative').

Exam tips

• Specify the stimulus and the sign, for example negative phototaxis or positive chemotaxis.
• If data show rate of turning or speed, identify it as a kinesis; if showing direction, it is a taxis.