Taxis & Kinesis (AQA A Level Biology)

Taxis & Kinesis

• An organism must find favourable living conditions in order to survive

• For complex organisms like monkeys and wolves, this seems like an easy process

  • They possess eyes, ears and nervous systems that allow them to observe and process important information about their environment

• Organisms like flatworms that do not possess complex nervous systems are still able to flee from trouble and find ideal living conditions

  • These organisms display behaviours known as taxes and kineses

• Taxes and kineses are simple responses that enable mobile organisms to stay in a favourable environment

Kinesis

• Kinesis is a non-directional response to a stimulus

• The rate of movement of an organism is affected by the intensity of the stimulus

• Flatworms called planarians possess a network of neurones and simple eye-like structures that have light-sensitive cells

• Planarians display kinesis when removed from their usual dark environment

  • Planarians are found on the underside of stones, hidden from daylight

  • When a stone is removed or turned over the planarians begin to move in random directions

  • Once these random movements eventually bring them back into the darkness they stop moving

  • This type of responsive behaviour helps them to protect themselves from predators

• In the scenario above, the light-sensitive cells are detecting light when the stone is overturned but the planarian has no way of detecting the nearest shaded space, therefore it moves randomly until the eye detects a low level or no light

  • The planarian uses kinesis to ensure it is in its favourable environment - darkness

Taxis

• Taxis is a directional response to a stimulus

• The organism moves directly away from or towards the stimulus

• A single-celled organism called Euglena which is commonly found in ponds exhibits taxis

  • It has chloroplasts for photosynthesis and a flagellum to help it swim

  • The flagellum has a receptor close to its base that is sensitive to light

  • Euglena swims directly towards the light, this is known as phototaxis

  • This behaviour is highly valuable as it brings the organism towards the light where it can photosynthesise

Investigating taxes and kineses

• Taxes and kineses behaviour in small animals can be studied using special apparatus

• Choice chambers and mazes are common pieces of apparatus that are used

• Woodlice and maggots are often the organisms studied

• It can be difficult to distinguish taxis from kinesis in these experiments

  • The animals need to be observed during the experiment to see if turning frequency or movement rate changes in different environments

  • If movement is directional then the turning frequency would decrease when the organism detects the stimulus

Choice chambers

• An experiment was conducted to investigate whether maggots exhibited negative phototaxis

  • This would mean that they moved away from bright light (not randomly)

• One half of the transparent choice chamber was covered in an opaque material to prevent light from entering

• 30 maggots were placed into the chamber via the hole in the centre of the lid

• 10 minutes later the number of maggots found in each half of the chamber were counted

• This was repeated several times

• The results showed that there was always more maggots in the shaded half of the chamber at the end of the experiment

  • As the maggots were not observed during the experiment it can not be said whether kinesis or taxis has occurred

  • However, the results do conclude that maggots have the ability to detect bright light and respond by moving until they reach a more favourable environment

The choice chamber can be altered in different ways to create subsections with different abiotic conditions