AQA A Level Biology复习笔记6.1.4 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