AQA A Level Biology复习笔记6.1.4 Taxis & Kinesis

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

     

Choice-chamber

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

Exam Tip

Plants, bacteria, fungi and smaller organisms do not have the ability to think like humans. Make sure not to use language such as "they want" when answering questions in relation to their responses to the environment.

 

 

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