Organisms Respond to Changes in Environments

• Taxis


  A simple direct response where the direction of response is determined by direction of stimuli
• Kinesis


  A random response where the movement is not related to the direction of the stimulus (not moving towards or away)

  In favourable conditions organisms move around and change direction less. This allows them to remain in favourable conditions for longer

  The more unpleasant the stimulus the more the organism moves and the more it changes direction. This reduces the time spent in unfavourable conditions
• Simple response

  A stimulus is a detectable change in the internal or external environment of an organism that produces a response in the organism

  A receptor detects the stimulus

  A coordinator formulates a suitable response to a stimulus

  An effector produces a response
• Importance of stimuli response


  Increases the chance of survival

  Can detect and move away from harmful stimuli

  Can detect and move towards a source of food

  Organisms that can respond and survive have a greater chance of producing more offspring and passing on alleles
• Tropisms


  A growth response in plants where direction of response is determined by direction of stimulus

  Part of the plant moves

  In plants not in mobile organisms
• The response to light of shoots - positive phototropism


  Indoleacetic acid (IAA) is produced in the shoot tip

  Light causes the IAA to diffuse away from the light - to the shady side of the shoot

  On this side, the IAA causes elongation of the shoot cells whilst on the illuminated side are unaffected

  The shoot therefore bends towards the light for more photosynthesis
• The movement of roots in response to gravity - positive gravitropism

  In roots, the IAA diffuses to the bottom cells and inhibits cell elongation

  The upper cells elongate normally, causing the roots to bend downwards in the direction of gravity

  This is to absorb more mineral ions for growth e.g nitrates
• Simple reflexes


  Rapid automatic response to a stimulus which is not under conscious control

  They usually have survival value
• Advantages of simple reflexes


  Fast - short pathway with few neurones and synapses

  Protect us from harmful stimuli

  Involuntary - leaving the brain free to carry out more complex responses

  Innate - they don't have to be learned so are immediately protecting the organism from birth
• Resting potentials and the polarisation of the neurone membrane


  Occurs when the neurone is a rest

  A condition where the outside of the membrane is positively charged compared to the inside which is more negatively charged

  Neurone is said to be polarised

  Neurone has a voltage difference of -70mV
• How is resting potential maintained


  At rest, the sodium ion channels are closed

  Membrane is 100 times more permeable to K+ ions causing some to diffuse out

  The 'sodium-potassium' pump pumps 2K+ ions in for 3Na+ ions pumped out (active transport) this further creates a charge difference

  This causes outside of the membrane to be more positively charged compared to the inside. The inside of the membrane is more negative compared to the outside

  An electrochemical gradient is established
• Action potential


  The mechanism by which neurones send impulses. They are comprised of electrical impulses generated at the cell body and moving along the axon toward the axon terminals (in motor neurones)
• The action potential occurs in two stages


  Depolarisation

  Repolarisation
• Depolarisation in action potential 1

  Prior to depolarisation, the stimulus causes sodium ion channels to open and sodium ions diffuse in (the membrane is more permeable to them). If enough diffuse in then the change in p.d causes all the voltage gated sodium ion channels to open. This is an example of positive feedback.

  When the neurone is excited past its 'threshold' the following events occur:

  Sodium ions diffuse quickly into the axon as the voltage gated sodium ion channels open

  The inside of the axon becomes temporarily more positive while the outside become temporarily more negative
• Depolarisation in an action potential 2


  The reversal of charge is known as 'depolarisation'

  Adjacent vg sodium channels open to continue depolarisation
• Repolarisation

  This is restoring of the + charge on the outside of the axon membrane and more negative on the inside

  Both happen at the same time:

  1. Voltage gated potassium ion channels open and potassium floods out (facilitated diffusion) - this progressively makes the outside of the membrane more positive
  2. Voltage gated sodium ion channels close (no + ions can get inside)

  The sodium/potassium ion pump pumps sodium ions out of the cell

  This further creates the more positive charge outside with a more negative charge inside