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• What is a stimulus?


  A detectable change in the internal or external environment, leading to a response in the organism.
• how do organisms increase their chances of survival

  by responding to changes in the environment
• why do organisms carry out a response

  so conditions are kept optimal for metabolism
• What is a receptor?


  - A specialised cell that detects a stimulus

  - They are specific to one type of stimulus
• What is taxis?


  Taxis is a directonal response towards a stimulus
• Give an example of taxis.

  - Single-celled algae moving towards light (positive phototaxis).

  - Earthworms moving away from light (negative phototaxis).

  - Both increase their chance of survival.

  - Bacteria move to a region of highly concentrated glucose ( positive chemotaxis)- also moves towards chemicals to aid survival
• What is kinesis?


  A non-directional response towards a stimulus
• Why is kinesis carried out? Give an example of kinesis.

  - It is carried out in order to increase the chance that the organism will enter more favourable conditions more rapidly.

  - woodlice lose water in their body in dry conditions so they move more rapidly and change direction more often

  - this increases their chance of moving into a more favourable area
• What is tropism?


  Tropism is the growth response of a plant to a directional stimulus.
• what is the growth hormone in plants called

  auxins- IAA
• What tropisms do plant shoots show?

  - Plant shoots grow towards the light (positive phototropism).

  - They grow away from gravity (negative gravitropism).

  - This allows the leaves to be in an optimum position for capturing light.
• What tropisms do roots show?

  - Plant roots grow away from light (negative phototropism).

  - They grow towards gravity (positive gravitropism).

  - This increases the probability roots will grow into the soil.

  - Plant roots also grow towards water (positive hydrotropism).
• What are plant growth factors?

  - They are hormones that affect growth and are made by cells.

  - Example: Indoleacetic acid (IAA) - Auxin.
• In flowering plants, where is IAA produced and where is it transported?

  - Cells in the tip produce IAA, which is transported down the shoot.

  - Initially transported throughout the shoot evenly.
• In plant roots, where is IAA produced and where is it transported?

  - Cells in the tip of the root produce IAA, which is then transported along the root.

  - It is initially transported evenly.
• How does IAA affect the plasticity of the cell wall?

  - It increases the plasticity of the cell walls of young cells.

  - As the plant cells mature, they develop rigidity so older parts will not be able to respond.
• What does the CNS consist of?

  - Brain

  - Spinal cord
• What is the peripheral nervous system?


  Pairs of nerves that originate from the CNS.
• What is the function of sensory neurones?


  - Carry nerve impulses from receptors towards the CNS.
• What is the function of motor neurones?


  - Carry nerve impulses away from the CNS to effectors.
• What is the voluntary nervous system?

  It carries nerve impulses to body muscles under voluntary control.
• What is the autonomic nervous system?


  It carries nerve impulses to glands, smooth muscle and cardiac muscle and is not under voluntary control.
• What is a reflex?


  - A rapid, automatic response that can protect an organism from harmful stimuli.

  - It is involuntary.
• What is a reflex arc?


  - The pathway of neurones involved in a reflex.

  - It involves 3 neurones and bypasses the brain.

  - It involves the spinal cord (spinal reflex arc).
• why does negative phototropsm and positive gravatropism important

  to increase the probability that roots will grow into the soil where they can better able to absorb water and mineral ions
• how does negative phototropism occur

  in roots

  1) IAA diffuses to the shaded side

  2)inhibits cell elongation here so the root bends downwards
• why does positive phototropism and negative gravatropism important

  so leaves can be in a favourable position to capture light for photosynthesis
• how does positive phototropism happen

  light is detected by receptors in the shoot

  IAA diffuses to the shaded side

  IAA increases on shaded side

  IAA causes cells on the shaded side to elongate so shoot bends towards the light (upwards)
• how does negative gravotropism occur

  IAA diffuses to the lower to the lower side of the shoot

  IAA increasases on the lower side

  IAA causes cell on this side to eleongate so the shoot bends upwards
• how does positvie gravotropism happpen

  in roots

  -IAA diffuses to the lower side

  - Inhibits cell elongation root bends downwards
• darwins investigations of response to plant shoots to light

  light towards plant shoot

  - what happens and why

  shoot bends towards light

  - becuase the shoot is positivley phototropic - bending occurs behind the tip
• darwins investigations

  -light towards plant, shoot tip removed, tip discarded

  - what happens and why

  no response

  -the tip must either detect a stimulus or produce the messenger or both

  - so the removal prevents any response
• darwins investigations

  light towards plant and light cover is placed over intact tip of the shoot

  - what happens and why

  no respnse

  - becuase the light stimuilus must be detected by tip
• darwins investigations

  - tips are removed and replaced but dispalced on one side

  what happens and why

  - shoot bends towards the side where no tip is presnet
• Why are reflex arcs important ?

  1) Since it is an involuntary response it doesn't require any decision-making powers of the brain

  
  

  2) They protect the body from harmful stimuli.

  
  

  3) They are fast since the neurone pathways are short with only one or two synapses.

  
  

  4) they are effective from birth so dont have to be learnt
• How do simple reflexes occur?

  1) receptor detects a stimulus

  2)sensory neurone carries impulse from receptor to relay neurone

  3) relay neurone carries impulse diresctly from sensory neurone to motor neurone

  4)motor neuron carries impulse from relay neuron to the effector

  effector carries out a response to a muscle or galnd
• how is a generator potential established

  if stimulus is large enough it leads to a genertor response being establisehd which can cause a response
• Where is the Pacinian corpuscle located?

  dermis of the skin and joints
• what is the function of a pacinian corpsucle


  respond to changes in mechanical pressure
• structure of the paccinian corpsucle

  - made up of lamellae which surround the end of a sensory neuron