Topic 6

• Stimulus


  A change in the internal or external environment
• Receptors


  Specialised cells or proteins in cell membranes which detect stimuli - they are specific to one type of stimulus
• Effectors


  Cells which bring about a response to a stimulus
• Nervous System


  • Communicates using electrical impulses
  • Made up of specialised cells known as neurones

  
• Types of neurones


  • Sensory neurons
  • Motor Neurones
  • Relay Neurone

  
• Sensory neurons transmit impulses from receptors to the CNS or relay neurone

  
• Motor Neurones transmit impulses from CNS/relay to the effectors

  
• Relay Neurone transmit impulses from sensory to motor neurones - only in reflexes

  
• The autonomic nervous system controls unconscious activities such as reflex responses and the response of heart rate to changes in carbon dioxide concentration or pressure
• Reflex


  A rapid, automatic response which does not need to be learnt
• Reflexes


  • Blinking
  • Contraction of the pupils when exposed to light
  • Withdrawal of the hands when one touches an hot object
  • Sneeze reflex when the nose is irritated
• Reflex arc


  • Pathway linking receptors to effectors
  • Consists of three neurones: sensory, relay and motor

  
• Example of a reflex arc: withdrawing your hand after touching a very hot object


  1. Stimulus: hot object

  2. Receptor: temperature and pain receptors in the skin

  3. Sensory neurone sends an impulse to the spinal cord via the dorsal root

  4. Relay (connector) neurone connects the sensory neurone to the motor neurone

  5. Motor neurone sends impulse to an effector via the ventral root

  6. Effector: arm muscles that contract to move hand away
• The brain can override the reflex via the relay neurone

  
• Importance of a reflex arc

  • Rapid
  • Protect against damage to body tissues
  • Do not have to be learnt
  • Help escape from predators
  • Enable homeostatic control
• Taxes


  Organisms move towards or away from a directional stimulus e.g. light
• Positive taxis


  Whole organism moves towards a favourable stimulus
• Negative taxis


  Whole organisms moves away from a unfavourable stimulus
• Taxes


  • Positive phototaxis as single celled algae move towards light (for photosynthesis)
  • Negative phototaxis as earthworms move away from light (to take them into the soil to conserve water, find food and avoid predators)
  • Positive chemotaxis as bacteria move towards high glucose concentration (more food)

  
• Kinesis


  The organisms' movement is affected by a non-directional stimulus e.g. humidity
• The organism responds to an unpleasant stimulus by increasing speed of movement and more rapid change in direction, both slow down when favourable conditions are met
• Kinesis


  • Woodlice lose water from their body in dry conditions so less likely to survive. If in dry conditions they move rapidly and change direction more often. When more moist conditions are encountered, speed and direction changes slow down, so more time is spent in these conditions
• Examination questions could ask about a variety of different environmental variables and organisms
• Using a choice chamber for woodlice

  1. Organism: woodlice

  2. Environmental variable: humidity

  
• Using a choice chamber for maggots

  1. Organism: maggots

  2. Environmental variable: light

  
• Taxis


  Organisms move towards or away from a directional stimulus
• Kinesis


  Organisms' movement is affected by a non-directional stimulus
• A p-value less than 0.05 (typically ≤ 0.05) is statistically significant. It indicates strong evidence against the null hypothesis, as there is less than a 5% probability the null is correct (and the results are due to chance). Therefore, we reject the null hypothesis.
• The probability of the differences being due to chance is less than 5%
• Skills in the examination questions

  • Identifying control variables
  • Describing and explaining observed distribution of organisms
  • Evaluating a conclusion
  • Adapting a given method to investigate a different variable
• General questions


  • Write a null hypothesis for the investigation
  • Name the type of behavioural response exhibited by the organism in this investigation. Give a reason for your answer
  • Suggest and explain one advantage of this behaviour to the organism
  • Suggest why the organism was left for 5 minutes before recording the results
• Tropism


  A response in a flowering plant to a directional stimulus. Plants respond to this stimulus by regulating their growth.
• Positive tropism


  Towards the stimulus
• Positive tropisms


  • Positive phototropism in leaves so that they can capture more light for photosynthesis
  • Positive gravitropism (geotropism) in roots so that they grow into the soil to absorb water and mineral ions

  
• Negative tropism


  Away from the stimulus
• Negative tropisms


  • Negative phototropism in roots to grow into the soil

  
• Growth factors


  Chemicals which speed up or slow down plant growth. They are produced in the growing regions of the plant (the shoot or root tips) and move to other tissues to regulate growth in response to directional stimuli.

  
• Auxins


  Growth factors which stimulate the growth of shoots by cell elongation. They cause cell walls to become 'loose and stretchy' so the cells elongate and become longer.

  
• How auxins stimulate cell elongation

  1. Auxins stimulate proton pumps in the cell membrane

  2. Protons are pumped into the cell wall and activate proteins called expansins

  3. Expansins break some of the hydrogen bonds between chains of cellulose

  4. Cell walls become 'loose and stretchy'

  5. Potassium channels are stimulated and K+ move into the cell, reducing the water potential of the cytoplasm

  6. Water moves in by osmosis, increasing cell volume and causing the cell to stretch and elongate

  
• Indoleacetic Acid (IAA)


  An important auxin produced in the meristem (tips) of shoots and roots of flowering plants. IAA stimulates growth by effecting the elongation of cells in roots and shoots.