topic 6

• ORGANISMS RESPOND TO CHANGES IN THEIR INTERNAL & EXTERNAL ENVIRONMENTS
• Stimuli are detected & lead to a response
• Tropism


  Response of plants to stimuli via growth. Can be positive (growing towards) or negative (growing away)
• Tropisms


  • Plants respond to light, gravity and water
  • Controlled by specific growth factors like indoleacetic acid (IAA)
• Indoleacetic acid (IAA)


  A type of auxin that can control cell elongation in shoots and inhibit growth in roots
• Phototropism


  1. IAA diffuses to shaded side of shoot, causing more elongation and bending towards light

  2. In roots, high IAA inhibits elongation on lighter side, causing bending away from light
• Gravitropism


  • In shoots, IAA diffuses to lower side causing upward growth
  • In roots, IAA moves to lower side causing downward growth
• Taxes


  Simple response where an organism moves its entire body towards a favourable stimulus or away from an unfavourable one
• Kinesis


  When an organism changes its speed of movement and rate of direction changes in response to stimuli
• Phototaxis


  Movement towards a light stimulus
• Chemotaxis


  Movement towards a chemical stimulus
• Receptor


  Cells that detect stimuli
• Pacinian corpuscle


  • Receptor that responds to pressure changes, found deep in skin
  • Contains a sensory neurone wrapped in layers of tissue separated by gel
  • Has stretch-mediated sodium channels that open when pressure is applied
• Rods


  Photoreceptors in the retina that detect light intensity but not colour

  
• Cones


  Photoreceptors in the retina that detect colour and require high light intensity

  
• Rods have retinal convergence, allowing detection of low light levels
• Cones have one-to-one connection to bipolar cells, allowing high visual acuity
• Fovea has high concentration of cones, rest of retina has more rods

  
• Sinoatrial node (SAN)


  Pacemaker of the heart, located in right atrium

  
• Atrioventricular node (AVN)


  Transmits wave of depolarisation from atria to ventricles
• Bundle of His


  Conducts wave of depolarisation through septum to ventricles
• Purkyne fibres


  Conduct wave of depolarisation through walls of ventricles
• Cardiac cycle


  1. SAN triggers wave of depolarisation across atria

  2. AVN transmits to ventricles after delay

  3. Bundle of His and Purkyne fibres conduct to ventricles

  4. Cells then repolarise and relax

  
• Medulla oblongata


  Controls heart rate via autonomic nervous system

  
• Chemoreceptors


  Detect changes in blood pH
• Baroreceptors


  Detect changes in blood pressure
• Response to high blood pressure

  1. Pressure receptors detect stretch

  2. Medulla oblongata receives signals

  3. Parasympathetic signals sent to SAN to decrease heart rate

  
• Response to low blood pressure

  1. Pressure receptors detect less stretch

  2. Medulla oblongata receives signals

  3. Sympathetic signals sent to SAN to increase heart rate

  
• Response to low blood pH

  1. Chemoreceptors detect low pH

  2. Medulla oblongata receives signals

  3. Sympathetic signals sent to SAN to increase heart rate

  
• Organisms are more likely to survive if they can respond to stimuli
• The Pacinian corpuscle links to diffusion and protein channels
• Chemoreceptors triggering response to low blood pH links to enzymes denaturing
• Pressure receptors triggering response to high blood pressure links to structure and function of arteries
• Diffusion of IAA through plants links to transport across membranes
• Neurone structure


  • Cell body contains organelles and makes proteins/neurotransmitters
  • Dendrites carry action potentials
  • Axon is conductive fibre
  • Schwann cells form myelin sheath

  
• Pacinian corpuscle


  Links to diffusion and protein channels
• Chemoreceptors triggering a response to low blood pH


  Links to enzymes denaturing
• Pressure receptors triggering a response to high blood pressure


  Links to the structure and function of arteries, as these can be damaged by high blood pressure
• Diffusion of IAA through plants

  Links to transport across membranes
• Cell body of the neurone

  Contains the organelles found in a typical animal cell, including the nucleus. Proteins and neurotransmitter chemicals are made here