Response to changes in environment

• Sub-topics


  • 6.1 Stimuli, both internal and external, are detected and lead to a response
  • 6.2 Nervous coordination
  • 6.3 Skeletal muscles are stimulated to contract by nerves and act as effectors
  • 6.4 Homeostasis is the maintenance of a stable internal environment
• Stimulus


  A change in an organism's internal or external environment
• Importance of organisms responding to stimuli

  • Organisms increase their chance of survival by responding to stimuli
• Tropism


  Growth of a plant in response to a directional stimulus
• Positive tropism


  Growth towards a stimulus
• Negative tropism


  Growth away from a stimulus
• Role of growth factors in flowering plants

  1. Specific growth factors (hormone-like growth substances) eg. Auxins (such as IAA) move (via phloem or diffusion) from growing regions eg. shoot / root tips where they're produced

  2. To other tissues where they regulate growth in response to directional stimuli (tropisms)
• Effect of IAA on cells in shoots and roots

  • In shoots, high concentrations of IAA stimulate cell elongation
  • In roots, high concentrations of IAA inhibit cell elongation
• Gravitropism in flowering plants


  1. Cells in tip of shoot / root produce IAA

  2. IAA diffuses down shoot / root (evenly initially)

  3. IAA moves to lower side of shoot / root (so concentration increases)

  4. In shoots this stimulates cell elongation whereas in roots this inhibits cell elongation

  5. So shoots bend away from gravity whereas roots bend towards gravity
• Phototropism in flowering plants

  1. Cells in tip of shoot / root produce IAA

  2. IAA diffuses down shoot / root (evenly initially)

  3. IAA moves to shaded side of shoot / root (so concentration increases)

  4. In shoots this stimulates cell elongation whereas in roots this inhibits cell elongation

  5. So shoots bend towards light whereas roots bend away from light
• Taxis


  Directional response, movement towards or away from a directional stimulus
• Kinesis


  Non-directional response, speed of movement or rate of direction change changes in response to a non-directional stimulus
• Protective effect of a simple (eg. 3 neurone) reflex

  • Rapid as only 3 neurones and few synapses (synaptic transmission is slow)
  • Autonomic (doesn't involve conscious regions of brain) so doesn't have to be learnt
  • Protects from harmful stimuli eg. escape predators / prevents damage to body tissues
• Using a choice chamber to investigate the effect of an environmental variable on the movement of an animal

  1. Set up choice chamber (different compartments) to create different environmental conditions

  2. Control other environmental conditions

  3. Use a teaspoon to place 12 animals eg. woodlice on centre of mesh platform and cover with lid

  4. After a set amount of time eg. 10 minutes record the number of animals in each section

  5. Repeat after gently moving woodlice back to centre
• Reasons for leaving animals for 15 minutes before recording movement in a choice chamber
• Ensuring safe and ethical handling of animals
• Reason for using a mesh platform when investigating the effect of humidity
• Using a maze to investigate the effect of an environmental variable on the movement of an animal

  1. Change environment at one end of T shape eg. add food source

  2. Place animal eg. maggot in stem of T

  3. Record whether animal turns towards or away from food source

  4. Repeat with a large number of maggots, wiping / cleaning maze between trials

  5. Repeat with food on other side of T
• Reasons for not using the same organism more than once
• Reason for using a clean petri dish / maze each time
• Statistical test to analyse results
• Pacinian corpuscle

  Receptor that responds to mechanical pressure
• How a generator potential is established in a Pacinian corpuscle


  1. Mechanical stimulus eg. pressure deforms lamellae and stretch-mediated sodium ion (Na+) channels

  2. Na+ channels in membrane open and Na+ diffuse into sensory neurone

  3. Greater pressure causes more Na+ channels to open and more Na+ to enter

  4. This causes depolarisation, leading to a generator potential

  5. If generator potential reaches threshold it triggers an action potential

  
• What the Pacinian corpuscle illustrates


  • Receptors respond only to specific stimuli
  • Stimulation of a receptor leads to the establishment of a generator potential
• Sensitivity to light for rods and cones in the retina

  • Rods are more sensitive to light
  • Cones are less sensitive to light
• Visual acuity for rods and cones in the retina

  • Rods give lower visual acuity
  • Cones give higher visual acuity
• Sensitivity to colour for rods and cones in the retina

  • Rods allow monochromatic vision
  • Cones allow colour vision
• Myogenic

  Cardiac muscle can contract and relax without receiving electrical impulses from nerves
• Myogenic stimulation of the heart and transmission of a subsequent wave of electrical activity

  1. Sinoatrial node (SAN) acts as pacemaker → sends regular waves of electrical activity across atria

  2. Non-conducting tissue between atria / ventricles prevents impulse passing directly to ventricles

  3. Waves of electrical activity reach atrioventricular node (AVN) which delays impulse

  4. AVN sends wave of electrical activity down bundle of His, conducting wave between ventricles to apex where it branches into Purkyne tissue

  
• Location of chemoreceptors and pressure receptors

  Chemoreceptors and pressure receptors are located in the aorta and carotid arteries
• Roles of chemoreceptors, pressure receptors, the autonomic nervous system and effectors in controlling heart rate

  1. Baroreceptors detect [fall / rise] in blood pressure and / or chemoreceptors detect blood [rise / fall] in blood CO2 conc. or [fall / rise] in blood pH

  2. Send impulses to medulla / cardiac control centre

  3. Which send more frequent impulses to SAN along [sympathetic / parasympathetic] neurones

  4. So [more / less] frequent impulses sent from SAN and to / from AVN

  5. So cardiac muscle contracts [more / less] frequently

  6. So heart rate [increases / decreases]

  
• Resting potential

  Inside of axon has a negative charge relative to outside
• Autonomic nervous system


  Not specifically the sympathetic or parasympathetic pathways

  
• Medulla oblongata


  Controls heart rate via the autonomic nervous system
• Sympathetic nerves

  Involved in increasing heart rate
• Parasympathetic nerves

  Involved in decreasing heart rate
• The medulla / cardiac control centre sends impulses to the SAN along sympathetic / parasympathetic neurones.: 'This is not wrong, but in the context of controlling heart rate, the emphasis needs to be on the frequency of impulses. When more frequent impulses are sent along sympathetic neurones, heart rate increases. When more frequent impulses are sent along parasympathetic neurones, heart rate decreases.'
• Resting potential

  Inside of axon has a negative charge relative to outside (as more positive ions outside compared to inside)
• How a resting potential is established across the axon membrane in a neurone

  1. Na+/K+ pump actively transports (3) Na+ out of axon AND (2) K+ into axon

  2. Creating an electrochemical gradient: higher K+ conc. inside AND higher Na+ conc. outside

  3. Differential membrane permeability: more permeable to K+ → move out by facilitated diffusion, less permeable to Na+ (closed channels)

  
• How changes in membrane permeability lead to depolarisation and the generation of an action potential

  1. Stimulus: Na+ channels open; membrane permeability to Na+ increases, Na+ diffuse into axon down electrochemical gradient (causing depolarisation)

  2. Depolarisation: If threshold potential reached, an action potential is generated, as more voltage-gated Na+ channels open (positive feedback effect), so more Na+ diffuse in rapidly

  3. Repolarisation: Voltage-gated Na+ channels close, Voltage-gated K+ channels open; K+ diffuse out of axon

  4. Hyperpolarisation: K+ channels slow to close so there's a slight overshoot – too many K+ diffuse out

  5. Resting potential: Restored by Na+/K+ pump