Stimuli and Response - BWO

• What is a stimulus


  A change in an organisms internal or external environment
• Why is important for organisms to respond to stimuli

  Increase their chance of survival
• Role of growth factors in flowering plants

  Specific growth factors e.g. auxins such as IAA move (via phloem or diffusion) from growing regions e.g. shoot/root tips where they're produced

  To other tissues where they regulate growth in response to directional stimuli (tropism)
• What is a tropism


  Growth of a plant in response to a directional stimulus

  Positive tropism = towards a stimulus, negative tropism = away from a stimulus
• Describe how indoleacetic acid (IAA) affects cells in roots and shoots

  In shoots, high conc of IAA stimulates cell elongation

  In roots, high conc of IAA inhibits cell elongation
• Explain gravitropism in flowering plants

  Cells in tip of shoot/root produce IAA

  IAA diffuses down shoot/root evenly (initially)

  IAA moves to lower side of shoot/root (so conc increase)

  In shoots this stimulaties cell elongation whereas in roots this inhibits cell elongation

  So shoots bend away from gravity whereas roots bend towards gravity
• Explain phototropism in flowering plants

  Cells in tip of shoot/root produce IAA

  IAA diffuses down shoot/root even;y (initially)

  IAA moves to shaded side of shoot/root so conc increases

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

  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 directional change changes in response to a non-directional stimulus

  Depending on intensity of stimulus
• Protective effect of a simple reflex

  Rapid as only 3 neurones and few synapses

  Autonomic (doesnt involve conscious regions of brain) so doesnt have to be learnt

  Protects from harmful stimuli e.g. escape predators
• How is a generator potential established in a pacinian corpusle

  Mechanical stimulus e.g. pressure deformes lamellae and stretch mediated sodium ion channels

  So Na+ channels in membrane open and Na+ diffuse into sensory neuron (greater pressure causes more Na+ channels to open and more Na+ to open)

  This causes depolarisation, leading to a generator potential

  If generator potential reaches threshold it triggers an action potential
• What does the pacinian corpuscle illustrate

  Receptors only respond to specific stimuli (pacinian corpuscle only responds to mechanical pressure)

  Stimulation of a receptor leads to the establishment of a generator potential

  When threshold reached, action potential sent (all-or-nothing principle)
• Explain differences in sensitivity to light for rods and cones in the retina

  Rods are more sensitive to light - several rods connected to a single neurone, spatial summation to reach threshold quicker

  Cones are less sensitive to light - each cone connected to a single neurone, no spatial summation
• Explain differences in visual acuity for rods and cones in the retina

  Rods give lower visual acuity - several rods connected to a single neurone, so send a single set of impulses to brain (so cant distinguish between seperate sources of light)

  Cones give higher visual acuity - each cone connected to a single neurone, so send seperate sets of impulses to brain (so can distinguish between 2 seperate sources of light)
• Explain the differences in sensitivity to colour for rods and cones in the retina

  Rods allow monochromatic vision - one type of rod/only one pigment (rhodopsin)

  Cones allow colour vision - 3 types of iodopsin (red/blue/green) which absorb different wavelengths, different combinations of cones give range of colour
• Rods/Cones connect to bipolar neurones, which connect to ganglion cells, which send impulses to the brain via the optic nerve
• What does myogenic mean (cardiac muscle)


  It can contract and relax without receiving electrical impulses from nerves
• Pacinian Corpuscle

  A) Lamella

  B) Stretch mediated sodium channels

  C) Gel

  D) Nerve ending

  E) Sensory neurone

  F) axon + myelin sheath

  
• Rods and Cells

  A) optic nerve

  B) Cones

  C) Rods

  D) Bipolar neurone

  E) Ganglion cell

  
• Heart rate diagram

  A) Sinoatrian node (SAN)

  B) Bundle of His

  C) Purkyne Tissue

  D) Atrioventricular node (AVN)

  
• Stimulation of the heart + transmission of wave of electrical activity

  1. SAN acts as pacemaker - sends regular waves of electrical activity across atria, causing atria to contract simultaneously

  2. Non-conducting tissue between atria/ventricles prevents impulse passing directly to ventricles (preventing immediate contraction of ventricles)

  3. Waves of electrical activity reach AVN which delays impulse, allowing atria to fully contract and empty before ventricles contract

  4. AVN sends wave of electrical activity down bundle of His, conducting wave between ventricles to apex where it branches into Purkyne tissues, causing ventricles to contract simultaneously from base up
• Where are chemoreceptors and pressure receptors located?

  Chemoreceptors and pressure receptors are located in the aorta and carotid arteries
• What do baroreceptors and chemoreceptors detect

  Baroreceptors - change in blood pressure

  Chemoreceptors - change in blood pH
• Describe roles of chemoreceptors, pressure receptors, autonomic nervous system and effectors in controlling heart rate

  Baroreceptors detect fall / rise in blood pressure and / or chemoreceptors detect rise / fall in blood CO2 conc or blood pH

  Send impulses to medulla oblongata

  Which send more frequent to SAN along sympathetic / parasympathetic neurones

  So more / less frequent impulses sent from SAN to AVN

  So cardiac muscle contracts more / less frequently

  So heart rate increases / decreases
• What do sympathetic nerves do

  Increase heart rate
• What do parasympathetic nerves do

  Decrease heart rate