Response to stimuli

• Behaviour


  The way an organism responds to a stimulus
• Kinesis


  behaviour that results in the change in the random movement of an animal, where an organisms either increases or decreases the number of turns it makes
• Stimulus


  Change in the internal or external environment of an organism
• Taxis


  behaviour that results in the change of direction of an organism, where the organism either moves towards or away from a stimulus
• stimulus


  detectable change in the internal or external environment that leads to a response
• Receptor


  detects stimulus, each receptor is specific to one type of stimulus
• Coordinator


  Formulates a suitable response to a stimulus
• Effector


  carries out the response to a stimulus
• Response

  change to adapt to a stimulus
• Tropisms


  a growth response of a plant towards or away from a stimulus
• Phototropism


  growth response to the direction of light
• Gravitropism


  growth response to gravity
• Hydrotropism


  growth response to water
• is growth towards the stimulus positive or negative

  positive
• is growth away from the stimulus positive or negative

  negative
• what does IAA do in the shoots of plants?

  promotes cell growth and elongation
• What does IAA do in the roots?

  inhibits cell growth and elongation
• How does positive phototropism occur?

  IAA produced in the tips of shoot cells

  IAA diffuses down the shaded side of the shoot so that there is a higher conc. in the shaded side

  IAA stimulates cell elongation

  Shaded side of the shoot grows more rapidly that the other side and the shoot bends towards the light
• How does negative gravitropism in shoots occur?

  IAA produced in the tips of cells and diffuses to the lower side of the shoot

  higher conc. of IAA in the lower side

  IAA stimulates cell elongation

  lower side of the shoot grows more rapidly than the upper side

  Shoot bends upwards
• How does positive gravitropism in roots occur?

  IAA produced in the tip of root cells

  IAA diffuses into the lower side of the root

  higher conc. of IAA in the lower side

  IAA inhibits cell elongation

  upper side of the shoot grows more rapidly that the lower side

  root bends downwards
• How does IAA work?

  IAA binds to a receptor protein on the cell surface membrane

  Stimulates ATPase protein pump to actively transport hydrogen ions from the cytoplasm into the cell wall

  Lowers the pH of the cell wall

  Activates expansins, proteins which loosen the bonds between cellulose microfibrils

  K+ channel opens

  Increases K+ conc. in the cytoplasm which decreases WP

  Water moves into the cell by osmosis, which is stored in the vacuole

  Increases pressure inside the cell which causes the cell wall to stretch and the cell to elongate.
• Reflex arc

  Stimulus --> Receptor --> Sensory neurone --> Relay neurone --> Motor neurone --> Effector --> Response
• Pacinian corpuscle


  receptor that responds to changes in pressure, found in the skin
• Pacinian corpuscle structure


  a single sensory neurone wrapped with layers of connective tissue separated by a gel containing Na+

  stretch-mediated Na+ channels in the membrane

  
• How is a generator potential established in a Pacinian corpuscle

  Pressure caused the stretch-mediated Na+ channels to deform causing Na+ to move into the axon through facilitated diffusion

  This causes depolarisation which causes a generator potential
• Receptors in the eye

  Rods and cones
• What do rods detect?

  light intensity
• What do cones detect?

  different wavelengths of light (colour)
• what is found in the retina?

  contains light receptor cells
• function of the optic nerve

  sensory neurone that carries impulses between the eye and the brain
• pigment in rod cells

  rhodopsin
• pigment in cone cells

  iodopsin
• visual acuity


  ability to distinguish between 2 seperate points
• why do rod cells have a lower visual acuity?

  As multiple rod cells synapse to the same bipolar cell and multiple bipolar cells synapse to a ganglion cell

  The brain is unable to interpret which impulse comes from which rod cell, this lowers the resolution of the image produced
• why do cone cells have a higher visual acuity?


  One cone cell synapses with one bipolar cell, which synapses to one ganglion cell, The brain is able to interpret which cone cell the impulse came from so the image produced has a higher resolution
• summation in rod cells

  one rod is unable to produce a large enough generator potential to stimulate the bipolar cell, so multiple generator potentials are combined to reach the threshold and stimulate a bipolar cell
• Benefit of summation in rod cells

  enables organisms to see in dimmer light
• types of cone cells

  Red-sensitive

  Blue-sensitive

  Green-sensitive
• myogenic


  muscle tissue (heart muscle) that generates its own contractions.
• what is the sinoatrial node and what does it do?

  it is a group of cells in the wall of the right atrium which initiates a wave of depolarisation that causes the atria to contract (atrial systole)