module 6 Flashcards
what is a stimuli
a change in the internal or external environment
why do organisms need to respond to stimuli
for survival ( predator/prey awareness, homeostasis)
how do simple organisms respond to stimuli
taxis and kenesis
what is taxis
directional response to a stimuli (towards or away)
what is kinesis
non-directional movement from an unfavourable area to a favourable area
organism moves rapidly and randomly in unfavourable area until they reach favourable areas where they move more slowly and less randomly
so spends more time in favourable area
example of response to stimuli in plants
tropism
what is tropism
directional growth in plants in response to stimuli
(towards= positive, away=negative
light = photo, water= hydro, gravity=geo)
shoot shows positive phototropism and negative geotropism
root shows positive geotropism and positive hydrotropism
controlled by a plant growth factor= indoleactic acid (IAA)- auxin
what is a plant growth factor
equivalent to animal hormones
difference- made by cells throughout the plant, only affects cells locally, affects growth
what are the affects of IAA
promotes growth in the shoot
inhibits growth in the root
how does positive phototropism in the shoot take place
normaly- shoot tip produces IAA sending it down both sides causing the shoot to grow forwards
if light is present on one side, the IAA redistributes to the opposites side (shaded side)
this causes the opposite side to grow faster
so the shoot bends towards the light
how does negative geotropism in the shoot take place
if gravity is present on one side the IAA redistributes to the same side
this causes the same side to grow faster
so the shoot bends away from gravity towards the light
how does geotropism/hydrotropism in the root take place
if gravity/water is present on one side, the IAA to the same side
this causes the same side to grow slowly so the opposite side grows faster
so the root bends towards the gravity/water
evidence for tropism (positive phototropism in shoot)
removing or covering shoot tip prevents tropism (tip causes tropism)
placing micin (prevents movements of chemicals eg IAA) accross shoot inhibits tropism (tropism caused by movements of chemicals)
placing gelatine (prevents movements of electrical signals) accros the shoot does not affect tropism( tropism not caused by movement of electrical signals)
if shoot tip is moved to one side that side grows faster and the shoot bends the other way ( IAA promotes growth in shoot)
when in light or darkness the overall levels of IAA remain the same (light does not inhibit or breakdown IAA but rather distributes
