Module 6 (Response to Stimuli) Revision Notes
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 Kinesis
What is Taxis? directional response to a stimuli (towards or away from)
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
area where they move slowly and less randomly
so spends more time in favourable area, less time in unfavourable area
Example of Response to Stimuli in Plants? Tropism
What is Tropism?
directional growth in plants in response to a 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 = Indoleacetic 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?
normally: 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 opposite 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
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 Kinesis
What is Taxis? directional response to a stimuli (towards or away from)
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
area where they move slowly and less randomly
so spends more time in favourable area, less time in unfavourable area
Example of Response to Stimuli in Plants? Tropism
What is Tropism?
directional growth in plants in response to a 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 = Indoleacetic 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?
normally: 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 opposite 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
