Response to stimuli
A stimulus is a detectable change in the internal or external environment of an organism.
A response is a change brought about by a stimulus.
The responses are generally designed to enhance the organisms’ chance of survival.
Responses
Taxes - Organism responds to a directional stimulus
® Eg. Light (phototaxis)
® Allows organism to move towards favourable conditions or away from danger.
Kinesis - Organism responds to a non-directional stimulus
® Eg. Humidity
® Move faster and more random when stimulus is unpleasant so the animal is
more likely to find favourable conditions.
Tropisms - Plant growth in response to a directional stimulus
Plant Growth
! Plants respond to light, gravity and water.
® Using growth factors – chemicals that speed up or slow down plant growth.
® Produced in growing regions and move to where needed.
® Growth factors called auxins stimulate cell elongation
! IAA is an important auxin
® Controls plant elongation
® High levels increase cell elongation in shoots
® High levels inhibit cell elongation in roots
Phototropism in flowering plants
• IAA is produced in the shoot tip
• IAA diffuses down the shoot
• IAA builds up on the shady side
• IAA stimulates shoot cell elongation
• Shoot bends towards the light
Gravitropism in flowering plants
• IAA is produced in the root tip
• IAA diffuses through the root
• IAA builds up on the lower side of the root
• IAA inhibits root cell elongation
• Root bends down
• In shoots, IAA on the lower side increases cell
elongation, causing the shoot to bend upwards.
A stimulus is a detectable change in the internal or external environment of an organism.
A response is a change brought about by a stimulus.
The responses are generally designed to enhance the organisms’ chance of survival.
Responses
Taxes - Organism responds to a directional stimulus
® Eg. Light (phototaxis)
® Allows organism to move towards favourable conditions or away from danger.
Kinesis - Organism responds to a non-directional stimulus
® Eg. Humidity
® Move faster and more random when stimulus is unpleasant so the animal is
more likely to find favourable conditions.
Tropisms - Plant growth in response to a directional stimulus
Plant Growth
! Plants respond to light, gravity and water.
® Using growth factors – chemicals that speed up or slow down plant growth.
® Produced in growing regions and move to where needed.
® Growth factors called auxins stimulate cell elongation
! IAA is an important auxin
® Controls plant elongation
® High levels increase cell elongation in shoots
® High levels inhibit cell elongation in roots
Phototropism in flowering plants
• IAA is produced in the shoot tip
• IAA diffuses down the shoot
• IAA builds up on the shady side
• IAA stimulates shoot cell elongation
• Shoot bends towards the light
Gravitropism in flowering plants
• IAA is produced in the root tip
• IAA diffuses through the root
• IAA builds up on the lower side of the root
• IAA inhibits root cell elongation
• Root bends down
• In shoots, IAA on the lower side increases cell
elongation, causing the shoot to bend upwards.
