Survival and Response Flashcards
What is a Stimulus?
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 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 the unfavourable area until they reach the favourable area where they move slowly and less randomly
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
The shoot shows positive phototropism and negative geotropism
Root shows positive geotropism and positive hydrotropism
What is a Plant Growth Factor?
Indoleacetic Acid (IAA) - auxin
equivalent to animal hormones
difference: made by cells throughout the plant, only affects cells locally, affects growth
What are the effects of IAA?
Promotes growth in the shoot, inhibits growth in the root
How does positive phototropism in the shoot take place?
Normally: the shoot tip produces IAA, sending it down both sides and causing the shoot to grow forwards
If the 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 toward 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 positive geotropism/hydrotropism in the root take place?
If gravity/water is present on one side, the IAA redistributes 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 the shoot)
Removing or covering the shoot tip prevents tropism [tip causes tropism]
Placing micin (prevents movement of chemicals e.g. IAA) across the shoot inhibits tropism [tropism caused by movement of chemicals]
Placing gelatine (prevents movement of electrical signals) across the shoot does not affect tropism [tropism not caused by movement of electrical signals]
If the shoot tip is moved to one side, that side grows faster and the shoot bends the other way [IAA promotes growth in the shoot]
When in light or darkness the overall levels of IAA remain the same [light does not inhibit or breakdown IAA but rather redistributes it]
