14.1 - 14.5 Response to stimuli Flashcards
Define tropism
The growth of part of a plant in response to a directional stimulus
Positive = towards stimulus
Negative = away from stimulus
Define phototropism
The growth of a plant in response to light
Define geotropism
The growth of a plant in response to gravity
Describe how positive geotropism occurs
- IAA produced in root tip
- IAA accumulates on lower side of shoot
- IAA inhibits cell elongation, causing root to bend downwards
What is a plant growth factor?
- Hormones that effect growth
- Usually made by cells throughout the plant rather than by a particular organ
What is the main plant growth factor?
IAA (indoleacetic acid)
This belongs to a group of substances called auxins
Where is IAA produced?
By cells in the tip of the shoot or root
Describe how positive phototropism occurs
- IAA is produced in shoot tip
- IAA accumulates on shaded side
- IAA causes elongation of shoot cells, bending shoot towards the light
Describe how negative phototropism occurs
- IAA produced in root tip
- IAA accumulates on shaded side
- IAA inhibits cell elongation, bending root away from light
Describe how negative geotropism occurs
- IAA produced in shoot tip
- IAA accumulates on the lower side of the shoot
- IAA causes cell elongation, causing shoot to bend upwards
How does IAA affect plant cells?
- Increases plasticity of plant cell walls
- Only affects young plant cells as as cells mature they develop greater rigidity
Define stimulus
A change in the internal or external environment
Define taxis (plural is taxes)
A directional response to stimulus
* Negative = move away from
* Positive = move towards
Define phototaxis
Taxis in response to light
Define chemotaxis
Taxis in response to the concentration of a substance
Define kinesis
A non-directional movement in response to a stimulus
Organisms tend to move faster and change direction more frequently in less favourable environments
List the five stages of a reflex arc
- Stimulus - detectable change
- Receptor - detects stimulus
- Coordinator - formulates response
- Effector - Produces response
- Response
Describe central nervous system
Made up of the brain and spinal cord