B6-STIMULI Flashcards
DEFINE stimulus
a change in the internal or external environment
DEFINE tropism
a directional response, growth of a plant toward or away from a stimulus
DEFINE positive response
movement towards stimulus
DEFINE negative response
movement away from stimulus
DEFINE phototropism
a growth response to light
DEFINE gravitropism
a growth response to gravity
DEFINE hydrotropic
a growth response to water
Give an example of taxis.
- Single celled algae moving towards light (positive phototaxis).
- Earthworms moving away from light (negative phototaxis).
- Both increases their chance of survival.
Kinesis is a form of response where an organism changes the speed at which it moves and the rate at which it changes direction.
- It is important when a stimulus is less directional such as humidity and temperature.
Why is kinesis carried out? Give an example of kinesis.
- It is carried out in order to increase the chance that the organism will enter more favourable conditions more rapidly.
- A woodlouse will speed up and change direction more rapidly in dry conditions to increase its chance of entering a damp region.
What is tropism?
Tropism is the growth of part of a plant in response to a directional stimulus.
What tropisms do plant shoots show?
- Plant shoots grow towards the light (positive phototropism).
- They grow away from gravity (negative gravitropism).
- This allows the leaves to be in an optimum position for capturing light.
What tropisms do roots show?
- Plant roots grow away from light (negative phototropism).
- They grow towards gravity (positive gravitropism).
- This increases the probability roots will grow into the soil.
- Plant roots also grow towards water (positive hydrotropism).
Difference between plant SHOOTS and ROOTS
SHOOTS=towards light
ROOTS= away light
What are plant growth factors?
- They are hormones that exert their influence by affecting growth and are made by cells
- Example: Indoleacetic acid (IAA) - Auxin.
how IAA works
causes active transport of H+ into cell walls of plants, this lowers the pH and disrupts the hydrogen bonding between microfibrils, makes cell wall more elastic and so can elongate
In flowering plant shoots, where is IAA produced and where is it transported?
- Cells in the tip produce IAA, which is transported down the shoot.
- Initially transported throughout the shoot evenly.
In terms of IAA, what happens when asymmetric illumination is detected by the plant?
- Asymmetric illumination is detected causing IAA to move to the darker side of the plant.
- IAA promotes cell elongation on the darker side.
- The darker side elongates faster than the lighter causing the shoot tip to bend towards the light.
In plant roots, where is IAA produced and where is it transported?
- Cells in the tip of the root produce IAA, which is then transported along the root.
- It is initially transported evenly.
How does gravity affect IAA in the roots and how does this influence the growth of the roots?
- Gravity influences the movement of IAA from the upper side to the lower side of the root.
- IAA inhibits the elongation of root cells so the lower side elongates less than the upper side.
- This causes the root to bend downwards as there is a greater elongation of cells on the upper side.
