Section 6 - 14 Response to stimuli Flashcards
Define - Stimulus
Detectable change in the internal or external environment of an organism that leads to a response in the organism.
Define - Selection pressure
External agents which affect an organism’s ability to survive in a given environment.
Define - receptors
Specific to one type of stimulus. A stimulus is detected by a receptor.
What does an effector do?
Produce a response.
What are the bodies two ways of communication?
The nervous system
the hormonal system.
What is the central coordinator?
Acts as a switchboard connecting information from reach receptor with the appropriate effector,
Summarise the sequence of events of communication
Stimulus -> receptor -> coordinator -> effector -> response
Define - Taxis
A simple response whose direction is determined by the direction of the stimulus.
What are the two types of taxis?
Positive - movement towards the stimulus
Negative - Away from the stimulus
Define - Kineses
Form of response in which the organism does not move towards or away from a stimulus. Instead, it changes speed and rate of change in direction to stay in optimum conditions more.
Define - Tropism
The growth of part of a plant in response to a directional stimulus.
What are the two types of tropism?
Positive - towards the stimulus
Negative - away from stimulus
How do shoots grow?
Towards the light - positive phototropism and Away from gravity - negative gravitropism
How do roots grow?
Away from light - negative phototropism. and towards gravity - positive gravitropism
Give examples of stimuli plants respond to
Light, gravity and water.
What is a plant growth factor?
External stimuli a plant response to: Exert influence by affecting growth made by cells throughout the plant rather than in particular organs affect the tissues that release them rather than acting on a distant target organ
Give an example of a plant growth factor
Indoleacetric acid (IAA) - a type of auxin controlling plant cell elongation.
Define - positive phototropism
young shoots growing towards the light that is directed at it from one side - unilateral light
Describe phototropism in flowering plants
- cells at tip of shoot produce IAA - transported down the shoot
- IAA initially transported evenly throughout all regions as it begins to move down the shoot
- light causes movement of IAA from the light side to the shaded side of the shoot
- greater conc. of IAA builds up on the shaded side of the shoot than on the light side.
- IAA causes elongation of shoot cells and there is a greater concentration on the shaded side the cells on this side elongate more
- shades side of shoot elongates faster than the light side causing the shoot tip to bend towards the light.
Explain the gravitropism in flowering plants
- cells in tip of root produce IAA transported along root
- IAA initially transported to all sides of the root
- gravity influences the movement of IAA from the upper side to the lower side of the root
- greater conc. of IAA builds upon the lower side of root than upper
- IAA inhibits elongation of root cells there is a greater conc of IAA on the lower side the cells on this side elongate less than those on the upper side
- relatively greater elongation of cells on the upper side compared to the lower side causes the root to bend downwards towards the force of gravity.
How does phototropism effect shoots?
High IAA conc. stimulates cell elongation causing positive phototropic response.
What effect does phototropism have on roots?
high IAA conc. inhibits cell elongation causing a negative phototropic response
What effect does gravitropism have on shoots?
high IAA conc. stimulates cell elongation causing a negative gravitropic response
What effect does gravitropism have on roots?
high IAA conc. inhibits cell elongation causing positive gravitropic response.
What effects does IAA have on plant cells?
- increase plasticity - ability to stretch cell walls
- elongate young cell walls.
- As older the cell walls become more rigid for older parts are unable to respond.
What is the acid growth hypothesis?
The proposed explanation of how IAA increases plasticity of cells.
- Active transport of hydrogen ions from cytoplasm into spaces in the cell wall causing the cell wall to become more plastic allowing the cell to elongate by expansion.