6. Organisms Respond To Changes In Their Environment Flashcards
Define stimulus
A stimulus is a detectable change in the internal or external environment of an organism that leads to a response in the organism
Give the sequence of events of the response to a stimulus
Stimulus —> receptor —> coordinator —> effector —> response
What is a taxis
A simple response whose direction is determined by the direction of a stimulus.
As a result a motile organism responds directly to environmental changes by moving its whole body either towards a favourable stimulus or away from an unfavourable one
What is meant by positive/negative taxis
Movement towards the stimulus = positive taxis
Movement away from stimulus = negative taxis
What is a kinesis
A kinesis is a form of response in which the organism does not move toward or away from a stimulus.
Instead it changes the speed at which it moves and the rate at which it changes direction
What is a tropism
A tropism is the growth of a plant in response to a directional stimulus
What are plant growth factors referring to
The hormone like substances involved in the responses of plants to external stimuli
Why do we refer to the hormone-like substances as plant growth factors over hormones ?
They exert their influence by affecting growth
They may be made by cells located throughout the plant rather than in particular organs
Unlike animal hormones, some plant growth factors affect the tissue that release them rather than acting on distant target organs
Give an example of a plant growth factor and its role
Indoleacetic acid (IAA)
- controls plant elongation
What is growth of a plant towards the light referred to as
Positive phototropism
or
Negative gravitropism
Describe the steps that explain positive phototropism
- Cells in the tip of the shoot produce IAA, which is then transported (initially evenly) down the shoot
- Light causes the movement of IAA from the light side to the shaded side of the shoot
- A greater conc of IAA builds up on the shaded side compared with the light side
- As IAA causes elongation of shoot cells, cells on the shaded side (side with greatest conc of IAA) elongate faster than the light side
- Therefore shoot tip bends towards the light
Describe the steps that explain positive gravitropism in plants
- Cells in the root tips produce IAA which is initially transported evenly to all sides of the root
- Gravity influences the movement of IAA from the upper side to the lower side of the root
- A greater conc of IAA builds up on the lower side of the root compared with the upper side
- As IAA inhibits elongation of root cells, the cells on the lower side of the root (region with greater IAA conc) elongate less than those on the upper side
- Therefore root bends downwards towards the force of gravity