Survival Flashcards
Define stimulus.
A detectable change in the internal or external environment of an organism that leads to a response in the organism.
What is the purpose of responding to a stimuli?
The ability to respond to stimuli is a characteristic of life and increases the chance of survival.
In what way is there a selection pressure favouring organisms with appropriate responses to stimuli?
The organisms that survive (by responding to stimuli) have a greater chance of raising offspring and passing their alleles onto the next generation. Therefore, there is a selection pressure which favours organisms with more appropriate responses.
Stimuli are detected by ___________
Receptors.
A _______________ forms a suitable response to a stimulus.
Coordinator.
Coordination may be at a molecular level or involved a large organ eg the brain.
A response is produced by an _________.
Effector.
What are the two means of communication throughout the body to respond to stimuli?
The nervous system and the hormonal system.
Outline the sequence of events after detecting a stimulus.
Stimulus -> Receptor -> Coordinator -> Effector -> Response
Define taxis.
A simple response whose direction is determined by the direction of the stimulus.
How does an organism respond to a taxis?
Motile organisms respond directly to environmental changes by moving its whole body either towards a favourable stimulus or away from an unfavourable one.
Define kinesis.
The organism doesn’t move towards or away from a stimulus. Instead, it changes the speed at which it changes direction.
The stimulus is less directional (eg humidity / temperature).
Define tropism.
The growth of a plant in response to a directional stimulus.
Give an example of why a plant might exhibit tropisms.
Positive phototropism - so their leaves are in the most favourable position to capture light for photosynthesis.
Positive geotropism - increases the probability that roots will grow into the soil, where they’re better able to absorb water and mineral ions.
Name 3 things plants respond to.
Light, gravity and water.
Why are shoots positively phototrophic?
Because light is needed for photosynthesis.
Why are roots positively hydrotrophic?
In order to absorb water for use in photosynthesis and other metabolic processes, as well as for support.
Give an example of a plant growing factor.
IAA (indoleacetic acid). This is an auxin.
Outline IAA in the shoot for phototropism.
In the shoot, IAA concentration increases on the shaded side. Cells elongate and the shoot bends towards the light.
Outline IAA in the root for phototropism.
IAA concentration increases in the shaded side.
As a result, growth is inhibited so the root bends away from the light.
Outline IAA in the shoot for geotropism.
In the shoot, IAA concentration increases on the lower side - so cells elongate so the shoot grows upwards.
Outline IAA in the roots for geotropism.
In the root, IAA concentration increases in the lower side - growth is inhibited so the root grows downwards.
