Survival and response Flashcards
What is a stimulus
A change in the internal or external environment of an organism that leads to a response in the organism.
How does an organisms response to stimuli link to natural selection
- The ability to respond to stimuli increases an organisms chance of survival.
- Those organisms that survive have a greater change of raising offspring and and passing on their alleles.
- There is always a selection pressure favouring organisms with more appropriate responses.
What are receptors
Receptors detect stimuli and are specific to one type of stimulus.
What does a coordinator do
- A coordinator formulates a suitable response to a stimulus.
- Coordination may be at the molecular level or involve a large organ such as the brain.
What does an effector do
- Carries out the response to the stimulus
- This response could be at the molecular level to involve the behaviour of a whole organism.
What are the two key systems by which the body responds to stimuli
- The hormonal system
- The nervous system.
List the sequence of events that occurs in either chemical control or a nervous response
- Stimulus
- Receptor
- Coordinator
- Effector
- Response
What is a taxis
- A simple response whose direction is determined by the direction of a stimulus.
- Motile organisms respond to environmental changes by moving its whole body either towards a favourable stimulus or away from an unfavourable one.
How are taxes classified
They are classified according to whether the movement is towards the stimulus (positive taxis) or away from the stimulus (negative taxis)
What is phototaxis
A taxis response to light
What is chemotaxis
A taxis response to chemical changes
What is a kinesis
A form of response on which the organism responds to a stimuli by changing the speed at which it moves and the rate at which it changes direction.
Explain the advantage of kinesis
- In kinesis, the organism does not move towards or away from a stimulus.
- Instead it changes the speed at which it moves and the rate at which it changes direction.
- If an organism crosses a sharp dividing line between a favourable and an unfavourable environment, it’s rate of turning increases.
- This raises the changes of a quick return to a favourable environment.
- However if it moves a considerable distance into an unfavourable environment it’s rate of turning may slowly decrease so that it moves in long straight lines before it turns, often very sharply.
- This type of response tends to bring the organism into a new region with favourable conditions.
- It is important when the stimulus is less directional.
What is a tropism
The growth of part of a plant in response to a directional stimulus.
Describe the tropisms that plant shoots and roots exhibit
- Plant shoots grow towards light (positive phototropism) and away from gravity (negative gravitropism) so that their leaves are in the most favourable position to capture light for photosynthesis.
- Plant roots grow away from light (negative phototropism) and towards gravity (positive gravitropism). In both cases the response increases the probability that the roots will grow into the soil, where they are better able to absorb water and mineral ions.