How Do Organisms DetectChanges in Their Environment? Lesson Objectives: •What are a stimulus and response? •What is the advantage to organisms of being able to respond to stimuli? •What are taxes, kineses an tropisms? •How does each type of response increase an organisms chance of survival?
What is a stimulus and response?• Using the text books available, make a list of all the stimuli and responses you can think of.• Do all animals respond in the same way and to the same stimuli?
Responding to light: Maggots• You are going to carry out an investigation into how the stimulus light affects the movement of maggots.• What sort of response is this?
Two types of response• Nervous• Hormonal• Which is quicker? Why is this important?
Nervous response• What can you remember from year 10?• Stimulus receptor coordinator effector response
The simplest kinds or responses andhow they can increase an organisms chance of survival • Taxes • Kineses • Tropisms
Taxes• A taxis is a simple response whose direction is determined by the direction of the stimulus.• This results in a motile animal responding directly to an environmental change by moving its whole body either towards a favourable stimulus of away from an unfavourable one.
Taxes Classification• Taxes are classified according to whether the taxis is:• Away from a stimulus = negative taxis• Towards a stimulus = positive taxis• Or by the nature of the stimulus.
Example of taxis• Single-celled algae move towards light (positive phototaxis). How does this increase their chance of survival?• Earthworms move away from light (negative phototaxis). How does this increase their chance of survival?• Some bacteria move towards a high conc. Of glucose (positive chemotaxis). How does this increase their chance of survival?
Kineses• A form of response in which the organisms doesnt move towards or away from a stimulus.• Instead the more un pleasant the stimulus the more rapidly it changes direction.• A kinesis therefore results in an increase in random movements.• This type of response is designed to bring the animal back to favourable conditions.• It is important when the stimulus is less directional e.g. humidity and temperature.
Kinesis Example: Woodlice• Woodlice loose water from their bodies in dry conditions.• When they are in a dry area they move more rapidly and change direction more often.• This increases their chance of moving into a different area.• If this area happens to be moist they slow down and change direction less often.• This means they are likely to stay where they are.
Kinesis Example: Woodlice• How does this increase their chance of survival?• They spend more time in favourable moist conditions then less favourable drier ones.• This prevents them drying out and so increases their chance of survival.
Tropisms• A tropisms is a growth or movement of part of a plant in response to a directional stimulus.• In almost all cases the plan part grows towards (positive response) or away from (negative response) the stimulus.• The type of response is named after the stimulus.
Tropisms examples• Plant shoots grow towards light (positive phototropism) so 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 geotropism) increasing the probability they will grow into soil.• Plant roots grow towards water (positive hydrotropism) so that the root system will develop where there is most water.
Questions• For each of the following statements, name the type of response described and the survival value of the response:1. Some species of bacteria move away from the waste products that they produce.2. The sperm cells of a moss plant are attracted towards a chemical produced by the female reproductive organ of another moss plant.3. The young stems of seedlings grow away from gravity.