1. How Do Organisms Detect
Changes 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?
2. 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?
3. 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?
4. Two types of response
• Nervous
• Hormonal
• Which is quicker? Why is this important?
5. Nervous response
• What can you remember from year 10?
• Stimulus receptor coordinator effector
response
6. The simplest kinds or responses and
how they can increase an organisms
chance of survival
• Taxes
• Kineses
• Tropisms
7. 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.
8. 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.
9. 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?
10. Kineses
• A form of response in which the organisms
doesn't 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.
11. 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.
12. 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.
13. 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.
14. 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.
16. 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.
