Unit 6 - Organisms respond to changes in their internal and external environments
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Summary response to stimuli
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Unit 6 - Organisms respond to changes in their internal and external environments
Institution
AQA
Summary notes for AQA A-level Biology response to stimuli. Includes clear information on stimuli, tropisms, IAA, reflex arcs, receptors, eye, rods and cones, control of heart rate. Summarised from class notes and the official course textbook. From an A* student.
Unit 6 - Organisms respond to changes in their internal and external environments
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Response to stimuli
Survival and response
• A stimulus is a detectable change in the external or internal environment of an organism
that leads to a response in the organism.
• The ability to respond to stimuli is important for the survival of an organism and so there
is a selection pressure favouring organisms with more appropriate responses
• Stimuli are detected by receptors which are specific to one type of stimulus
• A co-ordinator formulates a suitable response to a stimulus
• A response is produced by an effector
• One means of communication in large organisms is hormones, and another is the
nervous system
Sequence of events
Taxes
• A taxis is a simple response which is determined by the direction of the stimulus
• They are classified according to whether the movement is towards (positive taxis) or
away from (negative taxis) the stimulus
• Direction rather than intensity of stimulus is important
• e.g. Single-celled algae will move towards light (positive phototaxis). This increases their
chances of survival since, being photosynthetic, they require light to manufacture their
food
Kineses
• A kinesis is a form of response where the organism change the speed at which it moves
and the rate at which it changes direction
• Intensity rather than direction of stimulus is important
• e.g. Woodlice loose water from their bodies in dry conditions. When they move from a
damp area into a dry one, they move more rapidly and change direction more often. This
increases their chance of moving back into the damp area. Once back in the damp area,
they slow down and change direction less often. This means they are more likely to stay
within the damp area.
Tropisms
• A tropism is the growth of apart of a plant in response to a directional stimulus
• Towards (positive) or away from (negative) the stimulus
• e.g. 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 growth factors
Plants respond to:
1. Light: shoots grow towards the light for photosynthesis = positive phototropism
2. Gravity: plants need to be firmly anchored in the soil and roots grow in the direction of
gravity = positive geotropism
3. Water: almost all roots grow towards water to absorb it for support and metabolic
processes = positive hydrotropism
Plant growth factors
• Plant growth factors are how plants respond to external stimuli.
• They affect growth
• made by cells throughout the plant
• Some factors affect the tissues that release them
• Produced in small quantities
• Example is indoleacetic acid (IAA), a type of auxin which controls plant cell elongation
Phototropism
A young shoot will grow towards light directed at it from one side = unilateral light. This is
a positive phototropic response
1. Cells in the tip of the shoot produce IAA which is transported down the shoot
2. The IAA moves down the shoot and is transported evenly throughout all regions
3. Light causes the movement of IAA from the light side to the shaded side of the shoot
4. IAA builds up on the shaded side of the shoot
5. The cells on this side elongate more and so the shoot tip bends towards the light
In roots a high concentration of IAA inhibits cell elongation and so roots bend away from
light and are negatively phototrophic
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