Unit 6 - Organisms respond to changes in their internal and external environments
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Summary Organisms respond to changes in their environment notes
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Unit 6 - Organisms respond to changes in their internal and external environments
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AQA
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Unit 6 - Organisms respond to changes in their internal and external environments
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Sunday, 4 September y
ORGANISMS RESPOND TO CHANGES IN THEIR ENVI-
RONMENTS
Response to Stimuli
Plants
Tropism
- a movement in response to a directional stimulus
Shoots responding to light
- IAA produced in the meristem of the plant shoot
- IAA moves to shaded side of the shoot in response to light
- Promotes cell elongation
- Elongation is facilitated by allowing water into the cell by osmosis
- this is a positive phototropism
Starch grains in roots
- sink to the bottom due to gravity as they are heavy
Why would scientists spray solutions on the bottom of leaves
- thicker cuticle on the top and no stomata
- so there is a longer diffusion distance
Types of movement
Taxis = a directional response to a stimulus (away / towards)
Kinesis = a non-directional response (response depends on intensity of the stimulus)
Reflexes
Simple reflex
Advantages
- Rapid responce
- Allows us to escape from predators
- is an instinct (does not need to be learnt)
Resting potential
How is it maintained when no pressure is applied
1
, Sunday, 4 September y
- sodium potassium pump (3:2 - more sodium pumped out than potassium moves in)
- the cell membrane of the sensory neurone is more permeable to potassium ions than
sodium
- sodium ions are actively transported out while potassium ions are pumped in
Pacinian corpuscle
Structure
- found at the end of a sensory neurone
- Covered in layers of connective tissue
- Separated by viscous gel (contains sodium ions)
- the membrane layers contain stretch mediated sodium ion channels
How are they involved in establishing a generator potential
- when pressure is applied it deforms and the stretch mediated sodium ion channels are
stretched and open
- allows sodium ions to diffuse into the axon
- a greater pressure causes more channels to open
- Causes depolarisation of the
mem- brane - causing a gener-
ator po- tential
Eye
Rod cells - sensitive to light - rhodopsin
2
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