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Summary AQA A level Biology Paper 2 topic 6 notes
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A detailed summary of the AQA A level Biology paper 2 topic 6 which has been edited to learn help the content and the mark scheme too.
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🌿T6: organisms respond to
changes in their internal and
external environment
responses
describe the reflex arc and its importance
-stimulus, receptor, sensory neuron, coordinator, relay neuron, motor
neuron, effector, response
-importance= rapid as there are only 3 neurones= autonomic as it
doesnt have to be learnt= protects from harmful stimuli
what does positive and negative mean?
-towards and away from
what is meant by kinesis?
-speed at which organism changes direction in response to stimulus
what is meant by taxis?
-directional response to stimulus
what is a stimulus?
-detectable change in the external or internal environment that leads to
a response in the organism
what is a tropism?
-growth in response to a stimulus
plant growth factors
what is indoleacetic acid (IAA)?
-belongs to family of auxins
-produced in tips of roots and shoots and controls cell elongation
T6: organisms respond to changes in their internal and external environment 1
, explain how a phototropism would work? shoots
1- IAA produced in tips of shoots and diffuses evenly
2- light causes IAA to diffuse from light to dark side so more IAA on
dark side
3- stimulates cell elongation in shoots= cells on shady side elongate
more
4- shoot tip bends towards light
explain how a gravitropism would work? roots
1- IAA produced in tips of root and diffuse evenly
2- gravity causes IAA to move from upper to lower side
3- IAA inhibits cell elongation in lower side of roots
4- root bends down towards gravity
what is the difference between the action of IAA in roots and shoots?
-in roots, IAA inhibits elongation
-in shoots, IAA stimulates elongation
explain how IAA works
1- synthesised in meristems and diffuses elsewhere to cause elongation
2- IAA binds to receptors on cells
3- IAA auxins produce second messenger signal molecules which
change gene expression
4- transcription for metabolic enzymes begins
5- acidification of cell wall because proton pumps stimulated to move
H+ from cytoplasm to cell wall
6- acidic conditions= expansins (enzyme) breaks bonds between cross
links of cellulose= weakens cell wall
7- changing conc of ions= uptake of water by osmosis
8- cell elongates
receptors
what are receptors?
-specific to single stimulus
T6: organisms respond to changes in their internal and external environment 2
, -transducers= convert between energy forms to produce generator
potential
-magnitude of generator potential varies with size and direction of
stimulus
what is the pacinian corpuscle?
-a receptor that only responds to mechanical pressure
what happens when it is at rest?
-channels are too narrow so Na+ cant diffuse through
-outside is more negatively charged than inside
-known as resting potential
what happens when its stimulated?
-pressure causes lamellae to stretch and deform
-stretch mediated Na+ channels open wider
-Na+ diffuses into neuron making it more positively charged
(depolarised)
-generator potential produced creating an action potential
what happens when light enters the eye?
-light rays enter the eye through the pupil
-light rays reach retina and are detected by photopigments in the rod
(rhodopsin) and the cone cells (iodopsin)
T6: organisms respond to changes in their internal and external environment 3
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