Summary
Summary Full AQA biology a level - responding to changes in internal and external environment unit 5 full notes
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, Int!nal and ext!nalstimuli are detected and
lead to a response
,Plants
● Chemicals that regulate plant growth response to directional stimuli.
● Produced in plant growing regions (apical meristems).
● Diffuse from cell to cell/ phloem mass transport.
, Plants - phototropism
1. Indoleacetic acid (IAA) diffuses to shaded side of shoot tip.
2. As IAA diffuses down shaded side, it causes active transport of H+ ions into cell wall.
3. Disruption to H-bonds between cellulose molecules & action of expansins make cell more
permeable to water. (acid growth hypothesis).
4. Cells on shaded side elongate faster due to higher turgor pressure.
5. Shoot bends towards light.
1. Gravity causes IAA to accumulate on lower side of the root.
2. IAA inhibits elongation of root cells.
3. Cells on the upper side of the root elongate faster, so the root tip bends downwards.
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