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Plant Responses Revision…read more

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· Tropism ­ A growth-controlled directional response to a stimulus, in which
the direction of growth is related to the direction of the stimulus.
· Nastic Movements ­ Non-directional response to a stimulus generally
caused by changes in the turgor of specialised cells.
· Positive phototropism ­ Growth of shoots towards light
· Positive Geotropism ­ Growth of roots downwards in the direction of
· Negative Geotropism ­ Growth of shoots upwards in the opposite direction
to gravity.
Plant hormones ­ Substance produced by
plants that coordinates growth in the plant. Not
made in glands and do not travel far from point
of synthesis.…read more

Slide 3

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How does Auxin cause positive
· Phototropins = substances that acts as receptors for blue light.
1. Phototropins hit by blue light from one side of the plant
2. Causes phosphorylation of the phototropin (addition of a phosphate)
3. Phosphorylation activates a transporter protein in cell membranes to
allow auxin to travel sideways in the plant
4. More Auxin ends up on the shady side.
5. Auxin binds to receptors in plasma membrane of shady cells.
6. Affects transport of ions, so there is a build-up of hydrogen ions in the
cell walls.
7. Low pH activates enzymes called expansins ­ break cross links between
cellulose molecules.
8. Hydrogen bonding disrupted in cellulose cell walls.
9. Less cross-links = cells have larger ability to elongate.
10. Water into cell by osmosis. Increased cell swelling and cell elongates.…read more

Slide 4

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Apical Dominance
· The prescence and growth of the apex of the shoot (containing the
apical meristems) inhibits the growth of lateral buds.
· Evidence for Auxin's role in apical dominance:
1. Cut tip off 2 shoots and apply synthetic auxin to one. The one with the
added auxin will continue to display apical dominance, whereas lateral
buds are able to develop on the other.
2. If a growing shoot is turned upside down, apical dominance is no longer
exhibited. Auxin cannot be transported upwards against gravity, so
cannot reach lateral buds and prevent them from developing.
Gardeners prune to allow
side shoots to develop
(apex chopped off).…read more

Slide 5

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Gibberellin and stem elongation
· Gibberellin causes rapid growth of internodes
· Moves through plasma membranes and binds to receptor protein
· This binds to other receptor proteins, eventually causing DELLA proteins to
break down
· DELLA proteins normally bind to transcription factors, which are needed
to transcribe genes
· As the DELLA protein has been broken down, the transcription factor is
released and the gene can be transcribed.
· At internodes, cell elongation can occur due to loosening of cell walls and
cell division occurs due to increased production of a protein that controls
the cell cycle.…read more

Slide 6

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Mendel's peas and the Le allele
· If a low gibberellin conc was applied to dwarf peas, there was a rapid
growth of internodes. This is because dwarf plants lack the gene for
gibberellin production.
· Higher GA conc = taller plant
· Tall pea plants have the LeLe genotype
· Dwarf pea plants have the lele genotype
· The Le allele allows the plant to produce the enzyme to convert:
G20 GA in gibberellin production
Dwarf varieties of wheat have deliberately been grown without the gene for
GA production, so they put more energy into producing grain instead of
long stems.…read more

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