Download 9.1 Auxins and Growth

Hormones and Plant Growth
IB Assessment Statement
Explain the role of auxin in
phototropism as an example of
the control of plant growth.
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Patterns of Plant Growth
Patterns of Plant Growth
Biologists have discovered that plant cells send
signals to one another that indicate the following:
•
•
when to divide and when not to divide,
and when to develop into a new kind of cell.
Plant Hormones
How do plants know they need to grow?
Plant Hormones
A hormone is a substance that is produced in one
part of an organism and affects another part of the
same individual.
Plant Hormones
Hormone
producing cells
The portion of an
organism affected by
a particular hormone
is known as its
target cell or target
tissue.
Movement of
hormone
Target cells
Plant Hormones
To respond to a hormone, the target cell must contain
a receptor to which the hormone binds.
If the receptor is present, the hormone can influence
the target cell by:
•
changing its metabolism
•
affecting its growth rate
•
activating the transcription of certain genes
Auxins
Auxins
Charles Darwin and his son Francis carried out the
experiment that led to the discovery of the first
plant hormone.
They described an experiment in which oat
seedlings demonstrated a response known as
phototropism—the tendency of a plant to grow
toward a source of light.
Auxins
Darwin’s Experiment
In the experiment, they
placed an opaque cap
over the tip of one of the
oat seedlings.
This plant did not bend
toward the light, even
though the rest of the
plant was uncovered.
Auxins
However, if an opaque
shield was placed a few
centimeters below the
tip, the plant would bend
toward the light as if the
shield were not there.
Auxins
The Darwins suspected that the tip of each
seedling produced substances that regulated cell
growth.
Forty years later, these substances were
identified and named auxins.
Auxins
When light hits one side
of the stem, the shaded
part develops a higher
concentration of
auxins.
This change in
concentration stimulates
cells on the dark side to
elongate (get longer).
Auxins
As a result, the stem
bends away from the
shaded side and
toward the light.
Recent experiments
have shown that
auxins migrate
toward the shaded
side of the stem.
Location of Auxins in Plant….auxins are found in
•the embryo of seeds
•meristems of apical buds
•And young leaves
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What cells do auxin affect?:
• Auxins will only affect cells that have auxin
receptors
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How does auxins affect cells?:
• Cells with auxin will elongate
• In stems auxin concentrate on the shaded side.
• Auxin causes transport of hydrogen ions from
cytoplasm to cell wall
• This decrease in pH / H+ pumping breaks bonds
between cell wall fibres;
• This makes cell walls
flexible/extensible/plastic/softens cell walls and
makes cells enlarge/grow;
• This elongation of the stem will cause the plant to
bend towards the light.
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