Transcript Biology 102 - announcments

Biology 103 - Main points/Questions
1. How can plants respond to stimuli?
2. What stimuli will they respond to?
3. What are some of the chemicals that they
use to communicate?
A season of change at a Rhode
Island Stream...
Signal pathways link signals to
response
• Plants have cellular receptors that detect
changes in their environment
An example in potatoes
• A potato left growing in darkness
produces shoots that look unhealthy and
lacks elongated roots
• This helps the potato grow out of the soil
into the light, called etiolation
• But after exposure to light, a potato
undergoes changes called de-etiolation,
in which shoots and roots grow normally
(a) Before exposure to light
(b) After a week’s exposure to
natural daylight
A potato’s response to light is an example of
cell-signal processing
• Stimulus detected (increased light exposure)
• A signal is sent out and then…
• Some response occurs – in this case
changes in growth patterns and greening
• Response needs to be coordinated across
entire organism
Plant hormones
• Hormones are chemical signals that
coordinate different parts of an organism
• In plants many different hormones
coordinate a plants response to its
environment
This plant is growing in a window and is exhibiting a
growth pattern called positive phototropism!
Light!
The Discovery of Plant Hormones
• Any response resulting in curvature of
organs toward or away from a stimulus is
called a tropism
• Early experiments in tropisms led to the
discovery of the first plant hormones
• In the late 1800s, Charles Darwin and his
son Francis conducted experiments on
phototropism, a plant’s response to light
• They observed that a growing grass
seedling could bend toward light
Phototropism – growth in response to light
Darwin and his son
discovered that you
How could Opaque
you tell
could block
cap
oversensed?
tip.
phototropism if you where is light
covered the tip of
the plant
Clear cap
over tip.
They performed this control
experiment Opaque
- Why sleeve
do you
over
bending
think they did
this?
(what
region.
does it tell you?)
Porous gelatin
placed
between
Other
biologists
tip and shoot.
showed that the signal could
pass through gelatin from the tip to the lower part
of the plant.
Porous gelatin
placed between
tip and shoot.
Light
But that the signal didn’t
pass through an
Impenetrable
impenetrable
barrier
barrier between
tip and shoot.
Light
Porous gelatin
placed between
tip and shoot.
Impenetrable
barrier between
tip and shoot.
What does this tell you?
Light
Porous gelatin
placed between
tip and shoot.
Impenetrable
barrier between
tip and shoot.
Scientists decided this meant there was a
chemical signal that diffused through the gelatin!
• Later, in the 1920’s, a biologist used a
similar experiment to investigate the
chemical signal
• He removed the tip of several growing
plants then placed them on agar (a
substance a little like gelatin)
Tips placed
on agar.
• After some time he placed these agar
blocks, now infused with the signal, on
the plants that had had their tips removed
• What do you think will happen to these
plants? Why?
Agar without treatment
has no effect on plants
but agar that has been
under tips…
Figure 24.10 How Went demonstrated the effects of auxin
on plant growth
Agar that has been
treated causes cells to
elongate
Figure 24.10 How Went demonstrated the effects of auxin
on plant growth
If these treated agar
blocks are placed on
the edge the shoot
curves just like in
phototropism!
Figure 24.10 How Went demonstrated the effects
of auxin on plant growth
How phototropism works
• A chemical signal, Auxin, is produced in
the tip of the growing shoot.
• This signal causes cells below the tip to
elongate
• If there is more light on one side the
auxin moves to the shaded side of the
stem
• Excess auxin on
the shaded side
causes the curving
response
Figure 24.11 Auxin causes cells to elongate
Other Plant Hormones
• While auxin was the first hormone
discovered there are many other plant
hormones including:
• Gibberellins are synthesized in the apical
portions of roots and shoots and affect stem
elongation
The plant on the right
was treated with
giberellin the one on
the left was not
Figure 24.12 The effect of a gibberellin
Other Plant Hormones
• Gibberellins
• Cytokinins stimulate cell division in plants
and help determine the course of
differentiation
• Cytokinins work with Auxin to determine
what cells differentiate into
What are Axillary buds?
• What keeps them
dormant (not growing)?
• Apical dominance!
• Auxin from the structures
above – so remove the
structures…
Fig. 24.13.a
• The axillary buds grow!
• Only with cytokinin
around though…
Fig. 24.13.b
• This shoot development
occurs because there is
excess cytokinin and no
auxin.
• What do you think would
happen if you cut the tip
but added auxin?
Fig. 24.13.b
Lateral branches
“Stump” after
removal of
apical bud
• Excess auxin on the
tip blocks branch
growth
Apical bud removed
• Without auxin the lateral branches form –
what if you add auxin?
Auxin added to decapitated stem
Other Plant Hormones
• Gibberellins
• Cytokinins – important for root shoot
balance.
• Ethylene, when applied to fruit, hastens
ripening and can cause leaf senescence
Ripening tomatoes
• Depends on ethylene gas
• Tomatoes picked green are ripened after
shipping!
Figure 24.14 The effects of ethylene
Other Plant Hormones
• Gibberellins
• Cytokinins
• Ethylene
• One more…
• Just before dawn guard
cells are closed – but
light causes them to
pump potassium into
the cells. What will this
do?
• Potassium draws water & cells swell open!
• But what if plants need to conserve water?
Figure 24.15
Other Plant Hormones
• Gibberellins
• Cytokinins
• Ethylene
• Abscisic acid can cause plants close guard
cells in response to drought stress
Photoperiodism and Dormancy
• Photoperiodism plants sense seasonal
changes in day and night length
• three categories of plants
– long-day plants flower as days get longer
– short-day plants flower as days get shorter
– day-neutral plants use other cues to control
flowering
How photoperiodism works
•
• Short day
Long day plants flower
as
are
nights get shorter and
opposite
shorter
How photoperiodism works
•
Really it is
If you interrupt the night
night
length
though plants thinkthat
it is
is key!
two short nights… so who
flowers?
Photoperiodism
• Plants contain a pigment called phytocrome
that influences flowering
• this pigment exists in two interconvertible
forms Pr (inactive) and Pfr (active)
• in short-day plants, the presence of Pfr
suppresses flowering
Remember de-etiolation?
• Light is detected by
phytochrome!
(a) Before exposure to light
(b) After a week’s exposure to
natural daylight
1
Reception
2
Signal transduction
CYTOPLASM
NUCLEUS
Plasma
membrane
Phytochrome
activated
by light
Signal molecules inside
The cells of the potato
Cell
wall
Light
It triggers changes in the cell that
alter gene expression!
When phytochrome absorbes
light…
Signals in Animals
• Animals also need to coordinate activities in
a lot of different places
• As you know they can use the nervous
system to do this but…
• Animals use a large number of different
chemical signals
Signals in Animals
• Neurons use electrical changes for high
speed communication
• Diffusion of signal molecules important for
local communication
• Hormones are signal molecules that are
used over long distances
Local and long-distance cell communication in animals
Hormone Signals in Animals
• Used for longer term signals than neurons
• Different cells respond to different
hormones
• Hormones often key for homeostasis
33.02 The Timescale over
Which Chemical Messengers
Work
• CD33020.GIF
Hormone signaling is a series of simple steps
1. issuing the command
2. transporting the signal
–
most are transported through body by the blood
3. hitting the target
–
the hormone binds to a receptor on the target cell
4. having an effect
–
when the hormone binds, the protein changes shape
and triggers a change in cell activity
• Issuing
• Transport• “hit the
the
target”
command
Water vs. Lipid based
• Which is which?
– Steroids are lipids
– Peptide hormones are water soluble
• Peptide based
Fat-soluble
hormone
Watersoluble
hormone
– Bind to receptor
on cell
membrane
• Steroid
Signal receptor
Transport
protein
TARGET
CELL
(a)
Signal
receptor
NUCLEUS
(b)
– Transported
attached to a
protein
– Bind to receptor
inside the cell
• Peptide based
Fat-soluble
hormone
Watersoluble
hormone
Transport
protein
Signal receptor
TARGET
CELL
Cytoplasmic
response
OR
(a)
• Steroid
Signal
receptor
Gene
regulation
Cytoplasmic
response
NUCLEUS
(b)
– Signals are
often more
transient (just in
the cytoplasm)
– May alter gene
expression
Gene
regulation
– Mostly alter
gene
expression
– Tend to be long
lasting effects
Hormones are
produced in
glands
throughout
your body
Hormones are key players in
maintaining homeostasis
• Commonly used as signals in negative feedback
loops
• Remember Insulin & Glucagon?
• Insulin and glucagon are antagonistic hormones
that help maintain glucose homeostasis using
negative feedback