Transcript Water Movement Within a Plant - mvhs

Water Movement Within a Plant
AP Biology
Unit 5
Review: Properties of Water
• Water is a polar molecule
• Water molecules can form a
“column”
– due to hydrogen bonds
between them
• Cohesion = Water is
attracted to other water
molecules
• Adhesion = Water is
attracted to other polar
molecules
d-
O
d+
H
H
dd+
d+
d+
Hydrogen
bond
Question…
• Why do plants wilt?
• When there is enough
water  it flows into
plant cells  cells are
plump with water
(turgid)
• If there isn’t enough
water  it flows out of
the cell  cells are limp
(flaccid)
Movement Into Roots
• Water is absorbed into the roots
through osmosis
• Osmosis = diffusion of water
• Osmosis is a passive process– no
energy required
• Water potential (ψ) determines
the direction that osmosis will
occur
Water Potential
• Pure H2O has ψ = 0
• Water potential is a combination
of the pressure from solutes
(ψs)and the physical pressure
(ψp).
• The more solutes there are in a
solution, the more negative the
water potential will be.
Water Potential
• Water always moves
from areas of higher
water potential (less
solutes) to lower water
potential (more
solutes).
• Moves from less
negative ψ to more
negative ψ.
No net
water
movement
Pathways of Water
• Apoplast Pathway = water travels between
cells (outside of them)
• Symplast Pathway = water travels through
cells (inside them to get from cell to cell)
Water Movement in Roots
• Water and ions are able
to freely diffuse up to a
point in the root cells
• To get into the stele
(where the vascular
tissue is located), the
water and ions must pass
through the cells of the
endodermis (symplast
pathway).
Casparian Strip
• Blocks the water from
crossing the endodermis
through apoplast
pathway
• control what enters the
vascular tissue
• Prevents water and
solutes from “leaking”
out into the soil
Water Transport in Plant
• Water is transported
through the plant in the
xylem
• Cells that make up the
xylem:
– Tracheids– found in all
plants
– Vessel Elements – found in
some plants
Formation of xylem
1. Cell dies.
2. Cell contents disintegrate.
3. Water can move through
these hollow cells with
little resistance.
Capillary Action
• movement of H2O up a very narrow tube
• Does account for a little water movement up
the xylem, but not much (about 40 cm)
Image obtained without permission from http://ianrpubs.unl.edu/fieldcrops/graphics/soilh2o5.gif
Transpiration Pull
• Accounts for most of the
water movement up the
xylem.
• As H2O evaporates from the
stomata (=transpiration),
water from the xylem moves
into the leaf to take its place.
• Because of the hydrogen
bonds between water
molecules, water is “pulled”
up the xylem.
Tension-Cohesion-Evaporation Model
Question…
• When you receive a bouquet of flowers,
why is it important to cut them under water?
• Transpiration is still continuing, so if you
cut it under water, it will ensure that water
(not air) gets into the xylem.
• Air bubble in xylem would disrupt the
column of water.
Transpiration Rate
• Factors that affect transpiration
rate include: light, humidity,
temperature.
• The plant might also close its
stomata to limit transpiration
• We’ll look at this in more detail
in the upcoming dry lab.
Stomata and Guard Cells
• Stoma is Greek for "mouth" (plural =
stomata)
• Function of Stoma = to allow gas exchange
with the outside environment (CO2 and O2)
• Guard cells regulate the opening of the
stomata
Stomatal Opening and Closing
• When guard cells are
full of water (plump)
 stretch away from
each other  stoma is
open
• When guard cells don’t
have much water in
them (limp)  don’t
pull away from each
other  stoma is
closed
Role of
+
K
• K+ ions (potassium) control
the opening of stoma by
changing the water content
in the guard cells.
– K+ in : H20 follows by
osmosis  stoma open
– K+ out: H20 follows by
osmosis  stoma closed
Question…
• What conditions favor stomatal opening?
– Plentiful water
– Moderate temperatures
Hormonal Control of Stomata
• Abscissic Acid (plant hormone) acts on
guard cells to close stomata.
– “stress” hormone in plants
– Farmers can use it as an anti-transpirant
(reduces water loss and the need for irrigation).
– There are genetically engineered plants that
have a mutant era gene—this makes them
highly sensitive to abscissic acid.