Download Water Potential

Transport in plants occurs
across a network of vessels
and over long distances
1
Lecture 6 Outline (Ch. 36 & 37)
I.
Plant Transport Overview
II.
Driving Forces
A.
B.
C.
D.
Water potential
Transpiration & Bulk Flow
in Xylem
Stomata Control
Positive Pressure & Bulk
Flow in Phloem
III. Mineral Acquisition
IV. Essential Nutrients
V.
Relationships with other
organisms
VI. Preparation for next
lecture
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Transport in Plants
Physical forces drive the
transport of materials in plants
over a range of distances
Transport occurs on three scales
1. Within a cell – cellular level
2. Short-distance cell to cell –
tissue level
3. Long-distance in xylem &
phloem - whole plant level
Transport occurs by 3 mechanisms:
A. Osmosis & Diffusion
B. Active Transport
C. Bulk Flow
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Transport in Plants – Water Potential
Roots  xylem  stomata
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To survive
Water Potential
– Plants must balance water uptake and loss
• What is Osmosis? What is diffusion?
• Water potential : predicts water movement due to solute
concentration & pressure
– designated as psi (ψ)
Water molecules are
attracted to:
• Each other (cohesion)
• Solid surfaces (adhesion)
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Water Potential
• Free water flows from regions of high water
potential to regions of low water potential
Ψ changes with:
• Adding solutes
• Adding pressure
Water potential = Potential energy of water =
Energy per volume of water in megapascals (MPa)
ψTotal = ψsolute + ψpressure
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Water Potential
(a)
• Solutes added
 decreases ψ
0.1 M
solution
(water less likely to cross
membrane)
Pure
water
(in an open area, no
pressure, so ψp = 0)
H2O
 = 0 MPa
P = 0
S = 0.23
 = 0.23 MPa
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Water Potential
• Application of physical pressure
 increases ψ
(water more likely to cross membrane)
(b)
(c)
H2O
P = 0.23
S = 0.23
 = 0 MPa  = 0 MPa  = 0 MPa
H2O
P = 0.30
S = 0.23
 = 0.07 MPa
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Water Potential
Water Potential
ψ = ψs + ψp
Which
direction
will water
move?
ψcell = – 0.7 MPa + 0.5 MPa = – 0.2 MPa
ψsolution = –0.3 MPa (solution has no pressure potential)
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Water Potential
• Water potential
– Affects uptake and loss of water by plant cells
• If a flaccid cell is placed in an environment with a higher
solute concentration
– The cell will lose water and become plasmolyzed
0.4 M sucrose
solution:
Plasmolyzed cell
at osmotic
equilibrium
with its
surroundings
P = 0
S = 0.9
 = 0.9 MPa
P = 0
S = 0.9
 = 0.9 MPa
Initial flaccid cell:
P = 0
S = 0.7
 = 0.7 MPa
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Water Potential
Uses of turgor
pressure:
•
•
•
Inexpensive cell
growth
Hydrostatic
skeleton
Phloem
transport
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Water Route
Most plant tissues
- cell walls and cytosol are continuous cell to cell (via?)
- cytoplasmic continuum called the symplast
apoplast = continuum of cell walls plus extracellular spaces
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Water Route
How do water and minerals get from the soil to vascular tissue?
Symporters
(cotransporters)
contribute to the gradient
that determines the
directional flow of water.
Here, pumps in H+ and mineral ions
Water enters plants
via the roots.
H2O
Soil
Soil
Cytosol
Symporter
H+
Mineral
ions
Water
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Water Potential
Minerals & ions pumped
into root cells, then moved
past endodermis
What happens to ψ between soil
and endodermis?
Where is osmosis occurring?
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Water Potential
Once water & minerals cross the endodermis, they
are transported through the xylem to upper parts of
the plant.
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Xylem
Water exits plant
through stomata.
Smooth
surface
Rippled
surface
H2O
Water moves up
plant through xylem.
Water film that coats
mesophyll cell walls
evaporates.
Adhesion to xylem cells
Cohesion
between water
molecules
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16
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Transpiration = loss of water from the shoot
system to the surrounding environment.
Bulk Flow = movement of fluid due to pressure gradient
•
Transpiration drives bulk flow of xylem sap.
•
Water is PULLED up a plant.
•
Ring/spiral wall thickening protects against vessel collapse
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Xylem Ascent by Bulk Flow
• The movement of xylem sap is against gravity
– maintained by the transpiration-cohesion-tension
• Stomata help regulate the rate of transpiration
• Leaves generally have broad surface areas
• These characteristics
– Increase photosynthesis
– Increase water loss through stomata
20
µm
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Xylem
What happens if rate of transpiration nears zero?
i.e. – at night, water pressure
builds up in the roots
•
Guttation
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H+ pumped out
Stomata Control
K+ flow in
H2O flow in
Why?
stomata open
Why?
K+ channels, aquaporins and
radially oriented cellulose
fibers play important roles.
Cues for opening stomata:
Light
Depleted CO2
Internal cell “clocks”
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Phloem
•
•
•
Direction is source to sink
Near source to near sink
Phloem under positive
pressure
Phloem sap composition:
Phloem tissue
•
•
•
•
•
Sugar (mainly sucrose)
amino acids
hormones
minerals
enzymes
Are tubers and bulbs sources or sinks?
Aphid
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Phloem
Pressure Flow Hypothesis
Vessel
(xylem)
Sieve tube Source cell
(phloem) (leaf)
H2O
Sucrose
H2O
1
Where are sugars made?
2
Pressure flow
Water potential increased, turgor pressure
increased, sap PUSHED through phloem
Sugars removed (actively) at sink
 water potential decreased,
water leaves phloem
2
3
Transpiration stream
Sugars actively transported into
companion cells  plasmodesmata
to sieve tube elements
Via H+/sucrose
Water follows (WHY?!) cotransporters
1
4
Sink cell
(storage
root)
3
H2O
4
Sucrose
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Overview: A Nutritional Network
• Every organism
– Continually exchanges energy and materials with
its environment
• The branching root and shoot system provides high
SA:V to collect resources
– Plants’ resources are diffuse (scattered, at low
concentration)
What are these diffuse resources?
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Mineral Acquisition
What’s in dirt?!
Mineral Acquisition
• After heavy rainfall, water drains away from the
larger spaces in soil
– But smaller spaces retain water
– attraction to surfaces,
clay and other particles
• The film of loosely
bound water
available to plants
Soil particle surrounded by
film of water
Root hair
Water
available to
plant
Air space
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Mineral Acquisition
Soil particle
Cation Exchange
• Makes cations
available for
uptake.
K+ –
––
–
Cu2+ K+
– –
Mg2+
– +
K
– –
Ca2+
H+
CO2
H+
Root hair
H2O
Steps:
1. Roots acidify soil solution via respired CO2 and
H+/ATPase pumps
2. H+ attracted to soil particle (-) which “releases” cations
3. Roots absorb cations
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Essential Nutrients and Deficiencies
• Plants require certain chemicals to thrive
• Plants derive most organic mass from the CO2 of air
– Also depend on soil nutrients like water and minerals
Essential elements:
Required for a plant to
complete its life cycle
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Essential Nutrients and Deficiencies
• Photosynthesis = major source of plant nutrition
• Overall need
– Macronutrients – used in larger amounts
• Nine = C, O, H, N, K, Ca, Mg, P, and S
– Micronutrients – used in minute amounts
• Seven = Cl, Fe, Mn, Zn, B, Cu, and Mo
Healthy
Deficiency of any
one can have
severe effects on
plant growth
Phosphate-deficient
Potassium-deficient
Nitrogen-deficient
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Relationship with other organisms
•
•
•
•
Mycorrhizae
Root nodulation
Parasitic plants
Carnivorous plants
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Relationship with other organisms
• Symbiotic associations with mycorrhizal fungi are found
in about 90% of vascular plants
– Substantially expand the surface area available for
nutrient uptake
– Enhance uptake of phosphorus and micronutrients
The fungus gets: sugars from plant
Agriculturally, farmers and foresters
…Often inoculate seeds with
spores of mycorrhizae to promote
mycorrhizal relationships.
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Nitrogen, Soil Bacteria and Nitrogen Availability
• Plants need ammonia (NH3) or nitrate (NO3–) for: Proteins,
nucleic acids, chlorophyll…
• Nitrogen-fixing soil bacteria convert atmospheric N2 to
nitrogenous minerals that plants can absorb
N2
N2
Atmosphere
Soil
N2
Nitrogen-fixing
bacteria
Denitrifying
bacteria
H+
(From soil)
Soil
+
NH4
NH3
(ammonia)
–
+
NH4
(ammonium)
Organic
material (humus)
Nitrate and
nitrogenous
organic
compounds
exported in
xylem to
shoot system
Nitrifying
bacteria
NO3
(nitrate)
Ammonifying
bacteria
Root
Symbiotic relationships form between nitrogen-fixing bacteria
and certain plants - Mainly legume family (e.g. peas, beans)
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• Nodules: Swellings of plant
cells “infected” by Rhizobium
bacteria
Nodules
Bacteroids
within
vesicle
5 m
Roots
(a) Pea plant root
(b) Bacteroids in a soybean root
nodule. In this TEM, a cell from
a root nodule of soybean is filled
with bacteroids in vesicles. The
cells on the left are uninfected.
• Inside the nodule
– Rhizobium bacteria assume a
form called bacteroids, which
are contained within vesicles
formed by the root cell
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Epiphytes, Parasitic, and Carnivorous Plants
EPIPHYTES
Anchored on another
plant, self-nourished
PARASITIC PLANTS
Absorb sugar/minerals
from host plant
Staghorn fern,
an epiphyte
Pitcher plants
cavity filled with
digestive fluid
Venus flytrap
Mistletoe, a
photosynthetic parasite
To gain extra
nitrogen
Things To Do After Lecture 6…
Reading and Preparation:
1.
Re-read today’s lecture, highlight all vocabulary you do not
understand, and look up terms.
2.
Ch. 36 Self-Quiz: #2, 3, 4, 6, 7, 8, 9 (correct answers in back of book)
Ch. 37 Self-Quiz: #1, 2, 8, 9, 10 (correct answers in back of book)
3.
Read chapters 36 & 37, focus on material covered in lecture (terms,
concepts, and figures!)
4.
Skim next lecture.
“HOMEWORK” (NOT COLLECTED – but things to think about for studying):
1.
Explain the two components of water potential – which of these is due to
osmosis?
2.
Diagram the movement of water in a plant via xylem versus sugar
movement through phloem. List similarities and differences.
3.
Discuss how mycorrhizae and Rhizobium are different and the benefits
each provide to plants.
4.
Think about what types of environments might be more likely to have
carnivorous plants – what do plants gain by digesting insects?