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Transport in plants occurs across a network of vessels and over long distances
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Lecture 6 Outline (Ch. 36 & 37)
I. Plant Transport Overview
II. Driving Forces
A. Water potential
B. Transpiration & Bulk Flow in Xylem
C. Stomata Control
D. 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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Physical forces drive the transport of materials in plants
over a range of distances
Transport occurs on three scales
1. Within a cell – cellular level2. Short-distance cell to cell –
tissue level3. Long-distance in xylem &
phloem - whole plant level
Transport in Plants
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– 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 Potential
Water molecules are attracted to:• Each other (cohesion)• Solid surfaces (adhesion)
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• Free water flows from regions of high water potential to regions of low water potential
Water Potential
• Adding solutes
• Adding pressure
Water potential = Potential energy of water = Energy per volume of water in megapascals (MPa)
ψTotal = ψsolute + ψpressure
Ψ changes with:
0.1 Msolution
H2O
Purewater
P = 0
S = 0.23
= 0.23 MPa = 0 MPa
(a)
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• Solutes added
decreases ψ(water less likely to cross
membrane)
Water Potential
(in an open area, no pressure, so ψp = 0)
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• Application of physical pressure increases ψ
(water more likely to cross membrane)
H2OP
= 0.23S
= 0.23
= 0 MPa = 0 MPa
(b)
H2O
P = 0.30
S = 0.23
= 0.07 MPa = 0 MPa
(c)
Water Potential
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Water Potential
ψcell = – 0.7 MPa + 0.5 MPa = – 0.2 MPa
ψ = ψs + ψp
ψsolution = –0.3 MPa (solution has no pressure potential)
Water Potential
Which direction will water move?
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• 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:
Initial flaccid cell:
Plasmolyzed cellat osmotic equilibriumwith its surroundings
P = 0
S = 0.7
P = 0
S = 0.9
P = 0
S = 0.9
= 0.9 MPa
= 0.7 MPa
= 0.9 MPa
Water Potential
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Uses of turgor pressure:
• Inexpensive cell growth
• Hydrostatic skeleton
• Phloem transport
Water Potential
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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
Water Route
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Symporters (cotransporters) contribute to the gradient that determines the directional flow of water.
SoilH2O
Mineralions
Symporter
Water
Soil
Cytosol
H+
Water Route
Water enters plants via the roots.
How do water and minerals get from the soil to vascular tissue?
Here, pumps in H+ and mineral ions
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Minerals & ions pumped into root cells, then moved past endodermis
What happens to ψ between soil and endodermis?
Where is osmosis occurring?
Water Potential
Once water & minerals cross the endodermis, they are transported through the xylem to upper parts of the plant.
Water Potential
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Water exits plantthrough stomata.
Smoothsurface
Rippledsurface
Water film that coats mesophyll cell walls evaporates.
Water moves up plant through xylem.
Adhesion to xylem cells
Cohesion between watermolecules
H2O
Xylem
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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
Transpiration = loss of water from the shoot system to the surrounding environment.
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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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What happens if rate of transpiration nears zero?
• Guttation
Xylem
i.e. – at night, water pressure builds up in the roots
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Stomata ControlH+ pumped out
K+ flow in
H2O flow in
stomata open
Why?
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 tissue
• Direction is source to sink• Near source to near sink• Phloem under positive
pressure
Phloem
Are tubers and bulbs sources or sinks?
Phloem sap composition:
• Sugar (mainly sucrose)• amino acids• hormones• minerals• enzymes
Aphid
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Vessel(xylem)
H2O
H2O
Sieve tube(phloem)
Source cell(leaf)
Sucrose
H2O
Sink cell(storageroot)
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Sucrose
2
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1
2
3
4
Tra
nsp
irat
ion
str
eam
Pre
ssu
re f
low
Phloem
Pressure Flow Hypothesis
Where are sugars made?
Sugars actively transported into companion cells plasmodesmata to sieve tube elements
Via H+/sucrose cotransporters
Water potential increased, turgor pressure increased, sap PUSHED through phloem
Sugars removed (actively) at sink water potential decreased, water leaves phloem
Water follows (WHY?!)
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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?
What’s in dirt?!
Mineral Acquisition
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• 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 byfilm of water
Root hairWater available to plant
Air space
Mineral Acquisition
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H2O
Root hair
K+
Cu2+ Ca2+Mg2+
K+
K+
H+
H+
Soil particle–
– – – – – – ––
Mineral Acquisition
CO2
Steps:1. Roots acidify soil solution via respired CO2 and H+/ATPase pumps
2. H+ attracted to soil particle (-) which “releases” cations3. Roots absorb cations
Cation Exchange
• Makes cations available for uptake.
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27Essential 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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• 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
Essential Nutrients and Deficiencies
Phosphate-deficient
Healthy
Potassium-deficient
Nitrogen-deficient
Deficiency of any one can have severe effects on plant growth
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• Mycorrhizae• Root nodulation• Parasitic plants• Carnivorous plants
Relationship with other organisms
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• 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
Relationship with other organisms
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
Soil
N2 N2
Nitrogen-fixingbacteria
Organicmaterial (humus)
NH3
(ammonia)
NH4+
(ammonium)
H+
(From soil)
NO3–
(nitrate)Nitrifyingbacteria
Denitrifyingbacteria
Root
NH4+
Soil
Atmosphere
Nitrate and nitrogenous
organiccompoundsexported in
xylem toshoot system
Ammonifyingbacteria
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
(a) Pea plant root
Nodules
Roots
• Inside the nodule– Rhizobium bacteria assume a
form called bacteroids, which are contained within vesicles formed by the root cell
(b) Bacteroids in a soybean root nodule. In this TEM, a cell froma root nodule of soybean is filledwith bacteroids in vesicles. The cells on the left are uninfected.
5 m
Bacteroidswithinvesicle
Epiphytes, Parasitic, and Carnivorous PlantsStaghorn fern,
an epiphyteEPIPHYTESAnchored on another plant, self-nourished
PARASITIC PLANTSAbsorb sugar/minerals
from host plant
Mistletoe, a photosynthetic parasite
Pitcher plantscavity filled with digestive fluid
Venus flytrap
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?