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
2
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
3
Transport in Plants – Water Potential
Roots  xylem  stomata
4
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)
5
Water Potential
• Free water flows from
regions of high water
potential to regions of
low water potential
• Adding solutes
• Adding pressure
Water potential = Potential energy of water =
Energy per volume of water in megapascals (MPa)
ψTotal = ψsolute + ψpressure
6
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
7
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
8
If Ψ inside a plant cell is -0.5 Mpa and outside the
cell the solution Ψ is 0.2 Mpa, what will happen?
1.
2.
3.
4.
No net water movement
Water will move into the cell
Water will leave the cell
Solutes in the cell will increase
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)
10
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
11
Water Potential
• If the same flaccid cell is placed in a solution with a
lower solute concentration
– The cell will gain water and become turgid
Initial flaccid cell:
P = 0
S = 0.7
 = 0.7 MPa
Distilled water:
P = 0
S = 0
 = 0 MPa
Turgid cell
at osmotic
equilibrium
with its
surroundings
P = 0.7
S = 0.7
 = 0 MPa
12
Water Potential
Water will move until Ψcell = Ψsolution
13
Water Potential
14
15
Uses of turgor
pressure:
•
•
•
Inexpensive cell
growth
Hydrostatic
skeleton
Phloem
transport
15
For the situation below, what will be the final
solute potential inside the plant cell?
1.
2.
3.
4.
5.
0
-0.3
-0.1
0.1
-0.5
Solution
Ψs = -0.5
Ψp =
Ψs = -0.3
Ψp = 0.4
Ψ=
Ψ=
Final Cell
Ψs =
Ψp = 0
Ψ=
Initial Cell
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
17
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.
H2O
Soil
Soil
Cytosol
Symporter
H+
Water enters plants
via the roots.
Mineral
ions
Water
18
19
Water Potential
Minerals & ions pumped
into root cells, then moved
past endodermis
What happens to ψ between soil
and endodermis?
Where is osmosis occurring?
19
Water Potential
Once water & minerals cross the endodermis, they
are transported through the xylem to upper parts of
the plant.
21
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
21
21
22
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
22
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
23
24
Xylem
Transpiration = loss of water from the shoot system to the
surrounding environment.
What drives water loss?
24
25
Xylem
What happens if rate of transpiration nears zero?
i.e. – at night, water pressure
builds up in the roots
•
Guttation
25
Which plants are more likely to display guttation?
1.
2.
3.
4.
Tall plants in dry climates
Tall plants in moist climates
Short plants in dry climates
Short plants in moist climates
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”
27
28
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
28
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
29
Phloem
30
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?
31
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
33
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 surface “release” cations
3. Roots absorb cations
34
Which are more likely to be leached from soil
after heavy rains/watering: cations or anions?
1.
2.
3.
4.
cations: K+, H+, Mg+, Ca++
anions: NO3-, PO4-, SO4Both equally likely to be leached
Neither – ions are strongly attracted to the soil
36
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
36
Essential Nutrients and Deficiencies
• Photosynthesis is major source of plant
nutrition fixation
• 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
– Deficiency of any one can have severe effects
on plant growth
37
Symptoms of Mineral Deficiency
• The symptoms of mineral deficiency depend on
– nutrient’s function & mobility within the plant
• The most common deficiencies
Are those of nitrogen, potassium, and phosphorus
Healthy
Phosphate-deficient
Potassium-deficient
Nitrogen-deficient
38
39
Relationship with other organisms
•
•
•
•
Mycorrhizae
Root nodulation
Parasitic plants
Carnivorous plants
39
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.
40
Nitrogen, Soil Bacteria and Nitrogen Availability
• Plants need ammonia (NH3) or nitrate (NO3–)
• Nitrogen is needed for: Proteins, nucleic acids, chlorophyll…
• Nitrogen-fixing soil bacteria convert atmospheric N2
– To nitrogenous minerals that plants can absorb
Atmosphere
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)
41
• 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
42
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?