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Transport in Vascular Plants
http://bcs.whfreeman.com/thelifewire/content/chp00/00020.html
Chpt 35 : REVIEW SECONDARY GROWTH
Chpt 36: TRANSPORT IN PLANTS
http://glencoe.mcgrawhill.com/sites/9834092339/student_view0/chapter38/
Phloem loading
Water intake
Osmosis
Etc.
Text Summary of Transport
http://www.wou.edu/~bledsoek/103materials/chapter_notes/103Ch42b.pdf
Vascular land plants = plant body of roots
(absorb H O & minerals) & shoots (absorb
light and CO )
2
2
Transport sometimes over long distances
Xylem transports H2O
& minerals from roots
to shoots
Phloem transports
sugars, etc. from
where synth (source)
to where needed (sink)
I. Physical forces drive the transport of
materials in vascular plants
A. 3 levels of transport
1. Transport of water and solutes by individual cells,
e.g., root hairs via plasmodesmata
2. Short-distance transport of substances from cell to
cell at the levels of tissues and organs, e.g. loading
sugar from photosynthetic leaf cells
sieve tubes of
phloem
3. Long-distance transport within xylem & phloem
throughout whole plant
A variety of physical processes are involved
in the different levels of transport
5
Sugars are produced by
photosynthesis in the leaves.
4 Through stomata, leaves
take in CO2 and expel O2.
The CO2 provides carbon for
photosynthesis. Some O2
produced by photosynthesis
is used in cellular respiration.
CO2
H2O
O2
Light
Sugar
3 Transpiration, the loss of water
from leaves (mostly through
stomata), creates a force within
leaves that pulls xylem sap upward.
6
Sugars are transported as
phloem sap to roots and other
parts of the plant.
2
Water and minerals are
transported upward from
roots to shoots as xylem sap.
7
1 Roots absorb water
and dissolved minerals
from the soil.
Roots exchange gases
with the air spaces of soil,
taking in O2 and discharging
CO2. In cellular respiration,
O2 supports the breakdown
of sugars.
O2
H2O
Minerals
O2
CO2
1. Roots absorb H2O & minerals from soil
2. Transported up to shoots as xylem sap
H2O
H2O
Minerals
3. Transpiration (loss H2O) through stomata
creates force in leaves that pulls xylem sap up
CO2
H2O
H2O
Minerals
O2
4. Through stomata, leaves take in CO2 &
expel O2 (CO2 for photosynth, O2 from cell resp)
CO2
H2O
O2
Light
Sugar
6. Sugar transported
as phloem sap to
roots & other parts
where needed
H2O
Minerals
5. Sugar
made by
photosynth
in leaves
CO2
H2O
O2
Light
Sugar
7. Roots
exchange
gases with
air spaces
of soil (O2
O2
H2O
Minerals
CO2
used in
catabolism
of sugars)
B. Selective Permeability of Membranes
• Selective permeability of plant cell’s plasma
membrane controls movement of solutes
into & out of cell
• Specific transport proteins enable plant
cells to maintain internal environment
different from their surroundings
Review: passive and active transport compared
Passive transport. Substances diffuse spontaneously Active transport. Some transport proteins
down their concentration gradients, crossing a
membrane with no expenditure of energy by the cell.
No energy is required.
act as pumps, moving substances across a
membrane against their concentration gradients.
Energy for this work is usually supplied by ATP.
Gated channel
Diffusion. Hydrophobic
molecules and (at a slow
rate) very small uncharged
polar molecules can diffuse
through the lipid bilayer.
Facilitated diffusion.
Many hydrophilic
substances diffuse
through membranes
with the assistance of
transport proteins,
ATP
Most solutes cannot cx memb without help of transport proteins
B. Central Role of Proton Pumps
Most impt active transport prot in plants is
the proton pump in plant cells
- Creates a hydrogen ion gradient (a form of
potential energy that can be harnessed to do work)
- Contributes to a voltage known as a membrane
potential (also potential energy)
CYTOPLASM
–
ATP
–
–
EXTRACELLULAR FLUID
+
H+
+
H+
+ H+
H+
H+
H+
–
–
+
+
H+
Proton pump generates
membrane potential
and H+ gradient.
H+
Plants use energy stored in proton gradient & memb
potential to drive transport of different solutes
1. Uptake of K+
CYTOPLASM
–
–
K+
K+
+
EXTRACELLULAR FLUID
+
+
–
Cations ( K+ , for
example) are driven
into the cell by the
membrane potential.
K+
K+
K+
K+
K+
–
+
–
+
Transport protein
2. Cotransport couples downhill passage of
(H+) with concommitant uphill passage of
(NO3) this = active transport
H+
–
+
–
+
–
+
H+
H+
H+
H+
H+
H+
H+
–
+
–
+
–
+
H+
H+
H+
H+
Cell accumulates
anions ( NO3– , for
example) by
coupling their
transport to the
inward diffusion
of H+ through a
cotransporter.
3. Uptake of sucrose with contransport of H+
moving down its conc gradient in uptake of
sucrose by plant cells
–
H+
H+
+
H+
H+
–
+
–
+
Plant cells can
also accumulate a
neutral solute,
such as sucrose
H+
H+
S
H+
H+
H+
–
–
H+
+
+
–
–
steep proton
gradient.
H+
S
+
( S ), by
cotransporting
H+ down the
H+
C. Water Potential
To survive plants must balance water
uptake & loss
Osmosis (diffusion of H2O acx selectively permeable memb)
responsible for net uptake or loss of water
• Water potential (Ψ) is a measurement that
combines effects of solute conc & pressure.
to determine the direction of H2O movement
• Water flows from regions of high water
potential to regions of low water potential
• Solute contribution to water potential of a
solution is proportional to the number of
dissolved molecules
• Pressure contribution to water potential of
a solution is the physical pressure on a
solution (involves plant cell wall)
• Addition of solutes reduces Ψ
(a)
0.1 M
solution
Pure
water
H2O
 = 0 MPa
P = 0
S = 0.23
 = 0.23 MPa
• Application of physical pressure increases Ψ
(b)
(c)
H2O
H2O
 = 0 MPa
P = 0.23
S = 0.23
 = 0 MPa
 = 0 MPa
P = 0.30
S = 0.23
 = 0.07 MPa
• Negative pressure decreases Ψ
(d)
H2O
P = 0.30
S = 0
 = 0.30 MPa
P = 0
S = 0.23
 = 0.23 MPa
• If a flaccid cell is placed in an environment
with a higher solute conc (hypertonic soln),
the cell will lose water & plasmolyze (memb
will shrink away from its cell wall)
Flaccid = limp. A walled cell is flaccid in surroundings where
there is no tendency for water to enter
Hypertonic solution
Hypotonic solution
• If the same flaccid cell is placed in a soln with a solute
concentration lower than that in the protoplast, the cell
will gain water and become turgid (very firm) as cell
wall pushes back against enlarging memb. A walled
cell becomes turgid if it has a greater solute conc than
its surroundings, resulting in entry of water.
• Loss of turgor (due to loss of water in
environment) in plants causes wilting which
can be reversed when the plant is watered.
Healthy plants are
turgid most of the
time.
D. Aquaporin and Water Transport
● Aquaporins = transport prots in memb that
allow the passage of water
● Do not affect water potential
E. 3 Major Compartments Vacuolated Cells
Transport also regulated by compartmental structure of
plant cells
Cell wall
Cytosol
Vacuole
Ke
Symplast
Apoplast
Plasmodesma
Vacuolar membrane
(tonoplast)
Plasma membrane
1. cell wall (maintain cell shape) edge of space
2. plasma membrane (controls H2O in/out) &
edge of protoplast (contents of cell less wall)
3. vacuole
• Plasma membrane
- Directly controls the traffic of molecules
in/out protoplast
- Is a barrier between two major
compartments, cell wall & cytosol
• 3rd major compartment vacuole = a large
organelle that can occupy as much as 90% of
more of the protoplast’s volume
• Vacuolar membrane regulates transport between
the cytosol and the vacuole
Cell wall
Transport proteins in
the plasma membrane
regulate traffic of
molecules between
the cytosol and the
cell wall.
Cytosol
Vacuole
Transport proteins in
the vacuolar
membrane regulate
traffic of molecules
between the cytosol
and the vacuole.
Vacuolar membrane
Plasmodesma
Plasma membrane
Cell compartments. The cell wall, cytosol, and vacuole are the three main
compartments of most mature plant cells.
F. Short lateral transport via one of 3 ways:
Symplast = continuous cytosol, cell to cell via plasmodesmata
Apoplast = continuum cell walls & extracellular spaces Key
1. transmembrane route (out of memb-ax cell wall-into another cell)
Symplast
2. symplastic route (cell memb to cell directly vis dermatoplasmata)
Apoplast
3. apoplastic route (stay outside cells)
Transmembrane route
Apoplast
Symplast
Apoplastic route
Symplastic route
These 3 ways from root hairs to vascular cylinder
Substances may transfer from one route to another.
Water & minerals travel short distances from root
hairs to vascular cylinder of root via 3 lateral (not
up/down) routes
1. Transmemb: out of one cell, across a cell wall, & into another cell =
repeated crossing plasma memb
2. Via symplast == continum of cytosol; only one crossing of plasma
membrane & then cell-to-cell via plasmodermata
3. Along apoplast == continuem of cell wall; no entering protoplast
Can change from one route to another route
G. Bulk Flow for Long-Distance Transport
is movement of fluid in the xylem & phloem
driven by pressure differences at opposite
ends of the xylem vessels and sieve tubes
Diffusion OK for short distances,
but too slow for long distances ….
need bulk flow
Water & fluids move through tracheids & vessels of xylem
& sieve tubes of phloem
Tracheids = tapered cells with pits
Vessel elements = wider, long channels,
end walls perforated for easy flow
Sieve tube member = cell
with sieve plate
Companion Cell =
nonconducting cntd via
plasmodesmata
DEAD
ALIVE
Phloem: loading of sugar = high + pressure at opposite
end of sieve tube = mvt fluid
Xylem: negative pressure tension by transpiration from
leaves pulls sap up from roots
Cytoplasm of sieve-tube members almost
devoid of internal organelles & porous sieve
plates = easier flow
Dead tracheids & vessel elements (porous
end walls) have no cytoplasm to inhibit flow
Bulk flow due to pressure differences = way
long-distance transport of phloem sap & active
transport of sugar at cellular level maintains
pressure difference
II. Roots absorb water & minerals from soil
enter the plant through the
epidermis of roots, cx root
cortex, pass into vascular
cylinder & ultimately bulk
flow to shoot system
Root hairs account for much surface area of roots
• Most plants form mutually beneficial relationships
with fungi, which facilitate the absorption of water
and minerals from the soil
• Roots and fungi form mycorrhizae, symbiotic
structures consisting of plant roots united with
fungal hyphae = increase surface area of roots
2.5 mm
Lateral transport of minerals and water in roots
Casparian strip
Endodermal cell
Pathway
through
symplast
1
2
Uptake of soil solution by the
hydrophilic walls of root hairs
provides access to the apoplast.
Water and minerals can then
soak into the cortex along
this matrix of walls.
Casparian strip
Minerals and water that cross
the plasma membranes of root
hairs enter the symplast.
1
Plasma
membrane
Apoplastic
route
2
3
4
As soil solution moves along
the apoplast, some water and
minerals are transported into
the protoplasts of cells of the
epidermis and cortex and then
move inward via the symplast.
Vessels
(xylem)
Root
hair
Symplastic
route
Epidermis Cortex
5
Within the transverse & radial walls of each endodermal cell is the
Casparian strip, a belt of waxy material (purple band) that blocks the
passage of water and dissolved minerals. Only minerals already in
the symplast or entering that pathway by crossing the plasma
membrane of an endodermal cell can detour around the Casparian
strip and pass into the vascular cylinder.
Endodermis
Vascular cylinder
Endodermal cells & also parenchyma cells
within the vascular cylinder discharge water
and minerals into their walls (apoplast). The
xylem vessels transport the water &
minerals upward into shoot system.
Root hairs (extensions of epidermal cells) absorb
Passes freely apoplastic route into cortex
Cells of epidermis & cortex take up = into symplast
Endodermis: innermost layer cells in root cortex, surrounds
vascular cylinder & is last checkpoint for selective passage
of minerals from cortex into vascular tissue
Minerals/water from roots
into symplast of epidermis
or cortex, continue via
plasmodesmata of
endodermal cells into
vascular cylinder
Minerals/water from roots
into apoplast meet
Casparian strip (dead end
& cannot enter vascular
cylinder via apoplast) BUT
can enter symplast and
enter
Waxy belt in walls endodermal cells;
impervious to water/minerals
• III. Water and minerals ascend from roots
to shoots through the xylem
• Plants lose an enormous amount of water
through transpiration, the loss of water
vapor from leaves and other aerial parts of
the plant
• The transpired water must be replaced by
water transported up from the roots
Pulling Xylem Sap: The TranspirationCohesion-Tension Mechanism is the major
pressure driving flow of xylem sap up to
shoot system
• Water is pulled upward by negative
pressure in the xylem produced by
transpiration through stomata
• Cohesion of water (H bonding) transmits
upward pull along length of sylem to roots
IV. Stomata help regulate rate of transpiration
Leaves generally have broad surface areas &
high surface-to-volume ratios that increase
photosynthesis & increase water loss
● Plants lose a large amount of water by
transpiration
● If lost water is not replaced by absorption
through roots, plant will lose water & wilt
● Transpiration = evaporative cooling which
lowers T of leaf to prevent denaturation of
various enzs involved in photosynthesis &
other metabolic processes
Each stoma is flanked by guard cells that
control diameter by changing shape
V. Nutrients translocated through phloem
Phloem sap (an aqueous solution, mostly
sucrose) that translocated from source to sink
A sugar source : a plant organ that is a net
producer of sugar, such as mature leaves
A sugar sink: an organ that is a net consumer or
storer of sugar, such as a tuber or bulb
In angiosperms sap moves through a sieve tube by bulk
flow driven by positive pressure
Sucrose manufactured in mesophyll
cells can travel via the symplast (blue
arrows) to sieve-tube members.
Phloem loading may be via active
transport; proton pumping &
cotransport of sucrose & H+
Phloem sap flows from
source to sink via bulk
flow mediated by pressure
1. Load sucrose (green) into
sieve tube at source = ↓Ψ in
sieve tube = causes sieve tube to
take up water by osmosis
2. Uptake water = ↑ ΨP to force
sap to flow
3. At sink, sugar unloaded = ↓ ΨP
& water out via osmosis