Download 1. Overview of Transport in Plants 11/25/2014 1

11/25/2014
Chapter 36:
Resource Acquisition &
Transport in Vascular Plants
1. Overview of Transport in Plants
2. Transport of Water & Minerals
3. Transport of Sugars
1. Overview of
Transport in Plants
CO2
O2
Sugar
Light
H2O
H2O and
minerals
Resources
Needed by
Plants
O2
CO2
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Resources Needed by Plants
CO2 – carbon source used during photosynthesis
of sugars and other organic molecules
O2 – required for the synthesis of ATP by aerobic
respiration
Sunlight – source of energy for photosynthesis
Water – obtained primarily from the soil
Minerals & other Nutrients – obtained primarily
from the soil
Leaf Arrangement (Phyllotaxy)
Leaf arrangement and orientation evolved to:
• maximize light absorption
• reduce self shading (blocking light to lower leaves)
• avoid damage from intense light
Ground area
covered by plant
Leaf area index
represents % of
ground area
covered by plant:
• commonly >1 due to
multiple layers of
leaves
Plant A
Leaf area = 40%
of ground area
(leaf area index = 0.4)
Plant B
Leaf area = 80%
of ground area
(leaf area index = 0.8)
• plants self-prune
structures that don’t
receive enough light
More on Leaf Arrangement…
Plants must balance light absorption and
water loss:
• light absorption tends to correlate with water loss
• greater surface area for light absorption = greater
surface area for water loss
• leaf shape & arrangement reflect a balance
between the two:
• low light & high moisture =
larger, horizontal leaves
• harsh light & low moisture
= smaller, vertical leaves
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3 Modes of Transport
Apoplastic – through extracellular spaces
Symplastic – through cytosol, plasmodesmata
Transmembrane – across multiple plasma membranes
Cell wall
Apoplastic route
Cytosol
Symplastic route
Transmembrane route
Key
Plasmodesma
Apoplast
Plasma membrane
Symplast
Solute Transport Across
Plant Cell Membranes
CYTOPLASM
ATP
EXTRACELLULAR
FLUID
+
−
+
−
+
−
+
H+
S
H
Hydrogen
ion
H+
H+
−
+
−
+
−
+
H+
H+
H+
H+
H+
S
H+
H+
H+
H+
H+
H+
−
Proton pump
H+
S
H+
+
H+
−
+
H+/sucrose
cotransporter
(a) H+ and membrane potential
−
+
−
+
−
+
H+
Sucrose
(neutral solute)
(b) H+ and cotransport of neutral solutes
H+
H+
−
+
−
+
−
+
H+
K+
H+
H+
K+
H+
K+
Nitrate
H+
−
+
−
+
−
+
Potassium ion
K+
H+
K+
H+/NO3−
cotransporter
−
+
−
+
−
+
K+
K+
H+
H+
(c) H+ and cotransport of ions
H+
Ion channel
−
+
−
+
(d) Ion channels
Transport Through Ion Channels
•
plants have gated ion channels that, when opened,
allow ions to flow down the electrochemical gradient
K+
K+
K+
−
+
−
+
−
+
−
+
−
+
Potassium ion
K+
K+
K+
K+
Ion channel
Ion channels
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The Role of H+ in Cotransport
H+ ions are pumped by active transport to create an
electrochemical gradient (membrane potential)…
EXTRACELLULAR
FLUID
CYTOPLASM
ATP
−
+
−
+
−
+
H+
Hydrogen
ion
H+
H+
H+
H+
H+
Proton pump
−
+
−
+
H+
H+
H+ and membrane potential
H+ flow down its electrochemical gradient can be
coupled to the active transport (movement from low to
high conc.) of neutral solutes such as sugars…
H+
S
H+
S
−
+
−
+
H+
−
H+
H+
H+
+
H+
H+
H+
H+
S
H+/sucrose
cotransporter
−
+
−
+
−
+
H+
Sucrose
(neutral solute)
H+ and cotransport of neutral solutes
…or the active transport of ions such as nitrate (NO3-)
H+
H+
−
+
−
+
−
+ H+
H+
H+
H+
H+
H+/NO3−
cotransporter
−
+
−
+ H+
−
+
H+
Nitrate
H+
H+
H+ and cotransport of ions
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The Transport of Water
Osmosis is the diffusion of water across a cell
membrane.
The net direction of osmosis (water movement) in
plants depends on 2 factors:
• differences in the concentration of water & solutes across the
membrane (water diffuses from high to low concentration)
• differences in pressure (water moves from high to low
pressure)
The combination of these 2 factors (concentration &
pressure) is called water potential.
…more on
Water Potential
Initial flaccid cell:
ψP = 0
ψ S = −0.7
Environment
0.4 M sucrose solution:
ψP = 0
ψ S = −0.9
ψ = −0.7 MPa
Water potential (y) =
the sum of solute
potential (yS) and
pressure potential (yP)
ψ = −0.9 MPa
Final plasmolyzed cell at osmotic
equilibrium with its surroundings:
ψP = 0
ψ S = −0.9
ψ = −0.9 MPa
(a) Initial conditions: cellular ψ > environmental ψ
y = yS + yP
For pure water yS = 0,
the more solutes the
more negative the yS
yP can be + or – in
relation to atmospheric
pressure
Initial flaccid cell:
ψP = 0
ψ S = −0.7
Environment
Pure water:
ψP = 0
ψS = 0
ψ = −0.7 MPa
ψ = 0 MPa
Final turgid cell at osmotic
equilibrium with its surroundings:
ψP = 0.7
ψ S = −0.7
ψ =
0 MPa
(b) Initial conditions: cellular ψ < environmental ψ
Turgor Pressure in Plants
The protoplast (interior part) of plant cells normally has
a positive yP due to osmosis, a pressure called turgor
pressure which keeps cells turgid (opposite of flaccid).
Rate of osmosis is
increased by
aquaporins.
Normal plant
with turgid cells
Wilted plant
with flaccid cells
In extracellular compartments such as xylem, yP is
negative which aids in the movement of fluid up from
the root system.
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2. Transport of Water & Minerals
in Xylem
From Root Hairs to Xylem
Casparian strip
Endodermal cell
Pathway along
apoplast
4
Pathway
through
symplast
5
Plasmodesmata
1
Apoplastic
route
Casparian strip
1
2
Symplastic
route
Water
moves
upward
in vascular
cylinder
Plasma
membrane
Apoplastic
route
3
2
Symplastic
route
4
Root
hair
3
Transmembrane
route
4
The endodermis: controlled entry
to the vascular cylinder (stele)
Epidermis
5
Vessels
(xylem)
Endodermis Vascular
cylinder
(stele)
Cortex
5
Transport in the xylem
Water & Mineral Uptake by Roots
The transport of water, minerals and other nutrients
via xylem vessels begins at the interface of the root
tip & root hair epidermis and the surrounding soil.
• the root epidermal cells are permeable to the aqueous
soil solution which freely passes along the cell walls
(apoplastic route) to the root cortex
• once this material reaches the endodermis, water and
desired solutes are transported across the endodermal
cells to the vascular cylinder (stele)
• once in the stele, water and mineral nutrients enter the
tracheids and vessel elements of the xylem as xylem
sap to be transported throughout the plant
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How is Xylem Moved “Up”?
Xylem sap moves upward in the plant due to
a combination of the following:
ROOT PRESSURE (a minor factor)
• active transport of ions into the roots lowers
the water potential resulting in water flowing in
due to osmosis
TRANSPIRATION (the major factor)
• loss of water through the stomata of leaves
• adhesion of water to xylem vessels & cohesion
of water molecules to each other “pull” water
up to replace water lost through transpiration
Source of “Pull” in Transpiration
Diffusion of water vapor out of stomata starts the “pull”
which creates a negative water potential drawing water up:
4 Increased surface
tension pulls
water from cells
and air spaces.
5 Water from xylem pulled
into cells and air spaces.
Cuticle
Xylem
Upper
epidermis
Mesophyll
Air
space
3 Air-water
interface
retreats.
Microfibrils
in cell wall of
mesophyll cell
2 Water vapor
replaced
from water
film.
Lower
epidermis
Cuticle
Stoma
1 Water vapor diffuses outside
via stomata.
Microfibril
(cross section)
Water Air-water
film interface
Transpiration
Xylem sap
Outside air ψ
= −100.0 MPa
Mesophyll cells
Stoma
Water molecule
Atmosphere
Leaf ψ (air spaces)
= −7.0 MPa
Transpiration
Trunk xylem ψ
= −0.8 MPa
Water potential gradient
Leaf ψ (cell walls)
= −1.0 MPa
Xylem
cells
Adhesion by
hydrogen bonding
Cell
wall
Cohesion
by hydrogen
bonding
Cohesion and
adhesion in
the xylem
Water molecule
Trunk xylem ψ
= −0.6 MPa
Root hair
Soil particle
Water
Soil ψ
= −0.3 MPa
Water uptake from soil
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Guard Cell Control of Stomata
• when guard cells are turgid, they bend and as a
result open the stomata
• when guard cells are more flaccid the stomata are
closed
Guard cells turgid/
Stoma open
Guard cells flaccid/
Stoma closed
Radially oriented
cellulose microfibrils
Cell
wall
Vacuole
Guard cell
Changes in guard cell shape and stomatal
opening and closing (surface view)
Regulation of Guard Cells
Guard cell turgidity is controlled by K+ ions which
move in response to changes in membrane
potential due to active transport of H+:
Guard cells turgid/
• Stoma
pumping
open H+
H2O
K+
H2O
Guard cells flaccid/
out ofStoma
guard
cells
closed
H2O
H2O
H2O
H2O
H 2O
H2O
H2O
Role of potassium ions (K+) in stomatal
opening and closing
H2O
• H+ pumped out of guard
cells lowers the
membrane potential
(more negative) drawing
K+ ions into the cell
• the intracellular
increase in K+ lowers
the water potential and
water flows in
Plants open stomata by
pumping H+ in response
to light and low CO2
(provided there is
enough water)
3. Transport of Sugars
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Sugar Translocation via Phloem
The transport of photosynthetic products, a process
called translocation, proceeds through phloem
vessels in a direction opposite to that of xylem sap.
• photosynthetic products such as sucrose are produced
in photosynthetic organs such as leaves
• they are transported in phloem sap to sites of sugar use
or storage – sugar sinks
• e.g., fruits, tubers, growing shoot and root tips
• the transfer of sugars to phloem sieve tube elements or
companion cells occurs through both symblastic and
apoplastic routes…
Loading Sugars into Phloem
Sieve Tube Elements
• transport from apoplast to
sieve tube element symplast
involves cotransport with H+
Apoplast
Symplast
Companion
(transfer) cell
Mesophyll cell
High H+ concentration
Cell walls (apoplast)
Cotransporter
H+
Proton
pump
Sieve-tube
element
Plasma
membrane
S
Plasmodesmata
ATP
Mesophyll cell
Bundlesheath cell
H+
Phloem
parenchyma cell
Sucrose
H+
S
Low H+ concentration
(a) Sucrose manufactured in mesophyll cells
can travel via the symplast (blue arrows)
to sieve-tube elements.
(b) A chemiosmotic mechanism is
responsible for the active transport
of sucrose.
Bulk Flow of Phloem Sap
• this results in a net
diffusion of sugar
and movement of
water towards the
sinks
H2O
Sucrose
H2O
1
1 Loading of sugar
decreases water
potential
2
Bulk flow by positive pressure
• sugar concentration
decreases near the
sugar sinks due to
usage for energy or
addition to polymers
such as starch
Sieve
Source cell
tube
(leaf)
(phloem)
Vessel
(xylem)
Bulk flow by negative pressure
• unlike xylem sap,
phloem sap flows
toward sugar
sinks due to
positive pressure
2 Uptake of water
increases pressure
3 Unloading of sugar
Sink cell
(storage
root)
3
4
H2O
and loss of water
relieves the pressure
4 Recycling of water
Sucrose
9