Download Diffusion vs. Bulk Flow

Figure 37.2 The uptake of nutrients by a plant: an overview
Xylem and Phloem Flow in Plants
What makes up the mass of
plants?
1. Nutrient requirements of plants
1. Water makes up most of the
weight of a living plant
2. Overview of flow in plants
2. Carbon (organic)
compounds make up most of
the dry weight of plants
3. Lateral (short-distance flow)
4. Long-distance flow in the xylem
Thus, plants get most of their
mass from water and air, not
from soil mineral nutrients
(inorganic ions).
5. Phloem flow
Table 37.1 Essential Nutrients in Plants
Nutrients and Xylem Flow in Plants
1. Nutrient requirements of plants
2. Overview of flow in plants
3. Lateral (short-distance flow)
4. Long-distance flow in the xylem
Diffusion vs. Bulk Flow
• Diffusion: Net movement down a
concentration gradient due to the random
motion of individual molecules. (Note:
solutes may move independently of
water.)
• Bulk flow: Movement of water and solutes
together due to a pressure gradient.
• Osmosis: movement of water in or out
• Physical forces drive the transport of
materials in plants over a range of distances
• Transport in vascular plants occurs on three
scales
– Transport of water and solutes by individual
cells, such as root hairs
– Short-distance transport of substances from
cell to cell at the levels of tissues and organs
– Long-distance transport within xylem and
phloem at the level of the whole plant
1
Figure 36.2 An overview of transport in whole plants (Layer 4)
Note:
Short-distance, or
“lateral,” flow of fluids
in plants (e.g. from cell
to cell, or into the root
form the soil) happens
by diffusion
• The selective permeability of a plant cell’s
plasma membrane
– Controls the movement of solutes into and out
of the cell
Long-distance flow
(i.e. through the xylem
and phloem) can only
happen by “bulk flow,”
i.e. movement that is
driven by pressure
• Specific transport proteins
– Enable plant cells to maintain an internal
environment different from their surroundings
The Central Role of Proton Pumps
• Proton pumps in plant cells
– Create a hydrogen ion gradient that is a form
of potential energy that can be harnessed to
do work
– Contribute to a voltage known as a membrane
potential
+
–
–
+
+
H+
H+
H+
+
+
–
+
–
+
H+
H+
H+
–
3
–
H+
NO
+
–
+
–
+
3
–
+
–
+
– Is also responsible for the uptake of the sugar
sucrose by plant cells
Cell accumulates
NO3 –
anions (
, for
example) by
coupling their
transport to the
inward diffusion
of H through a
cotransporter.
+
–
+
–
+
–
+
Plant cells can
also accumulate a
neutral solute,
such as sucrose
H+
H+
H+
–
H+
H+
S
–
–
+
S
H+
S
–
( S ), by
cotransporting
down the
H+
steep proton
gradient.
+
H+
H+
H+
H+
H+
S
–
H+
H+
Transport protein
• The “coattail” effect of cotransport
H+
H+
NO 3
NO
K+
(a) Membrane potential and cation uptake
–
NO 3
H+
NO 3
K+
S
H+
H+
–
K+
Cations (
, for
example) are driven
into the cell by the
membrane potential.
K+
K+
H+
+
–
–
+
–
K+
S
N
K+
Proton pump generates
membrane potential
and H+ gradient.
• In a mechanism called cotransport: a
transport protein couples the passage of
one solute to the passage of another
–
O3
EXTRACELLULAR FLUID
+
–
H+
H+
–
–
H+
+
–
CYTOPLASM
K+
–
H+
H+
• Plant cells use energy stored in the proton
gradient and membrane potential to drive the
transport of many different solutes
EXTRACELLULAR FLUID
CYTOPLASM
ATP
Selective Permeability of
Membranes: A Review
H+
+
(c) Contransport of a neutral solute
(b) Cotransport of anions
2
Effects of Differences in Water
Potential
• To survive
– Plants must balance water uptake and loss
• Osmosis
• Water potential
– Is a measurement that combines the effects of
solute concentration and pressure
– Determines the direction of movement of water
• Water
– Determines the net uptake or water loss by a
cell
– Is affected by solute concentration and pressure
– Flows from regions of high water potential to
regions of low water potential
Water Potential
Water Potential
• Water moves from areas of higher (more
positive) potential to lower (more negative)
• Pressure potential is created by physical
pressure on water (positive) or a
vacuum/sucking (negative)
• Solute potential is created by a higher
concentration of solutes (= lower
concentration of water)
• Water moves from areas of higher (more
positive) potential to lower (more negative)
• Pressure potential is created by physical
pressure on water (positive) or a
vacuum/sucking (negative)
• Solute potential is created by a higher
concentration of solutes (= lower
concentration of water)
Ψ= Ψs + Ψp
Water pot. = solute pot. + pressure pot.
Quantitative Analysis of Water
Potential
• Application of physical pressure
– Increases water potential
(a)
• The addition of
solutes
0.1 M
solution
(b)
(c)
– Reduces water
potential
Pure
water
H 2O
H 2O
ψ = 0 MPa
ψP = 0
ψS = −0.23
ψ = −0.23 MPa
H 2O
ψ = 0 MPa
ψP = 0.23
ψS = −0.23
ψ = 0 MPa
ψ = 0 MPa
ψP = 0.30
ψS = −0.23
ψ = 0.07 MPa
3
• Negative
pressure
(suction)
• Water potential
(d)
– Affects uptake and loss of water by plant cells
• If a flaccid cell is placed in an environment
with a higher solute concentration
– Decreases
water potential
– The cell will lose water and become plasmolyzed
– Cellular potential is greater than environmental
Initial flaccid cell:
potential
ψ = 0
P
ψS = −0.7
Water flow in plants is
controlled by both of
these forces. For
example: potassium
concentration and
pressure from the cell
wall
0.4 M sucrose solution:
ψP = 0
ψS = −0.9
Plasmolyzed cell
at osmotic equilibrium
with its surroundings
H2 O
ψP = −0.30
ψS = 0
ψ = −0.30 MPa
ψP = 0
ψS = −0.23
ψ = −0.23 MPa
• If the same flaccid cell is placed in a
solution with a lower solute concentration
– The cell will gain water and become turgid
ψ = −0.7 MPa
ψ = −0.9 MPa
ψP = 0
ψS = −0.9
ψ = −0.9 MPa
Transport is also regulated by the
compartmental structure of plant cells
• The plasma membrane
Initial flaccid cell:
ψP = 0
ψS = −0.7
ψ = −0.7 MPa
Distilled water:
ψP = 0
ψS = 0
ψ = 0 MPa
Turgid cell
at osmotic equilibrium
with its surroundings
ψP = 0.7
ψS = −0.7
– Directly controls the traffic of molecules into
and out of the protoplast
– Is a barrier between two major compartments,
the cell wall and the cytosol
– Aquaporins are transport proteins that
facilitate water movement across membranes
ψ = −0 MPa
• A major compartment in most mature plant cells is the
vacuole, a large organelle that can occupy as much as
90% of more of the protoplast’s volume
• The 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.
Nutrients and Xylem Flow in Plants
1. Nutrient requirements of plants
2. Overview of flow in plants
3. Lateral (short-distance flow)
4. Long-distance flow in the xylem
Plasmodesma
(a)
Plasma membrane
Vacuolar membrane
(tonoplast)
Cell compartments. The cell wall, cytosol, and vacuole are the three main
compartments of most mature plant cells.
4
• In most plant
tissues
– The cell walls and
cytosol are
continuous from
cell to cell
• The cytoplasmic
continuum
Key
Short distance or lateral transport: movement within
tissues and organs radially:
3 routes
Symplast
Apoplast
1. Transmembrane route
Apoplast
The symplast is the
continuum of
cytosol connected
by plasmodesmata.
The apoplast is
the continuum
of cell walls and
extracellular
spaces.
Symplast
– Is called the
symplast
• The apoplast
– Is the continuum
of cell walls plus
extracellular
spaces
• Water and minerals can travel through a
plant by one of three routes
– Out of one cell, across a cell wall, and into
another cell (transmembrane route)
– Via the symplast
– Along the apoplast
Xylem transport: Root uptake
• Roots absorb water and minerals from the
soil
• These enter the plant through the
epidermis of roots and ultimately flow to
the shoot system through the xylerm
2. Symplastic route
3. Apoplastic route
(b) Transport routes between cells. At the tissue level, there are three passages:
the transmembrane, symplastic, and apoplastic routes. Substances may transfer
from one route to another.
Bulk Flow in Long-Distance
Transport
• In bulk flow
– Movement of fluid in the xylem and phloem is driven
by pressure differences at opposite ends of the xylem
vessels and sieve tubes
– Diffusion does not work well over long distances, such
as from roots to leaves
– In xylem it is negative pressure that drives flow
(transpiration)
– In phloem it is hydrostatic pressure in one end of the
sieve tube that forces sap to the opposite end
The Roles of Root Hairs,
Mycorrhizae, and Cortical Cells
• Much of the absorption of water and minerals occurs near root
tips, where the epidermis is permeable to water and where root
hairs are located
• Root hairs account for much of the surface area of roots, and
greatly enhance absorption
• Root hairs are extensions of epidermal cells
• The soil solution flows into the hydrophilic cell walls, along the
apoplast and into the root cortex
5
• 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
Lateral transport of minerals and water in roots
Note the important role of the
Casparian strip in the endodermis:
blocking the apoplastic pathway into
the stele. Why is this useful to the
plant?
2.5 mm
Lateral transport in roots
• Water can cross the cortex
– Via the symplast or apoplast
• The waxy Casparian strip of the endodermal
wall
– Blocks apoplastic transfer of minerals from the
cortex to the vascular cylinder
Nutrients and Xylem Flow in Plants
1. Nutrient requirements of plants
2. Overview of flow in plants
Lateral transport in roots
• Once soil solution enters the roots
– The extensive surface area of cortical cell
membranes enhances uptake of water and selected
minerals
• The endodermis is a selective sentry
– It is the innermost layer of cells in the root cortex
– Surrounds the vascular cylinder and functions as the
last checkpoint for the selective passage of minerals
from the cortex into the vascular tissue (via the waxy
casparian strip)
– All material must pass via the symplast
Ascent of water in a tree
(long-distance flow). Water
is pulled upward by
extreme negative water
potential generated by
evaporation out of stomata
(transpiration).
3. Lateral (short-distance flow)
4. Long-distance flow in the xylem
6
Figure 36.12 The generation of transpirational pull in a leaf
Cohesion and Adhesion in the
Ascent of Xylem Sap
• The transpirational pull on xylem sap
– Is transmitted all the way from the leaves to the root
tips and even into the soil solution
– Is facilitated by cohesion (water molecules to one
another via their polar bonds) and adhesion (to the
hydrophilic vessel walls)
– Small diameter of vessels and tracheids increases
adhesive surface
• The movement of xylem sap against gravity
– Is maintained by the transpiration-cohesiontension mechanism
At night, some xylem sap is also pushed up by root
pressure: roots pump ions in, and water follows,
creating high water potential in roots. Guttation (see
below) is one consequence of this.
– Stomata increase
photosynthesis
– Stomata increase water loss
– Closing them reduces
photosynthesis and can lead
to overheating in plants
However, root pressure cannot push very much xylem
sap. Most upward movement of xylem sap is due to
transpirational pull, not push by root pressure.
20 µm
Phloem flow
• Products of photosynthesis, organic nutrients
(sugars), are translocated through the phloem
• In angiosperms the specialized cells are called
sieve tube members (with companion cells)
• In gymnosperms these are sieve cells (with
albuminous cells)
• Phloem sap
– Is an aqueous solution that is mostly sucrose
– Travels from a sugar source to a sugar sink
– Also carries minerals, amino acids and
hormones
• A sugar source
– Is a plant organ that is a net producer of sugar,
such as mature leaves
• A sugar sink
– Is an organ that is a net consumer or storer of
sugar, such as a tuber or bulb
7
• Phloem sap moves from sugar
source to sugar sink
• Sugar must be loaded into
sieve-tube members before
being exposed to sinks
• In many plant species, sugar
moves by symplastic and
apoplastic pathways
Loading of sucrose into phloem: Notice that it can follow either the symplastic
or apoplastic pathway. Loading from the latter requires active transport.
Loading occurs in sugar sources, such as mature leaves.
Companion
(transfer) cell
Mesophyll cell
Sieve-tube
member
Cell walls (apoplast)
Plasma membrane
Plasmodesmata
Sucrose manufactured in mesophyll cells
can travel via the symplast (blue arrows)
to sieve-tube members. In some species,
sucrose exits the symplast (red arrow)
near sieve tubes and is actively
accumulated from the apoplast by sievetube members and their
companion cells.
Mesophyll cell
Phloem
parenchyma cell
Bundlesheath cell
Pressure flow in a sieve tube of
the phloem is from sugar sources
to sugar sinks.
• In many plants
– Phloem loading requires active transport
• Proton pumping and cotransport of
sucrose and H+
– Enable the cells to accumulate sucrose
High H+ concentration
Sources: Mature leaves, other
photosynthetic organs, storage
organs (such as roots)
Cotransporter
Sinks: Growing shoots, flowers,
developing fruits, root tips
H+
Proton
pump
Sugar loading at sources and
unloading at sinks creates a
pressure gradient that drives bulk
flow of the phloem sap.
S
Key
ATP
H+
Low H+ concentration
H+
Sucrose
S
Apoplast
Symplast
Tapping phloem sap with the help of an aphid, which is a phloem-feeder. An
aphid stylus, minus the aphid, functions as a phloem sap pressure probe
(right).
Sieve tube member
Stylet
Attach stylus near mature leaves, and another near base of plant.
Where would you predict sap pressure to be higher?
Things to remember about
water potential
• Water will diffuse from areas of low solute
concentration (high potential) to areas of
high solute concentration (low potential)
• Water will move from areas of high
pressure to areas of low pressure by bulk
flow
• Cell membranes are selective in the
solutes allowed to pass through, some
solutes are actively pumped
8