Chapter 36
Section 1: Physical forces drive the transport of materials
in plants over a range of distances
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Transport in vascular plants includes:
1 - transport of water and solutes by individual cells
2 - short-distance transport of substances from cell to
cell
3 - long-distance transport within xylem and phloem
Figure #1 (right)
1 – The roots absorb water and dissolved minerals from the soil
2 – Water and minerals are transported upward from roots to
shoots as xylem sap
3 – Transpiration, the loss of water from leaves (mostly through
stomata) create a force within leaves that pulls xylem sap
upward
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
5 – Sugars are produced by photosynthesis in the leaves.
6 – Sugars are transported as phloem sap to roots and other
parts of the plant
7 – Roots exchange gasses with the air spaces of soil, taking in
O2 and discharging CO2. In cellular respiration, O2 supports the
breakdown of sugar.
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Selective Permeability of
Membranes: A Review
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passive transport- diffusion across a membrane down a gradient; happens
without the cell directly using metabolic energy
 active transport- the pumping of solutes across membranes against their
electrochemical gradients, the combined effects of the concentration gradient
and the voltage (charge difference) across the membrane; the cell must expend
energy, usually in the form of ATP
 transport proteins- embedded in the membrane; help solutes cross the lipid
bilayer of the membrane; may bind selectively to a solute on one side of a
membrane and release it on the opposite side, or they may provide a selective
channel across the membrane
The Central Role of Proton Pumps
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proton pump- uses energy from ATP to pump hydrogen ions (H+) out of the
cell
 The pumping of hydrogen ions out of the cell results in a proton gradient with
a higher H+ concentration outside the cell than inside. The hydrogen ions
diffuse back into the cell. This “flow” can be harnessed as energy to do work.
 membrane potential- a separation of opposite charges across a membrane.
Charge separation is a form of potential energy that can be harnessed to
perform cellular work.
 Proton pumps contribute to membrane potential.
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In plant cells, energy stored in the proton gradient and
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membrane potential to drive the transport of many
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different solutes.
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cotransport- a transport protein couples the downhill
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passage of one solute to the uphill passage of another.
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Cotransport is responsible for the uptake of sucrose by
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plant cells.
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chemiosmosis- uses proton pumps; has a
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transmembrane proton gradient that links energy
releasing processes to energy-consuming processes
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in cells.
Effects of Differences in Water
 osmosis- the passive transport of water
Potential
across a membrane
 The net uptake or loss of water by a cell
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occurs by osmosis.
Water moves by osmosis from the
solution with a lower solute
concentration to the solution with a
higher solute concentration.
Physical pressure is also a factor in
osmosis.
water potential (abbreviated )- the
combined effects of solute concentration
and physical pressure; determines the
direction of movement of water
Free water (water that is not bound to
solutes or surfaces) moves from regions
of higher water potential to regions of
lower water potential if there is no
barrier to its flow.
megapascals (abbreviated MPa)- a unit
of pressure used to measure water
potential.
Effects of Differences in Water Potential: continued
How Solutes and Pressure Affect Water
 solute potential (abbreviated
S)- proportional to the
number of dissolved solute
molecules.
 Solute potential is also called
osmotic potential because
solutes affect the direction of
osmosis.
 pressure potential- the
physical pressure on a
solution. It can be positive or
negative relative to
atmospheric pressure
 tugor pressure- produced
when cell contents press the
plasma membrane against
the cell wall
Effects of Differences in Water Potential: continued
Quantitative Analysis of Water Potential
 In the absence of physical pressure (P = 0) the water potential
() will be equal to the solute potential (S), and the water will
flow towards the region of low solute potential (the region with
more dissolved solute molecules).
 If pressure equal to the positive value of the negative solute
potential is applied to the region with the dissolved solute, the
water potential would equal zero, and the water would be at
equilibrium if the other side of the membrane contained no
dissolved solutes (= 0).
 If pressure greater than the positive value of the negative solute
potential is applied to the region with the dissolved solute, the
water potential would be greater than zero, and if the other side
of the membrane contained no dissolved solutes (= 0), water
would flow towards the side with no dissolved solutes.
 If pressure was lessened on the side with no dissolved solute
molecules to less than the solute potential of the side with
dissolved solutes, the water would flow towards the side with no
dissolved solutes.
Quantitative Analysis of Water Potential:
Continued
 flaccid- limp; a cell that is flaccid has a water potential of 0.
 plasmolyze- occurs when the cell’s protoplast shrinks and pulls away
from the wall; This happens when the cell has a higher water potential
than the outside of the cell. Water then leaves the cell and the cell
plasmolyzes.
 turgid- very firm; occurs when the water potential of a cell is lower
than the water potential outside the cell. Healthy cells are turgid.
turgor contributes to support.
 wilting- the drooping of leaves and stems as a result of cells becoming
flaccid
Effects of Differences in Water Potential: continued
Aquaporin Proteins and Water Transport
 aquaporins- transport proteins that help water cross vacuolar and
plasma membranes
 Aquaporins control the rate at which water diffuses down its water
potential gradient. They do not affect the water potential gradient or
the direction of water flow.
 Rate of water movement through aquaporins may be regulated by
phosphorylation of the aqua Orin proteins induced by changes in
second messengers.
Aquaporin Water Channel
Three Major Compartments of
Vacuolated Plant Cells
 Transport is regulated by the compartmental structure of plant
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cells. The plasma membrane functions as a selectively permeable
membrane that controls the traffic of molecules into and out of the
protoplast. It is a barrier between the cell wall and the cytosol, two
major compartments of the cell.
The vacuole is a large organelle that can occupy as much as 90%
or more of the protoplast’s volume.
vacuolar membrane or tonoplast- regulates molecular traffic
between the cytosol and vacuolar contents (cell sap)
Cell walls and cytosol are continuous from cell to cell. They are
connected by plasmodesmata.
symplast- the cytoplasm continuum
apoplast- the continuum of cell walls plus the extra cellular spaces
The vacuole is the third cellular compartment, and is not shared
with neighboring cells
Functions of the Symplast and
Apoplast in Transport
 Short-distance transport, such as from the root hairs to the vascular
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cylinder of the root, is called lateral transport because it is usually along the
radial axis of plant organs, rather than up and down along the length of the
plant.
Lateral transport may be accomplished by three different routes.
1 - transmembrane route - substances move out of one cell, across the cell
wall, and into the next cell, which may then pass the substance along to the
next cell
2 - via symplast - requires only one crossing of a plasma membrane; After
entering one cell, solutes and water move from cell to cell via
plasmodesmata
3 - along the apoplast - water and solutes move from one location to
another within a root or other organ along the byways provided by the
continuum of cell walls; the solute never enters a protoplast
Bulk Flow in Long-Distance
Transport
 bulk flow- the movement of a fluid
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x y l e m
p h l o e m
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driven by pressure; contributes to
long-distance transport
In bulk flow, water and solutes
moves through the tracheids and
vessels of the xylem and through
the sieve tubes of the phloem.
In phloem, the loading of sugar
generates a high positive
pressure at one end of a sieve
tube, forcing sap to the opposite
end of the tube.
Xylem use tension (negative
pressure) to drive long distance
transport
Transpiration is the evaporation
of water from a leaf. It reduces
pressure in the leaf xylem,
creating tension that pulls xylem
sap upward from the roots.
Chapter 36
Section 2: Roots absorb Water and Minerals from the soil
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In the Apoplastic route, the hydrophilic walls
of the root hairs take in the water and
minerals from the soil, providing access to
the apoplast.
In the Symplastic route, the water and
minerals enter the symplast through the
plasma membrane of the root hairs.
During the Apoplastic route, some water
and soil enter the protoplasts of cells
through the symplast when passing trough
the epidermis and the cortex.
Endodermal cells contain a casparian strip
(shown here in purple) that does not allow
water or minerals to pass. In order to
overcome this barrier, the material must
enter the symplast ,if not already there, to
get into the vascular cylinder.
Endodermal and parenchyma cells
discarche water and mineralso inot the
apoplast, allowing the xylem vessels to
receive it and transport it to the shoot
system.
The Roles of Root Hairs,
Mycorrhizae, and Cortical Cells
 Root hairs are extensions of epidermal cells that
allow soil particles coated with water and minerals to
adhere to them. Once the soil solution is absorbed by
the hydrophilic walls, it is exposed to the symplast of
all the epidermal cells and the cortical cells.
 As material is being absorbed, roots can accumulate
vital minerals in concentrations hundreds of times
higher than the soil.
 Mycorrhizea are symbiotic structures formed by roots
and fungi. These structures allow an increased
surface area for water and select materials to be
absorbed.
The Endodermis: A Selective
Sentry
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The Endodermis is located at the end of the cortex. It serves as a
checkpoint before material is transmitted throughout the rest of the plant.
Minerals traveling through the symplastic route pass easily through the
endodermis, but those passing through the apaplastic route must enter
they symplast. This happens because endodermal cells contain the
Casparian strip.
 The Casparian strip is made of suberin which does not allow the passage
of water or any other minerals. This strip assures that only selected
minerals can pass through to the xylem vessels and solutes that have
accumulated in the xylem sap do not flow back into the soil solution.
 Once the minerals pass the Casparian strip, endodermal cells and
parenchyma cells discharge the minerals from the symplast to the
apoplast. This lets the material enter the tracheids and vessel elements of
the xylem. These are water conducting and part of the apoplast since they
do not contain a protoplast when mature. These parts make up the end of
the soil to xylem pathway and the material can now be passed to the
shoot system.
Chapter 36
Section 3: Water and minerals ascend from roots to
shoots through the xylem
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Leaves depends on the long-distance transport
system of xylem sap which is flowing upward
from roots throughout the shoot system to veins
that branch throughout each leaf for their supply
of water and mineral nutrients.
Transpiration is the loss of astonishing amount
of water vapor from leaves usually through
stomata, the microscopic pores on the surface of
a leaf and other aerial parts of the plant by
diffusion and evaporation.
When the transpiration is very low or zero, usually
at night, the root cells continue to pump mineral
ions into the xylem of the vascular cylinder. At the
same time endodermis helps prevent the ions
from leaking out. This accumulation of minerals
lowers the water potential within the vascular
cylinder.
Factors affecting the Ascent of Xylem sap
-Root pressure is an upward push of xylem sap
which allows water to flow in from the root cortex
and cause guttation by making more water to
enter the leaves than is transpired.
-Guttation is the exudation of water droplets
that can be seen in the morning on the tips of
grass blades or the leaf margins of some small,
herbaceous eudicots. It is different from dew.
(figure 34.11)
-In most of plants, root pressure is a minor
mechanism driving the ascent of xylem sap, at
most forcing water upward only a few meters.
The generation of
transpirational pull in a leaf
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1) In transpiration, water vapor
(shown as blue dots) diffuses
from the moist air spaces of the
leaf to the drier air outside via
stomata.
 2) At first, the water vapor lost
by transpiration is replaced by
evaporation from the water film
that coats mesophyll cells.
 3) Evaporation causes the airwater interface to retreat farther
into the cell wall and become
more curved as the rate of
transpiration increases. As the
interface becomes more curved,
the water film’s pressure
becomes more negative. This
negative pressure, or tension,
pulls water from the xylem,
where the pressure is greater.
Water and minerals ascend from
roots to shoots through the xylem
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The transpiration-cohesion-tension mechanism is responsible for pulling or transporting
xylem sap against its gravity:
-Transpiration pull: When the transpiration occurs severely, the degree of curvature and the
surface tension of the water molecules increases, accordingly the pressure at the air-water
interface becomes increasingly negative and more water molecules are pulled toward this region
to reduce the tension.
*Negative pressure (tension) lowers water potential and thus, it causes the “pull” in
transpirational pull.
-Cohesion and Adhesion facilitate the long-distance transport throughout the plant
*The cohesion of water due to hydrogen bonding between the water molecules makes it
possible to pull a column of
sap from above without the water molecules separating.
*The strong adhesion of water molecules (again by hydrogen bonds) to the hydrophilic
walls of xylem cells aids in
offsetting the downward pull of gravity.
*Some other plant structures, such as the thick secondary walls help to prevent
vessels from collapsing (structural support).
* Transpirational pull can extend down to the roots only through an unbroken chain of
water molecules
-Capitation is the formation of a water vapor pocket in a vessel breaks the chain;
the air bubbles resulting from cavitations expand and eventually become
embolisms, blockages of the water channels of the xylem.
-Root pressure enable small plants to refill embolized vessels in spring ; Only
youngest, outermost secondary xylem transports water while the older secondary
xylem does provide support for the tree.
Water and minerals ascend from
roots to shoots through the xylem
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Xylem Sap Ascent by Bulk Flow : In the
long-distance transport of water from roots
to leaves by bulk-flow, the movement of
fluid is driven by a water potential
difference at opposite ends of a conduit
and it is only depending on pressure. (In
case of plants, the conduits are vessels or
chains of tracheids.)
-The plant absorbs the sunlight to
drive transpiration by causing water to
evaporate from the moist walls of
mesophyll cells and by lowering the water
potential in the air spaces within a leaf
rather than expending any type of energy
to lift xylem sap by bulk flow.
- Unlike osmosis, which moves only
water, the bulk flow moves the whole
solution, water plus minerals and any
other solutes dissolved in the water.
The water potential difference is
generated at the leaf end by
transpirational pull, which lowers the
potential (increase tension) at the
“upstream” end of the xylem.
-Water potential gradients drive the
osmotic movement of water from cell to
cell within root and leaf tissue ;
* Difference in both solute
concentration and turgor pressure
contribute to this short-distance transport.
Chapter 36:
section4: Stomata help regulate the rate of transpiration
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Leaves generally have large surface areas and high surface area-tovolume ratios.
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 Large surface area  morphological adaptation that enhances the
absorption the absorption of light needed to drive photosynthesis
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 High surface area-to-volume ratio  aids in the uptake of carbon
dioxide during photosynthesis and well as the release of oxygen
produces as a by-product of photosynthesis
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Large surface areas and high surface area-to-volume ratios have the
serious drawback of increasing water loss by way of the stomata.
Guard cells  help balance the plant’s requirements to conserve water
with its requirement for photosynthesis.
Effects of Transpiration on
Wilting and Leaf Temperature
 A leaf may transpire more than its weight in water each
day, and water may move through the xylem at a rate as
fast as 75 cm/min.
 If transpiration continues to pull sufficient water upward to
the leaves, they will not wilt.
 Some evaporative water loss does occur even when the
stomata are closed.
 Transpiration also results in evaporative cooling, which
can lower the temperature of a leaf by as much as 1015C compared with the surrounding air.
Stomata: Major Pathways for Water Loss
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About 90% of the water a plant loses escapes through stomata, though these
pores account for only 1-2% of the external leaf surface.
Cuticle limits water loss through the remaining surface of the leaf
Guard cells control the diameter of the stoma by changing shape, thereby
widening or narrowing the gap between the two cells.
The stomatal density of a leaf is under both genetic and environmental control.
High light intensities and low carbon dioxide levels during leaf development tend to
increase stomatal density in many plant species.
When guard cells take in water from neighboring cells by osmosis, they become
more turgid and bowed.
The changes in turgor pressure that open and lose stomata results primarily from
the reversible uptake and loss of potassium ions by the guard cells.
The K+ fluxes across the guard cell membrane are coupled to the generation of
membrane potentials by proton pumps.
In general, stomata are open during the day and closed during the night.
This prevents the plant from losing water when it is too dark for photosynthesis.
A second stimulus causing stomata to open is depletion of carbon dioxide within
air spaces for the leaf, which occurs when photosynthesis begins in the mesophyll.
A third cue causing stomatal opening is an internal “clock” in the guard cells.
Cycles that have intervals of appx. 24 hours are called circadian rhythms.
Environmental stresses can cause stomata to close during the day time.
Guard cells arbitrate the photosynthesis-transpiration compromise on a momentto-moment basis by integrating a variety of internal and external stimuli.
Xerophyte Adaptations that
reduce transpiration
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Plants adapted to arid climates, called xerophytes, have various leaf
modifications that reduce the rate of transpiration.
 Many xerophytes have small, thick leaves, an adaptation that limits
water loss by reducing surface area relative to leaf volume.
 An elegant adaptation to arid habitats is found in succulents of the
family Crassulaceae, in ice plants, and in many other plant families.
 These plants assimilate carbon dioxide by an alternative photosynthetic
pathway known as CAM, for crassulacean acid metabolism
Chapter 36
section 5: Organic nutrients are translocated through the
phloem
 Translocation- the transport of organic
nutrients in a plant
 In order to transport sugars from leaves to
other parts of the plant xylem sap flows from
roots to leaves in the opposite direction
 Products of photosynthesis are transported via
the phloem, which is a second vascular tissues
Movement from Sugar Sources to
sugar sinks
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Sieve tube members are the specialized cells of phloem that function as the
conduits for translocation in angiosperms, these members when placed together
from sieve tubes.
Between the cells are sieve plates which allow the flow of sap along the sieve
tube.
Phloem sap is an aqueos solution that is different from xylem sap, it is primarily
made up of dissacharide sucrose. The sucrose concentration can be up to 30%
and gives the sap its thickness.
Sieve tubes carry sugar from the sugar source to the sugar sink
Sugar source – a plant organ that is the net producer of sugar
Sugar sink – a plant organ that is the net consumer of sugar
Some spieces move sugar from the mesophyll to the sieve tube via symplast,
which means passing the plasmodesmata. Then it moves into the apoplast and is
then accumulated by the sieve tube
Transfer cells- cells that enhance the transfer of solutes between apoplast and
symplast
The sugar level in the sink is always lower, because it is either consumed during
growth or converted into another polymer
Figure 1: Loading of sucrose into
phloem
Pressure Flow: The Mechanism of
Translocation in Angiosperms
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Phloem sap flows at a rate of 1 m/hr
 When scientist studied angiosperms, they determined that sap flows
through the sieve tube in bulk flow, and is driven by pressure flow
 Sugar transport can be seen on three levels
1.
Cellular level across the plasmadesmata- sucrose accumulation
via active transport
2.
Short-distance transport –sucrose migration from the mesophyll
to the phloem
3.
Long distance transport between organs – bulk flow in sieve
tubes.
Figure 2: Pressure flow in a
sieve tube