Land plants require a transport system, because unlike their
aquatic ancestors, photosynthetic plant organs have no direct access to water
I. The traffic of water and solutes occurs on cellular,
organ, and whole-plant levels: an overview of transport
Three levels of transport occur in plants:
1. Uptake of water and solutes by individual cells.
2. Short-distance cell-to-cell transport at the level of
tissues and organs.
3. Long-distance transport of sap in xylem and phloem at the
A. Transport at the Cellular Level
1. Review of Active and Passive Transport of Solutes
The plasma membrane's selective permeability controls the
movement of solutes between a plant cell and the extracellular fluids. Solutes
may move by passive or active transport .
Passive transport occurs when a solute molecule
diffuses across a membrane down a concentration gradient.
direct expenditure of energy by the cell.
proteins embedded in the membrane may increase the speed at which solutes
Transport proteins may facilitate diffusion by
serving as carrier proteins or forming selective channels.
proteins bind selectively to a solute molecule on one side of the
membrane, undergo a conformational change, and release the solute molecule
on the opposite side of the membrane.
channels are simply passageways by which selective molecules may enter and
leave a cell; some gated selective channels are stimulated to open or
close by environmental conditions.
Active transport occurs when a solute molecule is
moved across a membrane against the concentration gradient.
pump is an active transporter important to plants.
A proton pump hydrolyzes A TP and uses the energy to pump
hydrogen ions (H+) out of the cell.
a proton gradient with a higher concentration outside of the cell.
a membrane potential since the inside of the plant cell is negative in
relation to the outside.
membrane potential and the stored energy of the proton gradient are used
by the plant to transport many different molecules.
ions (K+) are pulled into the cells because of the electrochemical
(NO3 1 enters plant cells against the electrochemical gradient by cotransporting
with hydrogen ions.
2. Water Potential and Osmosis
Osmosis results in the net uptake or loss of water by
the cell and depends on which component, the cell or extracellular fluids, has
the highest water potential.
Water potential (Y) = The free energy
of water that is a consequence of solute concentration and applied pressure;
physical property predicting the direction water will flow.
will always move across the membrane from the solution with the higher
water potential to the one with lower water potential.
potential is measured in units of pressure called megapascals
(MPa); one MPa is equal to ten atmospheres of pressure.
water in an open container has a water potential of zero megapascals (Y=
of solutes to water lowers the Y into the negative range.
Increased pressure raises the Y into the positive range.
negative pressure, or tension, may also move water across a
membrane; this bulk flow (movement of water due to pressure
differences) is usually faster than the movement caused by different
NOTE: You may wish to call attention to Campbell, Figure
The effects of pressure and solute concentration on water
potential are represented by:
Y = Yp -Ys
·Yp = pressure potential.
·Ys = solute potential or osmotic potential.
·A 0.1M solution has a pressure potential of 0.23
MPa; in an open container, physical pressure is zero; the water potential of
this 0.1M solution would be – 0.23 (Y= 0 -0.23 = -0.23). Water would enter this solution due
only to osmotic pressure.
·The addition of pressure to the solution could
counter the affects of osmotic pressure by stopping net water movement (if P =
0.23) or by forcing water from the solution back into the pure water (if P >
·Similar changes would result if a negative
pressure were applied to the pure water side of the membrane.
Plant cells will gain or lose water to intercellular fluids
depending upon their water potential.
flaccid cell (P = 0) placed in a hyperosmotic solution will lose water by
osmosis; the cell will plasmolyze (protoplast pulls away from the cell
wall) in response.
flaccid cell placed in a hypoosmotic solution will gain water by osmosis;
the cell will swell and a turgor pressure develops; when pressure
from the cell wall is equal to the osmotic pressure, an equilibrium is
reached and no net water movement occurs (P = p;
3. The Role of the Tonoplast
Tonoplast = Membrane surrounding the large central
vacuole found in plant cells; is important in regulating intracellular
integral transport proteins that control the movement of solutes between
the cytosol and the vacuole.
membrane potential. Proton pumps in the tonoplast help the plasma membrane
maintain a low H+ concentration in the cytosol by moving H+ into
solutes are transported between the cytosol and vacuole due to this
membrane potential and proton gradient.
B. Short-Distance (Lateral) Transport at the Level of Tissues and Organs
Lateral transport is usually along the radial axis of
plant organs, and can occur by three routes in plant tissues and organs: (See
Campbell, Figure 32.5)
1. Across the plasma membranes and cell walls.
Solutes move from one cell to the next by repeatedly crossing plasma membranes
and cell walls.
2. The symplast route. A symplast is the continuum of
cytoplasm within a plant tissue formed by the plasmodesmata which pass through
pores in the cell walls. Once water or a solute enters a cell by crossing a
plasma membrane, the molecules can enter other cells by traveling through the
3. The apoplast route. An apoplast is the continuum
between plant cells which is formed by the continuous matrix of cell walls.
Water and solute molecules can move from one area of a root or other organ via
the apoplast without entering a cell.
Water and solute molecules can move laterally in a plant
organ by anyone of these routes or by a combination through switching from one
C. Long-Distance Transport at the Whole-Plant Level
This type of transport is usually along the vertical axis of
the plant (up and down) from the roots to the leaves and vice versa.
Vascular tissues are involved in this type of transport as
diffusion would be too slow.
flow (movement due to pressure differences) moves water and solutes
through xylem vessels and sieve tubes.
reduces pressure in the leaf xylem; this creates a tension which pulls sap
up through the xylem from the roots.
pressure develops at one end of the sieve tubes in the phloem; this forces
the sap to the other end of the tube.
II. Roots absorb water and minerals from soil
Water and minerals enter plants through the following
soil à epidermis à
root cortex à
xylem. (See Campbell, Figure 32.6)
Soil à Epidermis:
absorption occurs near root tips where the epidermis is permeable to
hairs, extensions of epidermal cells, increase the surface area available
Epidermis à Root cortex:
Lateral transport of minerals and water through the root is
usually by a combination of apoplastic and symplastic routes.
apoplastic route exposes parenchymal, cortex cells to soil solution.
=> Soil solution, containing
soil particles, water and dissolved minerals, flows into hydrophilic walls of
epidermal cells and passes freely along the apoplast into root cortex.
=> Compared to the epidermis,
the apoplastic route exposes greater membrane surface area for water and
mineral uptake into cytoplasm-
symplastic route makes selective mineral absorption possible.
=> As soil solution moves along
cell walls, some water and solutes cross theplasma membrane of epidermal and
=> Cells cannot absorb a
sufficient supply of mineral ions by diffusion alone -the soil solution is too dilute.
Active transport permits root cells to accumulate essential minerals in very
=> For example, transport
proteins of the plasma membrane and tonoplast actively transport K+ into root
cells as Na+ is pumped out.
Root Cortex à Xylem:
minerals using the symplastic route may move directly into the vascular
tissues. They have been previously selected by a membrane.
and water passing through apoplasts are blocked at the endoderm is by a Casparian
strip (a ring of suberin around each cell in the endodermis) and must
enter an endodermal cell.
Water and mineral entrance into the stele must be through
the cells of the endodermis.
strips ensure that all substances entering the stele pass through at least
one membrane, allowing only selected ions to pass into the stele. Also
prevents stele contents from leaking back into the apoplast and out into
and minerals enter the stele via symplast, but tracheids and xylem vessels
are part of the apoplast.
and parenchymal cells selectively discharge minerals into the apoelast so
they may enter the xylem. This action probably involves diffusion and
minerals and water which move into the apoplast are free to enter the
tracheids and xylem vessels.
III. The ascent of xylem sap depends mainly on
transpiration and the physical properties of water
The shoot depends upon an efficient delivery of its water
sap flows upward at ISm per hour or faster.
vessels are close to each leaf cell, because veins branch throughout the
Water transported up from roots must replace that lost by transpiration.
is the evaporation of water from the aerial parts of a plant.
upward flow of xylem sap also provides nutrients (minerals) to the shoot
A. Pushing Xylem Sap: Root Pressure
When transpiration is low, active transport of ions into the
xylem decreases the stele's water potential and causes water flow into the
stele. This osmotic water uptake increases pressure which forces fluid up the
xylem (= root pressure).
pressure causes guttation (exudation of water droplets at leaf
water droplets escape through specialized structures called hydrathode which
relieve the pressure caused by more water entering the leaves than is lost
Root pressure is not the major mechanism driving the ascent
of xylem sap.
keep pace with transpiration.
only force water up a few meters.
B. Pulling Xylem Sap: The Transpiration-Cohesion- Tension Mechanism
Transpiration pulls xylem sap upward, and cohesion of water
transmits the upward-pull along the entire length of xylem.
pull depends upon the creation of negative pressure.
water in damp intercellular leaf spaces diffuses into the drier atmosphere
lost water vapor is replaced by evaporation from mesophyll cells bordering
remaining water film, adhering to the hydrophilic cell walls, retreats
into the cell wall pores.
in this surface film of water resists an increase in the surface area of
the film -a surface tension effect.
water film forms a meniscus due to the negative pressure caused by the
adhesion and cohesion.
negative pressure pulls water from the xylem, through the mesophyll,
toward the surface film on cells bordering the stomata.
moves through symplasts and apoplasts to a region of low water potential.
results in water from the xylem replacing water transpired through the
Cohesion and Adhesion of Water:
transpirational pull on the xylem sap is transmitted to the soil solution.
Cohesion of water due to H bonds allows for the pulling of water from the
top of the plant without breaking the "chain."
adhesion of water (by H bonds) to the hydrophilic walls of xylem cells
also helps pull against gravity.
small diameter of vessels and tracheids is important to the adhesion
upward pull of sap causes tension (negative pressure) in xylem, which
decreases water potential and allows passive flow of water from soil into
Cavitation = Formation of a water vapor pocket in
the chain of water molecules and the pull is stopped.
cannot function again unless refilled with water by root pressure. (This
can only occur in small plants.)
between adjacent xylem vessels allow for detours around a cavitated area.
growth also adds new xylem vessels each year.
C. Review: The Long-Distance Transport of Water in Plants
by Solar-Powered Bulk
Bulk flow is the movement of fluid due to pressure
differences at opposite ends of a conduit.
The ascent of xylem sap is ultimately solar-powered, by
causing evaporative water loss and, thus, negative pressure.
vessels or chains of tracheids serve as the conduits in plants.
pull lowers the pressure at the upper (leaf) end of the conduit.
movement of water from cell to cell in roots and leaves are due to small
gradients in water potential caused by both solute and pressure gradients.
contrast, bulk flow through the xylem vessels depends only on pressure.
IV. Guard cells mediate the transpiration-photosynthesis compromise
Transpiration results in a tremendous water loss from the
plant. This water is replaced by the upward movement of water through the
xylem, and guard cells surrounding stomata balance the requirements for
photosynthesis with the need to conserve water.
A. The Photosynthesis- Transpiration Compromise
Large surface areas along a leaf's airspaces are needed for
CO2 intake for photosynthesis, but also results in greater surface area for
evaporative water loss.
surface area of a leaf may be 10x-30x the external surface area.
are more concentrated on the bottom of leaves, away from the sun; this
reduces evaporative loss.
waxy leaf cuticle prevents water loss from the rest of the leaf surface.
Transpiration:photosynthesis ratios measure efficiency of
water use. This ratio is 9 H2O
Lost/g CO2 assimilated into organic material.
of 600: 1 is common in C3 plants; 300: 1 in C4 plants.
plants can assimilate CO2at greater rates than C3 plants.
Benefits of transpiration:
in mineral transfer from roots to shoots.
cooling reduces risk of leaf temperatures becoming too high for enzymes to
If transpiration exceeds delivery of water by xylem, plants
can adjust to reduce risk of wilting.
the size of stomatal openings also reduces transpiration.
B. How Stomata Open and Close
Guard cells = Cells that flank stomata and control
stomatal diameter by changing shape.
turgid, guard cells "buckle" due to radially-arranged
microfibrils and stomata open; when flaccid, guard cells sag and stomatal
The change in turgor pressure that regulates stomatal
opening results from reversible uptake and loss of K+ by guard cells. (See Campbell,
of K+ decreases guard cell water potential so H2O is taken up, cells
become turgid, and stoma opens. The tonoplast plays a role as most of the
K+ and water are stored in the vacuole.
increase in positive charge is countered by the uptake of chloride (CI-),
export of H+ ions released from organic acids, and the negative charges
acquired by organic acids as they lose their protons.
of the stomata results when K+ exits the guard cells and creates an
osmotic loss of water.
Evidence from studies using patch clamping techniques
indicates that K+ fluxes across the guard cell membrane are likely coupled to
membrane potentials created by proton pumps.
opening correlates with active transport of H+ out of the guard cell.
resulting membrane potential drives K+ into the cell through specific
By integrating internal and external environmental cues,
guard cells open and close, balancing the requirements for photosynthesis with
the need to conserve water from
open at dawn in response to three cues:
=> Light induces guard cells to
take up K+ by:
a blue-light receptor which stimulates proton pumps in the plasma membrane.
¯Driving photosynthesis in guard cell chloroplasts, making A
TP available for the ATP-driven proton pumps.
=> Decrease of CO2 in leaf air
spaces due to photosynthesis in the mesophyll.
=> An internal clock of guard
cells will make them open even if kept in dark (a
circadian rhythm approximates a 24 hour cycle
cells may close stomata during the daytime if:
=> There is a water deficiency
resulting in flaccid guard cells.
=> Mesophyll production of
abscisic acid (a hormone) in response to water deficiency signals guard cells
=> High temperature increases
CO2 in leaf air spaces due to increased respiration, closing guard cells.
C. Evolutionary Adaptations that Reduce Transpiration
Xerophytes, plants adapted to arid climates, have
some of the following evolutionary adaptations that reduce transpiration:
thick leaves (reduced surface area: volume, so less H2O loss).
are in depressions on the underside of leaves to protect from water loss
due to drying winds.
shed leaves in the driest time of the year.
and others store water in stems during the wet season.
Plants of family Crassulaceae use CAM
(crassulacean acid metabolism) to assimilate CO2.
night, mesophyll cells assimilate CO2 into organic acids.
the day, the acids are broken down, releasing CO2 which is used to
synthesize sugars by the conventional C3 pathway.
stomata can close during the day, the plant conserves water, and there is
still an ample supply of CO2 for photosynthesis.
V. A bulk-flow mechanism translocates phloem sap from
sugar sources to sugar sinks
Translocation = The
transport of the products of photosynthesis by phloem to the rest of the plant.
angiosperms, sieve-tube members are the specialized cells of phloem
that function in translocation.
=> Sieve-tube members are
arranged end-to-end forming tong sieve tubes.
=> Porous cross walls called sieve
plates are in between the members and allow phloem to move freely along the
sap contains primarily sucrose, but also minerals, amino acids and
A. Source-to-Sink Transport
Phloem sap movement is not unidirectional; it moves through
the sieve tubes from a source (production area) to a sink (use or storage
Source = Organ where sugar is produced by
photosynthesis or by the breakdown of starch (usually leaves).
Sink = Organ that consumes or stores sugar (growing
parts of plants, fruits, non-green stems and trunks, and others).
Sugar flows from source to sink.
and sink depend on season. A tuber is the sink when stockpiling in the
summer, but is the source in the spring.
may also be transported to sinks.
sink is usuatty supptied by the nearest source.
of flow within a phtoem etement can change, depending on locations of the
source and sink.
B. Phloem Loading and Unloading
Sugar produced at a source must be loaded into sieve-tube
members before it can be translocated to a sink.
some plant species, the sugar may move through the s)'mplast from
mesophyll cells to the sieve members.
other species, the sugar uses a combination of symptastic and apoptastic
routes. (See Campbell, Figure
plants have transfer cells. These are modified companion cells
which have numerous ingrowths of their walls. These
structures increase the cells' surface area and enhances solute
transfer between apoplast and symptast.
plants such as corn, active transport accumulates sucrose in sieve-tube
members to the
concentration in mesophyll cells.
=> Proton pumps power this
transport by creating a H+ gradient-
=> A membrane protein uses the
potential energy stored in the gradient to drive the cotransport of sucrose by
coupling sugar transport to the diffusion of H+ back
Sucrose is unloaded at the sink end of sieve tubes.
some plants, sucrose is unloaded from the phloem by active transport.
other species diffusion moves the sucrose from the phloem into the cells
of the sink.
symplastic and apoplastic routes may be involved.
C. Pressure Flow
(Bulk Flow) of Phloem Sap
Phloem sap flows up to I meter per hour, too fast for just
diffusion or cytoplasmic streaming.
The flow is by a bulk flow (pressure-flow) mechanism;
buildup of pressure at the source and release of pressure at the sink causes
source-to-sink flow. (See Campbell,
the source end, phloem loading causes high solute concentrations.
=> Water potential decreases, so
water flows into tubes creating hydrostatic
=> Hydrostatic pressure is
greatest at the source end of the tube.
the sink end, the water potential is lower outside the tube due to the
unloading of sugar; osmotic loss of water releases hydrostatic pressure.
vessels recycle water from the sink to the source.
Aphids have been used to study flow in phloem. (See Campbell,
aphid stylet punctures phloem and the aphid is "force-fed" by
aphid is severed from its stylet and flow is measured through the stylet.
flow exerts greater pressure and has a higher sugar concentration the
closer the source.