Transport In Plants
Transport in plants occurs on three
(1) the uptake and loss of water and
solutes by individual cells, such as the
absorption of water and minerals from
the soil by cells of a root;
(2) short-distance transport of
substances from cell to cell at the level
of tissues and organs, such as the
loading of sugar from photosynthetic
cells of a mature leaf into the sieve
tubes of phloem;
(3) long-distance transport of sap
within xylem and phloem at the level of
the whole plant.
Transport at the cellular level
• This mode of transport depends mainly on the selective
permeability of membranes known as passive transport. This
process is facilitated and accelerated by transport proteins
within the cell membrane.
• Some of these facilitate diffusion by binding selectively to a
solute on one side of the membrane and releasing the solute
on the opposite side, while other transport proteins function as
selective channels, which are simply selective passageways
across the membrane.
• Active transport is the pumping of solutes across membranes
against their natural diffusion meaning that the cell must exert
energy to move the solutes and is performed by a special
class of membrane proteins, each responsible for pumping
• The most important active transporter in the plasma
membranes of plant cells is the proton pump, which
uses energy pump hydrogen ions (H+) out of the cell
causing membrane potential and is used as a form of
• Cotransport is when a transport protein couples the
downhill passage of one solute (H+) to the uphill passage
of another such asNO3-.
• Chemiosmosis is the process of creating a
transmembrane proton gradient, which links energy-
releasing processes to energy-consuming processes in
cells, ultimately allowing photosynthesis.
Differences in water potential
• The survival of plant cells depends on their ability to balance
water uptake and loss.
• The net uptake or loss of water by a cell occurs by osmosis,
the passive transport of water across a membrane. Water will
move by osmosis from hypotonic(lower solute concentration)
to hypertonic(higher solute concentration). But in the case of a
plant cell, the presence of a cell wall adds physical pressure
to the mix.
• Solute concentration and pressure--are incorporated into a
single measurement called water potential. Water potential
states that water will move across a membrane from the
solution with the higher water potential to the solution with the
lower water potential measured in megapascals (abbreviated
MPa ), where 1 MPa is equal to about 10 atmospheres of
• There is an inverse
relationship of water potential
to solute concentration, while
water potential is directly
proportional to pressure.
• The combined effects of
pressure and solute
concentration on water
potential are calculated by
adding the solute potential
and pressure potentials
•Turgor pressure is when the
cell begins to swell and push
against the wall creating
•In these experiments, identical
cells, initially flaccid, are placed
in two different environments.
(The protoplasts of flaccid cells
are in contact with their walls
but lack turgor pressure.) The
blue arrows indicate the initial
direction of water movement.
•In contrast to a flaccid cell, a
walled cell that has a greater
solute concentration than its
surroundings will be turgid, or
pushing against its cell wall.
• These specific channels for passive traffic of
water are transport proteins called aquaporins.
• Aquaporins do not affect the water potential
gradient or the direction of water flow, but rather
the rate at which water diffuses down its water
• Aquaporins may form gated channels that open
and close in response to variables such as
turgor pressure of the cell. Many laboratories are
now investigating how aquaporins work, and this
research is likely to increase our understanding
of how the cells of plants and other organisms
regulate water balance.
Three major compartments of
Vacuolated barrier between two major
The plasma membrane serves as a
compartments: the cell wall and the cytosol (the part of the cytoplasm
contained within the plasma membrane but outside the intracellular
organelles). Most mature plant cells have a third major compartment,
the vacuole, a large organelle that can occupy as much as 90% of the
• The membrane that bounds the vacuole, the tonoplast, regulates
molecular traffic between the cytosol and the vacuolar contents, called
• In most plant tissues, two of the three cellular compartments are
continuous from cell to cell. Plasmodesmata connect the cytosolic
compartments of neighboring cells, forming a continuous pathway for
transport of certain molecules between cells. This cytoplasmic
continuum is called the symplast.
• 6). The walls of adjacent plant cells are also in contact, forming a
second compartment at the tissue level. This compartment, the
continuum of cell walls, is called the apoplast.
• Vacuoles are not shared between cells.
• (a) The cell wall, cytosol,
and vacuole are the three
main compartments of most
mature plant cells. Specific
embedded in the plasma
membrane and tonoplast
regulate traffic of molecules
between the three
• (b) At the tissue level, there
are two compartments, the
symplast and the apoplast.
This anatomy provides
three routes for lateral
transport in a plant tissue or
organ. In this diagram,
substances seem confined
to one of the three routes;
in fact, substances may
transfer from one route to
another during their
commute across an organ.
Symplast and the Apoplast
Function in Transport
• Three routes are available for lateral(short
distance) transport .
• First, substances may move out of one cell,
across the cell wall, and into the neighboring
• Second, via the symplast, the continuum of
cytosol within a plant tissue, solutes and water
can then move from cell to cell via
• Third, with the apoplast, solutes and water move
along the extracellular pathways consisting of
cell walls and extracellular spaces.
Bulk flow functions in long-distance
• Water and solutes move through xylem vessels and
sieve tubes by bulk flow, the movement of a fluid driven
• In phloem, for example, hydrostatic pressure is
generated at one end of a sieve tube, forcing sap to the
opposite end of the tube.
• In xylem, it is actually tension (negative pressure) that
drives long-distance transport. Transpiration, the
evaporation of water from a leaf, reduces pressure in the
leaf xylem. This creates a tension that pulls xylem sap
upward from the roots.
ABSORPTION OF WATER AND
MINERALS BY ROOTS
• Water and mineral
salts from soil enter
the plant through
the epidermis of
roots, cross the root
cortex, pass into the
stele, and then flow
up xylem vessels to
the shoot system.
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, which are extensions of epidermal cells, account for much of the
surface area of roots.
• Water and mineral solution flows into the hydrophilic walls of epidermal cells
and passes freely along the apoplast into the root cortex.
• As the water and mineral solution moves along the apoplast into the roots,
cells of the epidermis and cortex take up water and certain solutes into the
symplast. The solution is usually very dilute, and roots can accumulate
essential minerals to concentrations that are hundreds of times higher than
the concentrations of these minerals in soil. Selective transport proteins of
the plasma membrane and tonoplast enable root cells to extract essential
minerals and nutrients.
• "Infected" roots form mycorrhizae, symbiotic structures consisting of the
plant’s roots united with the hyphae (filaments) of fungi. The hyphae absorb
water and selected minerals, transferring much of these resources to the
host plant greatly enhancing surface area at a proportion up to 300:1
• The endodermis, the innermost layer of cells in the root cortex,
surrounds the stele and functions as a last checkpoint for the
selective passage of minerals from the cortex into the vascular
tissue, ensuring that no minerals can reach the vascular tissue of
the root without crossing a selectively permeable plasma membrane
allowing roots to preferentially transport certain minerals from the
soil into the xylem.
• Minerals already in the symplast when they reach the endodermis
continue through the plasmodesmata of the endodermal cells and
pass into the stele. Whial those minerals that reach the endodermis
via the apoplast encounter a dead end that blocks their passage into
• In the wall of each endodermal cell is the Casparian strip, a belt
made of suberin, a waxy material that is impervious to water and
dissolved minerals. Which is why water and minerals cannot cross
the endodermis and enter vascular tissue via the apoplast.
• At night, when transpiration is very low or zero, the root cells are still
expending energy to pump mineral ions into the xylem. The endodermis
prevents the leakage of these ions back out of the stele causing lower water
potential there generating a positive pressure that forces fluid up the xylem
in a process called root pressure.
• Root pressure causes guttation, the exudation of water droplets that can
be seen in the morning on tips of grass blades or the leaf margins.
• At most, root pressure can force water upward only a few meters, and many
plants generate no root pressure at all. Even in plants that display guttation,
root pressure cannot keep pace with transpiration after sunrise.
• Transpiration provides the pull, and the cohesion of water due to hydrogen
bonding transmits the upward pull along the entire length of the xylem to the
roots hence its name Transpirational Pull. This is the main method of
transport in xylem.
• Transpirational pull can extend down to the roots only through an unbroken
chain of water molecules. Cavitation, the formation of a water vapor pocket
in a xylem vessel, such as when xylem sap freezes in winter, breaks the
chain stopping the flow, which in those plants without root pressure is
forms an unbroken
chain of water
from leaves all the way
to the soil. The force
that drives the ascent
of xylem sap is a
gradient of water
potential. For the bulk
flow over long
distance, the water
potential gradient is
due mainly to a
gradient of the
Transpiration results in
the pressure potential
at the leaf end of xylem
being lower than the
pressure potential at
the root end.
• Guard cells, by controlling the size of stomata, help balance the
plant’s need to conserve water with its requirement for
• About 90% of the water a plant loses escapes through stomata,
though these pores account for only 1-2% of the external leaf
surface. The waxy cuticle limits water loss through the remaining
surface of the leaf.
• One way to evaluate how efficiently a plant uses water is to
determine its transpiration-to-photosynthesis ratio, the amount of
water lost per gram of CO2 assimilated into organic material by
photosynthesis. For many plant species, this ratio is about 600:1,
meaning the plant transpires 600 g of water for each gram of CO 2
that becomes incorporated into carbohydrate.
• With the same concentration of CO2 within the air spaces of the leaf,
C4 plants can assimilate that CO2 at a greater rate than C 3 plants
can at a ratio closer to 300:1
• When transpiration exceeds the delivery of water by xylem, as when
the soil begins to dry out, the leaves begin to wilt as their cells lose
• Guard cells control the diameter
of the stoma by changing
shape, thereby widening or
narrowing the gap between the
two cells .When guard cells take
in water by osmosis, they
become more turgid and swell
causing an opening. When the
cells lose water and become
flaccid, they become less
bowed and close the space
•The changes in turgor pressure that open and close stomata result primarily from the
reversible uptake and loss of potassium ions (K +) by the guard cells. Stomata open
when guard cells actively accumulate K+ and close when they lose K+.
•In general, stomata are open during the day and closed at night. This prevents the
plant from needlessly losing water when it is too dark for photosynthesis. At least three
cues contribute to stomatal opening at dawn.
•First light itself stimulates guard cells to accumulate potassium and become turgid.
•Second, depletion of CO2 caused by photosynthesis triggers the opening.
• Third, all eukaryotic organisms have internal clocks that somehow keep track of time
and regulate cyclic processes, and those that have intervals of approximately 24
hours are called circadian rhythms.
•Thus, guard cells arbitrate the photosynthesis-transpiration compromise on a
moment-to-moment basis by integrating a variety of internal and external stimuli.
• Plants adapted to arid climates, called xerophytes, have
various leaf modifications that reduce the rate of
• Many xerophytes have small, thick leaves, reducing
surface area relative to leaf volume. A thick cuticle gives
some of these leaves a leathery consistency. The
stomata are concentrated on the lower (shady) leaf
• Some plants assimilate CO2 by an alternative
photosynthetic pathway known as CAM, for
crassulacean acid metabolism allowing them to take in
its CO2 at night, and close their stomata during the day,
when transpiration is most severe.
• Xylem sap flows in a direction that generally does not
allow it to function in exporting sugar from leaves to
other parts of the plant. A second vascular tissue, the
phloem, transports the organic products of
photosynthesis throughout the plant. This transport of
food in the plant is called translocation.
• Phloem sap is an aqueous solution that differs
markedly in composition from xylem sap. The sucrose
concentration may be as high as 30% by weight,
giving the sap a syrupy thickness. Phloem sap may
also contain minerals, amino acids, and hormones in
transit from one part of the plant to another.
Phloem Translocates Sugar
• A sugar source is a plant organ in which sugar is being produced
by either photosynthesis or the breakdown of starch. Mature leaves
are the primary sugar sources. A sugar sink is an organ that is a
net consumer or storer of sugar.
• A sugar sink usually receives its sugar from the sources nearest to
• Sugar from the mesophyll cells of a leaf and other sources must be
loaded into sieve-tube members before it can be exported to sugar
sinks. In some species, sucrose moves all the way from mesophyll
cells to sieve-tube members via the symplast, passing from cell to
cell through plasmodesmata. In other species, sucrose reaches
sieve-tube members by a combination of symplastic and apoplastic
• In some plants, companion cells have numerous ingrowths of their
walls, an adaptation that increases the cells’ surface area and
enhances the transfer of solutes between apoplast and symplast.
Such modified cells are called transfer cells.
• In corn and many other plants, sieve-tube members accumulate sucrose to
concentrations two to three times higher than concentrations in mesophyll, and
thus phloem loading requires active transport. Proton pumps do the work that
enables the cells to accumulate sucrose.
• Downstream, at the sink end of a sieve tube, phloem unloads its sucrose.
Phloem unloading is a highly variable process; its mechanism depends on the
plant species and the type of organ. Regardless of its exact mechanism, the
concentration of free sugar in the sink is lower than that in the sieve, thus sugar
molecules diffuse from the phloem into the sink tissues, and water follows by
• Phloem sap flows from source to sink at rates as
great as 1 m/hr, which is much too fast to be
accounted for by either diffusion or cytoplasmic
streaming meaning phloem sap moves by bulk
flow, which is driven by pressure.
• The building of pressure at one end of the tube
(source) and reduction of that pressure at the
opposite end (sink) cause water to flow from
source to sink, carrying the sugar along.
• Water is recycled back from sink to source by
• The pressure flow model
explains why phloem sap
always flows from a
sugar source to a sugar
sink, regardless of their
locations in the plant.
• The case for pressure
flow as the mechanism of
convincing. It is not yet
known, however, if this
model applies to other
1. What are the two main types of transfer proteins?(Be sure to specify the
role and two methods of transport for proton pumps)
2. What is water potential and how is it calculated in plants?
3. How does turgor pressure act on plant cells?
4. What is the purpose of aquaporins?
5. What are the tonoplast, symplast, and apoplast and what do they aid in?
6. What is bulk flow and what hydraulic properties does it make use of?
7. What are the main functions of roots?(Think absorption and concentration)
8. What is the purpose of the endodermis with its casparian strip?
9. What are the two main methods of transport in the xylem and why is
10. How does transpiration affect xylem transport?
11. How do guard cells function and what is their purpose in plants?
12. How do some xerophytes prevent water loss?
13. What is the phloem and how does it differ from the xylem?
14. What is a sugar source and sugar sink and how are they related?(location)
15. How does pressure flow work in the phloem?
Campbell Biology Sixth Edition