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Resource Acquisition and
Transport in Vascular Plants
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Transport Overview
• 1- uptake and loss of
water and solutes by
individual cells (root
cells)
• 2- short-distance
transport from cell to
cell (sugar loading
from leaves to phloem)
• 3- long-distance
transport of sap within
xylem and phloem in
whole plant
Figure 36.1 The Pathways of Water and Solutes in the Plant
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Whole Plant Transport
• 1- Roots absorb water and dissolved
minerals from soil
• 2- Water and minerals are transported
upward from roots to shoots as xylem sap
• 3- Transpiration, the loss of water from
leaves, creates a force that pulls xylem sap
upwards
• 4- Leaves exchange CO2 and O2 through
stomata
• 5- Sugar is produced by photosynthesis in
leaves
• 6- Sugar is transported as phloem sap to
roots and other parts of plant
• 7- Roots exchange gases with air spaces of
soil (supports cellular respiration in roots)
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Figure 5.8 Osmosis Modifies the Shapes of Cells
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Uptake and Movement of Water and Solutes
• For osmosis to occur, a membrane must be
permeable to water but not to the solutes.
• Plant cells have a rigid cell wall.
• As water enters the cell, the plasma membrane
presses against the cell wall, restricting
expansion.
• The opposing force exerted by the rigid cell wall
as water enters is called the pressure potential,
or turgor pressure.
• Water enters a plant cell until the pressure
potential exactly balances the solute potential.
The cell is then called turgid.
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• Tonoplast
vacuole membrane
• Plasmodesmata : cytosolic
connection
• Symplast route (lateral)
cytoplasmic continuum
• Apoplast route (lateral)
continuum of cell walls
• Bulk flow (long distance)
movement of a fluid by
pressure (phloem)
Transport within tissues/organs
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Figure 36.4 Apoplast and Symplast
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Uptake and Movement of Water and Solutes
• The endodermis cell walls have Casparian
strips—waxy, suberin-containing structures that
form a hydrophobic belt sealing the cell and
preventing movement of water and ions between
the cells.
• The Casparian strips thus separate the apoplast
of the cortex from the apoplast of the stele.
• Water and ions can enter the stele only by way of
the symplast—by entering and passing through
the endodermal cytoplasm.
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Figure 36.5 Casparian Strips
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• Transpiration: loss of water
vapor from leaves pulls water
from roots (transpirational pull);
cohesion and adhesion of
water
• Root pressure: at night (low
transpiration), roots cells
continue to pump minerals into
xylem; this generates pressure,
pushing sap upwards; guttation
Transport of Xylem Sap
Figure 36.7 Guttation
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Transport of Water and Minerals in the Xylem
• Eduard Strasburger cut trees at the base and
placed the cut ends into a bucket of water and
poison.
• Transport continued until the poison reached the
leaves, at which point it stopped.
• His experiment established three important points:
 “Pumping cells” are not responsible for
transport.
 The leaves play a crucial role in transport.
 The roots are not the cause of transport.
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Transport of Water and Minerals in the Xylem
• The transpiration–cohesion–tension mechanism:
• The concentration of water vapor is higher inside the
leaf than outside, so water diffuses out of the leaf
through the stomata. This process is called
transpiration.
• This creates a tension in the mesophyll that draws
water from the xylem of the nearest vein into the
apoplast surrounding the mesophyll cells.
• The removal of water from the veins, in turn,
establishes tension on the entire volume of water in
the xylem, so the column is drawn up from the roots.
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Figure 36.8 The Transpiration–Cohesion–Tension Mechanism
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Transport of Water and Minerals in the Xylem
• Hydrogen bonding between water molecules
results in cohesion, the tendency of water
molecules to stick to one another.
• The narrower the tube, the greater the tension the
water column can stand.
• The water column is also maintained by adhesion
of water molecules to the walls of the tube.
• This combination of cohesion and adhesion
creates capillary action
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Transport of Water and Minerals in the Xylem
• The key elements in water transport in xylem:
 Transpiration
 Tension
 Cohesion
• The transpiration–cohesion–tension mechanism
does not require energy.
• At each step, water moves passively toward a
region with a more negative water potential.
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Transpirational Control
• Photosynthesis-Transpiration compromise….
• Guard cells control the size of the stomata
• Xerophytes (plants adapted to arid environments)~ thick cuticle;
small spines for leaves
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Transpiration and the Stomata
• Leaf and stem epidermis has a waxy cuticle that
is impermeable to water, but also to CO2.
• Stomata, or pores, in the epidermis allow CO2 to
enter by diffusion.
• Guard cells control the opening and closing of
the stomata.
• Most plants open their stomata only when the light
is intense enough to maintain photosynthesis.
• Stomata also close if too much water is being lost.
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Figure 36.11 Stomata (Part 1)
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Transpiration and the Stomata
• Opening closing and of the stomata are regulated
by controlling K+ concentrations in the guard cells.
• Blue light activates a proton pump to actively pump
protons out of the guard cells. The proton gradient
drives accumulation of K+ inside the cells.
• Increasing K+ concentration makes the water
potential of guard cells more negative, and water
enters by osmosis.
• The guard cells respond by changing their shape
and allowing a gap to form between them.
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Figure 36.11 Stomata (Part 2)
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Transpiration and the Stomata
• The guard cells close when the process is
reversed; when active transport of protons ceases.
K+ diffuses out of the cell, and water follows.
• This occurs in the absence of blue light or when
abscisic acid is present.
• Abscisic acid is produced by the mesophyll cells on
hot, sunny, windy days so that guard cells will close
the stomata to prevent water loss.
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• Translocation: food/phloem transport
Translocation of Phloem Sap
• Sugar source: sugar production organ (mature
leaves)
• Sugar sink: sugar storage organ (growing
roots, tips, stems, fruit)
• 1- loading of sugar into sieve tube at source
reduces water potential inside; this causes
tube to take up water from surroundings by
osmosis
• 2- this absorption of water generates pressure
that forces sap to flow alon tube
• 3- pressure gradient in tube is reinforced by
unloading of sugar and consequent loss of
water from tube at the sink
• 4- xylem then recycles water from sink to
source
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Translocation of Substances in the Phloem
• Sugars, amino acids, some minerals, and other
solutes are transported in phloem and move from
sources to sinks.
• A source is an organ such as a mature leaf or a
starch-storing root that produces more sugars
than it requires.
• A sink is an organ that consumes sugars, such as
a root, flower, or developing fruit.
• These solutes are transported in phloem, not
xylem, as shown by Malpighi by girdling a tree.
Figure 36.12 Girdling Blocks Translocation in the Phloem
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Translocation of Substances in the Phloem
• Plant physiologists have used aphids to collect
sieve tube sap from individual sieve tube
elements.
• An aphids inserts a specialized feeding tube, or
stylet, into the stem until it reaches a sieve tube.
• Sieve tube sap flows into the aphid. The aphid is
then frozen and cut away from its stylet, which
remains in the sieve tube.
• Sap continues to flow out the sieve tube and can
be collected and analyzed by the physiologist.
Figure 36.13 Aphids Collect Sieve Tube Sap
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Translocation of Substances in the Phloem
• There are two steps in translocation that require
energy:
 Loading is the active transport of sucrose and
other solutes into the sieve tubes at a source.
 Unloading is the active transport of solutes
out of the sieve tubes at a sink.
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Translocation of Substances in the Phloem
• Sieve tube cells at the source have a greater
sucrose concentration that surrounding cells, so
water enters by osmosis. This causes greater
pressure potential at the source, so that the sap
moves by bulk flow towards the sink.
• At the sink, sucrose is unloaded by active
transport, maintaining the solute and water
potential gradients.
• This is called the pressure flow model.
Figure 36.14 The Pressure Flow Model
Table 36.1 Mechanisms of Sap Flow in Plant Vascular Tissues