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Resource Acquisition and
Transport in Vascular Plants
36
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
36
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)
36
Figure 5.8 Osmosis Modifies the Shapes of Cells
36
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.
36
• 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
36
Figure 36.4 Apoplast and Symplast
36
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.
36
Figure 36.5 Casparian Strips
36
• 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
36
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.
36
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.
36
Figure 36.8 The Transpiration–Cohesion–Tension Mechanism
36
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
36
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.
36
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
36
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.
36
Figure 36.11 Stomata (Part 1)
36
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.
36
Figure 36.11 Stomata (Part 2)
36
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.
36
• 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
36
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
36
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
36
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.
36
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