http://www.pearsoned.ca/school/science11/
biology11/sugartransport.html
http://dendro.cnre.vt.edu/forestbiology/cam
bium2_no_scene_1.swf
Objectives:
*describe the distribution of xylem and
phloem tissue in roots, stems and leaves of
dicotyledonous plants
**describe the structure of xylem vessel
elements and be able to recognise these
using the light microscope;
***relate the structure of xylem vessel
elements to their functions;
Uptake of water: the transpiration
stream
• The transpiration stream is the one-way
movement of water:
 from the soil into root hairs;
 across the root into xylem vessels;
 through root stem and leaf xylem into
mesophyll cells;
 by evaporation from mesophyll cell surfaces into
leaf air spaces;
 by diffusion from leaf air spaces through
stomata into the atmosphere
Uptake of water : the transpiration
stream
• The movement of water in the transpiration stream is
down a water potential gradient from soil solution to
atmosphere
• The transpiration stream is ‘driven’ by the evaporation of
water from mesophyll cell surfaces, each evaporating
molecule ‘pulling’ another one behind it because of the
cohesion of water molecules (due to hydrogen bonding)
• The ‘pull’ is transmitted from molecule to molecule in an
unbroken chain all the way down to the root: this is the
cohesion-tension hypothesis
• As well as cohesion, the adhesion of water molecules to
the vessel walls and the cellulose molecules in
mesophyll cell walls supports the column of water and
keeps it from breaking
• Mineral ions taken by active transport into root hairs
are carried passively in the transpiration stream
Root structure
Know these tissues!
•structure
•location
stele
•function
Root structure
Endodermis
Epidermis
Air space
Cortex
Stele
{
TS buttercup root
(low power)
Know these tissues!
•structure
•location
•function
Cortex
parenchyma cell
Pericycle
TS buttercup root Xylem
stele (high power)
Phloem
Passage of water across a root
Root hair
Epidermis
Cortex
Endodermis
Pericycle
Xylem
Passage of water across a root
Some water enters
the root hair vacuole
by osmosis, and
travels by osmosis
from vacuole to
vacuole across the
cortex.
This is the vacuolar
pathway.
The vacuolar
pathway presents
the most resistance
to water flow
(because of the
number of
membranes to be
crossed), the
apoplastic pathway
the least …
Some water (blue line) crosses the
cell surface membrane into the
cytoplasm and passes from cell to
cell via plasmodesmata: this is the
symplastic pathway.
Most water (red line) does
not enter the living cells at
all but passes along cells
walls and intercellular
spaces: this is the
apoplastic pathway.
… but at the endodermis
the apoplastic pathway is
completely blocked by a
strip of corky material (the
Casparian strip) around
the walls of the
endodermal cells.
Passage of water across a root
The Casparian
strip completely
blocks the
apoplast
pathway …
… so that only
the symplast
and vacuolar
pathways are
available.
Why is this important?
It allows the flow of water and dissolved minerals
into the plant to be controlled.
Movement through the xylem
• Water enters the xylem because its water
potential is reduced by the upward ‘pull’
(tension) on the water column it contains
• Adhsion of water molecules to the xylem
vessel walls also helps maintain the
column.
Structure of xylem
• Xylem is a compound tissue, consisting of:
• two types of conducting cell, vessels and
tracheids
• fibres (thin elongated cells with thick
woody walls and no living contents)
• Xylem parenchyma (living cells with thin
cellulose cell walls)
Vessels and tracheids
Vessels are short
hollow cells with
woody (lignified)
cell walls and no
living contents at
maturity.
Their end walls
break down, so that
water can flow freely
from one to the
next.
Many vessels have
pits allowing
sideways movement
of water from vessel
to vessel: this can
help by-pass
blockages.
Tracheids are
narrower lignified cells
with tapered ends that
overlap, transferring
water from cell to cell
via pits.
Xylem vessels
Xylem vessels show different
patterns of woody thickening
(lignification), giving them a
function in support as well as
water conduction.
Xylem parenchyma
Pitted vessel
LS
Fibre
Vessel with annular
thickening
TS
1 Water evaporates from the
surface of a mesophyll cell into
the leaf air space
2 By cohesion, another water
molecule is pulled into the cell
from the leaf xylem
The whole
picture
2
1
3
3
5 … so that water flows down a water potential gradient
from the soil across the vacuolar, symplastic and
apoplastic pathways in the root
3
4
5
3 By cohesion, the
pull is transmitted all
the way down the stem
and root xylem
4 The upward pull
lowers the water
potential in the root
xylem …
Use of a potometer to investigate
water uptake
1. Why is a potometer like the one above usually assembled under water?
2. What is the function of the central reservoir?
3. Describe how you would use the above apparatus to investigate the effect of
moving air on the rate of water uptake by a leafy shoot.
Use of a potometer to investigate
water uptake
4. The graph shows the results of
an experiment in which a
potometer was used to
measure the uptake of water by
a leafy shoot in three different
conditions: still dry air, still
humid air and dry air blown by a
fan. Suggest which curve was
obtained in which condition.
Give reasons for your answers.
5. Calculate (a) the mean rate of
water uptake by the shoot in
moving dry air, (b) the
percentage increase in mean
rate of uptake when changing
from still dry air to moving dry
air.
TRANSLOCATION
Objectives:
*
**
***
****
Translocation in phloem
• Phloem transports organic products of
photosynthesis from leaves or storage
organs to sites of use
• It also transports plant growth substances
and mineral ions
• Unlike xylem, the conducting elements of
phloem are living cells, and the transport
(called translocation) is an active
process
http://glencoe.mcgrawhill.com/sites/9834092339/student_view0/chapter38/animation__phloem_loading.html
Structure of phloem
In addition phloem contains
parenchyma cells and
phloem fibres
Phloem fibres
Phloem
In roots phloem is found between
the ‘arms’ of the star-shaped xylem
The conducting elements of phloem
are sieve cells, assisted by
companion cells
In stems phloem is found on the
peripheral side of xylem in vascular
bundles
Phloem structure
LS
Phloem parenchyma
Sieve plate
Plasmodesmata
Sieve cell. These join
end to end to form
sieve tubes,
connected by
perforated end walls
(sieve plates)
Sieve cells have little
cytoplasm, no nucleus and
very few organelles. Every
sieve cell is closely
associated with a
companion cell, the two
cells communicating through
many plasmodesmata
Companion cell
Sieve plate
TS
Phloem parenchyma
TRANSLOCATION
Objectives:
*
**
***
****
Phloem structure
Sieve plates are
associated with large
amounts of a protein Parenchyma
called P-protein. Its
cell
precise role is
unknown.(It is now
believed to be untrue)
Sieve cell
Companion
cell
Sieve plate
A sieve cell and its adjacent companion cell are produced by division of the same parent
cell.
The companion cell probably carries out metabolic functions for the sieve cell,
compensating for the sieve cell’s lack of organelles.
At the tips of leaf veins, companion cells have folded surfaces and act as transfer cells,
actively transporting sucrose from mesophyll cells into sieve cells.
Companion cells
• In between sieve tubes
• Large nucleus, dense
cytoplasm
• Many mitochondria to
load sucrose into sieve
tubes
• Many plasmodesmata
(gaps in cell walls
between companion cells
and sieve tubes) for flow
of minerals
Phloem Structure and Function:
Sieve Elements
•
•
•
•
•
Specialises in efficient transport of food.
Living cells but do not have a nucleus.
Long, narrow, thin walled living cells.
End walls are heavily perforated – called a sieve plate.
A series of sieve elements is called a sieve tube.
Companion Cells
•
•
•
•
Assist the sieve element in food transport.
Live narrow cells with a prominent nucleus.
Its nucleus also controls the sieve element.
Dense cytoplasm particularly rich in mitochondria.
Movement of Sugars
• Translocation: movement of assimilates (sugars
and other chemicals) through the plant
• Source: a part of the plant that releases sucrose
to the phloem e.g. leaf
• Sink: a part of the plant
that removes sucrose from
the phloem e.g. root
The mass flow hypothesis
for the movement of organic
solutes in phloem suggests
that as sucrose is actively
transferred into sieve cells at
its source, water follows it by
osmosis, raising the pressure
in the sieve cells at that point.
The mass flow
hypothesis
Where sucrose is actively
transferred out of sieve cells
(a sucrose ‘sink’), water
again follows by osmosis,
reducing the pressure in the
sieve cells at that point.
There is therefore a
pressure gradient pushing
sucrose and other solutes
from source to sink.
The contents of sieve cells
are under positive pressure,
as is shown by the feeding of
aphids.
The mass flow
hypothesis
Aphids plug their piercing
mouthparts (stylets) into
sieve cells, and the pressure
in the phloem pushes its
contents into the insect’s gut
– sometimes so quickly that it
exudes from the aphid’s
anus.
Sucrose Entering the Phloem
• Active process (requires energy)
• Companion cells use ATP to transport hydrogen
ions out of their cytoplasm
• As hydrogen ions are now at a high
concentration outside the companion cells, they
are brought back in by diffusion through special
co-transporter proteins, which also bring the
sucrose in at the same time
• As the concentration of sucrose builds up inside
the companion cells, they diffuse into the sieve
tubes through the plasmodesmata (gaps
between sieve tubes and companion cell walls)
Sucrose movement through
phloem
• Sucrose entering sieve tube lowers the water
potential (more negative) so water moves in by
osmosis, increasing the hydrostatic pressure
(fluid pushing against the walls) at the source
• Sucrose used by cells surrounding phloem and
are moved by active transport or diffusion from
the sieve tube to the cells. This increases water
potential in the sieve tube (makes it less
negative) so water moves out by osmosis which
lowers the hydrostatic pressure at the sink
Movement along the phloem
• Water entering the phloem at the source,
moving down the hydrostatic pressure
gradient and leaving at the sink produces
a flow of water along the phloem that
carries sucrose and other assimilates. This
is called mass flow. It can occur either up
or down the plant at the same time in
different phloem tubes
Evidence for translocation
• Radioactively labelled carbon from carbon dioxide can appear in the
phloem
• Ringing a tree (removing a ring of bark) results in sugars collecting
above the ring
• An aphid feeding on the plant stem contains many sugars when
dissected
• Companion cells have many mitochondria
• Translocation is stopped when a metabolic poison is added that
inhibits ATP
• pH of companion cells is higher than
that of surrounding cells
• Concentration of sucrose is higher at
the source than the sink
Evidence against translocation
• Not all solutes move at the same rate
• Sucrose is moved to parts of the plant at
the same rate, rather than going more
quickly to places with low concentrations
• The role of sieve plates is unclear