1. 9.2 Transport in the phloem of plants
Essential idea: Structure and function are correlated in
the phloem of plants
http://commons.wikimedia.org/wiki/File:Taraxacum_offici
nale,_central_leaf_vein,_Etzold_green_2.JPG
2. Understandings
Statement Guidance
9.2 U.1 Plants transport organic compounds from sources to
sinks.
9.2 U.2 Incompressibility of water allows transport along
hydrostatic pressure gradients.
9.2 U.3 Active transport is used to load organic compounds
into phloem sieve tubes at the source
9.2 U.4 High concentrations of solutes in the phloem at the
source lead to water uptake by osmosis.
9.2 U.5 Raised hydrostatic pressure causes the contents of
the phloem to flow towards sinks.
3. Applications and Skills
Statement Guidance
9.2 A.1 Structure–function relationships of phloem sieve tubes
9.2 S.1 Identification of xylem and phloem in microscope
images of stem and root
9.2 S.2 Analysis of data from experiments measuring phloem
transport rates using aphid stylets and radioactively-
labelled carbon dioxide
5. Lumen of the
sieve tube with
no organelles
that would
obstruct the
flow of sap
Cell Wall resists
high pressures
inside the sieve
tube
Sieve Plate
cross walls
strengthens the sieve
tube with pores that
allow sap to pass
through in either
direction
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Cell Organelles
necessary for
carrying out
metabolic
activities
The cytoplasm of
Sieve-tube
Membrane and
Companion cells
Is connected by
plasmodesmata
Phloem sap
(mostly Sugar)
Passes vertically
Through pores in
The wall between
Sieve-tube members
9.2 A.1 Structure–function relationships of phloem sieve tubes
8. • The transport of materials in
plants is called translocation. The
organic compounds synthesized
during photosynthesis are carried
in the phloem tube.
• Plants will not transport glucose
as it is used directly it must be
converted into sucrose.
• Sucrose move through the plant:
With increased water pressure
The use of ATP (active transport)
Sink is any tissue
that is accepting
sugar
Source is any tissue
that makes sugar
available to the plant
9.2 U.1 Plants transport organic compounds from sources to sinks.
9. • During the growing season
plant leaves produce glucose.
• Glucose is used to produce ATP
for metabolic activities.
• Excess glucose is converted into
sucrose and stored in the roots
of the plant (called the sink).
• At the end of a growing season
plant tubers becoming the
source of glucose, as the leaves
glucose production ceases.
During non growing seasons
plants convert sucrose back into
glucose to carry out metabolic
activities (sink becomes the
source).
Potatoes
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media/42/82542-004-4A7EA186.jpg
9.2 U.1 Plants transport organic compounds from sources to sinks.
11. Pressure-Flow Hypothesis
•Hydrostatic pressure is pressure in a liquid.
•The term water potential is used to
describe the tendency for water molecules
to move within and between cells.
•A net movement of water will occur from
one region to another as a result of a
difference in water potential.
•Water moves by osmosis from an area of
high water concentration to an area of low
water concentration.
•In the diagram water flow out of the xylem
and into the phloem due to this gradient.
•This change in water pressure in the
phloem helps push sucrose to the sink cell
(storage site) in the plant.
http://cnx.org/resources/cd8839949717d6b04f4f
ade2e6b0f7cf/Figure_30_05_07.jpg
9.2 U.2 Incompressibility of water allows transport along hydrostatic
pressure gradients.
13. • Phloem tissue is found in plants
(stems, roots & leaves).
• Phloem is composed of sieve tubes.
Sieve tubes along with companion
cells composes a column of
specialized cells making up most of
the phloem.
• The sieve tubes that make up
phloem tissue are composed of
living cells. These cells have
mitochondria which generate ATP
needed for active transport
• Sieve plates are found at either end
of a sieve tube. They contain by
perforated walls called sieve plate
pores
9.2 U.3 Active transport is used to load organic compounds into phloem
sieve tubes at the source.
15. 1. The source during the growing
season is the leaf which
produces organic molecules.
2. Photosynthesis produced
glucose in leaf cells.
3. Glucose is converted to sucrose
for transport.
4. Companion cell actively loads
the sucrose increasing the
concentration in the phloem.
5. Water flows from xylem by
osmosis into phloem (water
from the xylem helps transport
sucrose from the phloem). This
one reason why close proximity
is import between the two.
Combined Transpiration/ Translocation
9.2 U.4 High concentrations of solutes in the phloem at the source lead
to water uptake by osmosis
16. 6. SAP volume and pressure
increased leading to mass flow
(material moving together with
water).
7. Unload the organic molecules by
the companion cell occurs at the
sink (during the growing season
this is the roots).
8. Sucrose is converted and stored
as insoluble starch.
9. Water mixed with sucrose is
released and flows back into the
xylem by osmosis.
10. Water recycles as part of
transpiration to resupply sucrose
loading.
* Remember transpiration is the
movement of water from the
roots to the leaves of the plant.
Combined Transpiration/ Translocation
9.2 U.4 High concentrations of solutes in the phloem at the source lead
to water uptake by osmosis
17. 1.Translocation moves the
organic molecules (sugars,
amino acids) from their
source through the tube
system of the phloem to the
sink. Phloem vessels still
have cross walls called sieve
plates that contain pores.
2. Companion cells actively load
sucrose (soluble, not
metabolically active) into the
phloem.
3. Water follows the high solute
in the phloem by osmosis. A
pressure develops moving
the mass of phloem sap
forward.
Translocation theory
Click4biology.com
9.2 U.5 Raised hydrostatic pressure causes the contents of the phloem to flow towards sink.
18. 4. The sap must cross the
sieve plate.
5. The phloem still contains a
small amount of
cytoplasm along the walls
but the organelle content
is greatly reduced.
6. Companion cells actively
unload (ATP used) the
organic molecules
7. Organic molecules are
stored (sucrose as starch,
insoluble) at the sink.
Water is released and
recycled in xylem.Click4biology.com
Translocation theory
9.2 U.5 Raised hydrostatic pressure causes the contents of the phloem to flow towards sink.
19. • An understanding of sap flow rates through phloem has come from
the use of an insect called an aphid and its feeding technique
• Aphids have a long piercing mouthpart called a stylets, which they
insert into the plants sieve tube and draw out sap.
http://upload.wikimedia.org/wikipedia/commons/1/11/Aphid-giving-birth.jpg
9.2 S.2 Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively-
labelled carbon dioxide
20. http://plantsinaction.science.uq.edu.au/book/export/html/23
9.2 S.2 Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively-
labelled carbon dioxide
1. Plants grown in a lab with leaves
covered with a clear bag and then
exposed to CO2 containing the
radioactive isotope C14
.
2. The radioactive CO2 is taken and
incorporated into glucose by the
process of photosynthesis. The
glucose is then converted into
sucrose and move through the
phloem of the plant.
3. In the experiment colonies of
Aphids feed on the sucrose in the
phloem at different locations of
the plant stem.
4. When feeding sucrose drips down
into container below the colonies.
The distant between the colonies
is used to calculate sucrose flow
rates in the phloem.
21. Experiment numbers 1 2 3
Distance between
aphid colonies (mm)
650 340 630
Time for radioactivity
to travel between
colonies (hours)
2.00 1.25 2.50
Rate of movement
(mm hours -1
)
32.5 27.2 25.4
Example with two colonies
9.2 S.2 Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively-
labelled carbon dioxide
