9.2.11 Outline the Role of Phloem in
Active Translocation of Sugars
(sucrose) and amino acids from
source (photosynthetic tissue to
storage organs) to sinks (fruits,
seeds, roots)
Nutrient Transport
–Movement from Source to Sink
• A process of phloem transport moves sugars
through a plant from a source to a sink.
• A source is any cell in which sugars are produced
by photosynthesis.
• A sink is any cell where the sugars are used or
stored (fruits, roots, seeds).
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Vascular Tissue
PHLOEM Tissue
• Phloem Cells are alive.
• Sieve Tube elements (cells) lacks
nucleus & cytoplasm
• Sieve tube elements are
connected to each other via sieve
plates
• Sieve plates have holes which
allow movement of water and
dissolved organic molecules
•
Companion Cells has nucleus
and dense cytoplasm
• Companion cells are attached to
sieve tube element via
plasmodesmata
Sieve plate
Phloem Translocation Summary
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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 (active transport)
load sucrose (soluble, not metabolically
active) into the phloem.
3. Water follows the high solute in the
phloem by osmosis. A positive pressure
potential develops moving the mass of
phloem sap forward.
4. The sap must cross the sieve plate.
Current hypothesis do not account for this
feature.
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.
Phloem
STEP 1
•Sugars produced during
photosynthesis (source cells)
are actively (using ATP) pumped
into the sieve tubes.
•This loading of sugar is
accomplish by Active Transport.
Movement of
water
Movement of
sugar
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Sugar molecules
Source cell
Xylem
STEP 2
Phloem
•As sugar concentrations
increase in the sieve tubes, water
from the xylem moves into the
sieve tube by osmosis.
Sugar molecules
Source cell
Movement of
water
Movement of
sugar
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Xylem
STEP 3
Phloem
•This movement of water causes a
positive pressure to build up in the
sieve tube that results in a bulk flow
of phloem sap.
• This movement does not require
energy.
Sugar molecules
Source cell
Movement of
water
Movement of
sugar
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Xylem
STEP 4
•The pressure diminishes
by the removal of sugar
from the sieve tube at the
sink.
•This removal of sugar is
accomplished via Active
Transport (uses ATP)
•The sugar is converted into
starch and starch in
insoluble and exerts no
osmotic affect.
Movement of
water
Movement of
sugar
Sink cell
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Phloem Xylem
STEP 5
•Xylem recycles the
relatively pure water by
carrying it from the sink
back to the source.
Movement of
water
Movement of
sugar
Sink cell
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Phloem Xylem
Transport in Plants Summary
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1. Source produces organic molecules
2. Glucose from photosynthesis produced
3.Glucose converted to sucrose for transport
4. Companion cell actively loads the sucrose
5. Water follows from xylem by osmosis
6. Sap volume and pressure increased to give
Mass flow
7. Unload the organic molecules by the
companion cell
8. Sucrose stored as the insoluble and
unreactive starch
9. Water that is released is picked up by the
xylem
10. water recycles as part of transpiration to re
supply the sucrose loading
Watch the below animations on
translocation
Click on the links below to access animations:
• http://www.youtube.com/watch?v=b6dvKgWBVY&feature=related
• http://www.youtube.com/watch?v=MxwI63r
QubU&feature=related