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Module 2
Exchange and transport
1.2.19 Translocation
Phloem
Think, Pair, Share
Learning Objectives
• Describe the mechanism
of transport in phloem
involving active loading at
the source and removal at
the sink, and the evidence
for and against this
mechanism
• Explain translocation as
an energy-requiring
process transporting
assimilates between the
sources and sinks
Success Criteria
State examples of source and
sinks (Grade E-D)
Define key terms: source, sink &
translocation (Grade E-D)
Interpret the mechanism of
active loading (Grade C-A)
Evaluate evidence for this
mechanism (Grade C-A)
• Define key terms: source,
sink & translocation (Grade
E-D)
Translocation
• Definition:
– Transport of assimilates throughout the plant,
in the phloem tissue
•State examples of source
and sinks (Grade E-D)
•Define key terms: source,
sink & translocation (Grade
E-D)
• What is a source?
– Examples:
• Leaf Cell
• What is a sink?
– Examples:
• Root Cell
Source & Sink
Translocation
 Unlike the water in the xylem, the contents of the phloem can
move both up or down a plant stem. It helps to identify where
the sugar is being transported from (the source), and where to
(the sink).
 During the summer sugar is mostly transported from the
leaves, where it is made by photosynthesis (the source) to the
roots, where it is stored (the sink).
 During the spring, sugar is often transported from the
underground root store (the source) to the growing leaf buds
(the sink).
 Flowers and young buds are not photosynthetic, so sugars can
also be transported from leaves or roots (the source) to
flowers or buds (sinks).
•Interpret the mechanism of active loading (Grade C-A)
Active Loading at the Source
• Companion cells
–
–
–
–
Have lots of mitochondria to make ATP
ATP used to pump out protons (H+ ions)
This causes a diffusion gradient
Protons diffuse back in through cotransporter
proteins, bringing sucrose in with them
– This leads to a high concentration of sucrose
•Interpret the mechanism of active loading (Grade C-A)
Active Loading at the Source
• Sieve Tube Elements
– Sucrose moves in through
plasmodesmata
– Sucrose lowers the water
potential, so water moves
in by osmosis
– This increases the
hydrostatic pressure
•Interpret the mechanism of active loading (Grade C-A)
Removal at the Sink
• Sink
– Uses up sucrose in respiration or it is stored
as starch
– Lowers sucrose concentration
– Sucrose moves in by diffusion, lowering the
water potential
– Water moves into the sink by osmosis
– This lowers the hydrostatic pressure
• This high hydrostatic pressure near the source and
low hydrostatic pressure near the sink creates
movement in the phloem called mass flow
• Evaluate evidence for this
mechanism (Grade C-A)
Evidence
• Active loading and movement of sucrose
in the phloem is a theory
• Evidence supports this theory:
• Summarise the evidence for this theory
Learning Objectives
• State examples of source and sinks
(Grade E-D)
• Define key terms: source, sink &
translocation (Grade E-D)
• Interpret the mechanism of active loading
(Grade C-A)
• Evaluate evidence for this mechanism
(Grade C-A)