Chapters 34 & 35
Lecture 10
Transport in Plants: Phloem
Dr. Alan McElligott
Phloem
Aims:
• To introduce the structure of the
phloem
• To study the translocation of
substances in the phloem
Phloem
Aims:
• To introduce the structure of the phloem
• To study the translocation of substances in the phloem
These lecture aims form part of the knowledge
required for learning outcome 3:
Describe mechanisms for the life processes
(LOC3).
Phloem
• 34.2 How Are Plant Cells Unique?
(Phloem)
Essential reading
• pages 751-752
• pages 775-777
Figure 34.6 Three Tissue Systems Extend Throughout the Plant Body
34.2 How Are Plant Cells Unique?
Phloem transport cells—sieve tube elements, are
living. Cells meet end-to-end; transport
carbohydrates.
34.2 How Are Plant Cells Unique?
Plasmodesmata in end walls enlarge to form pores—
form sieve plates. Membrane that encloses central
vacuole (tonoplast) disappears; nucleus and some
cytoplasmic components break down, thus pores in
sieve plate are not clogged.
35 Transport in Plants
• 35.4 How Are Substances Translocated in
the Phloem?
35.4 How Are Substances Translocated in the Phloem?
Movement of carbohydrates and other
solutes through the phloem is
translocation.
Substances are translocated from
sources to sinks.
Sources (e.g., leaves, produce more
sugars than they require). A sink
consumes sugars for growth or storage.
35.4 How Are Substances Translocated in the Phloem?
In a classic experiment, a tree was
girdled; a ring of bark containing the
phloem was removed.
Organic solutes collect in the phloem
above the girdle, causing it to swell.
Eventually the bark, then roots below,
and whole tree die because sugars are
not being translocated downwards.
Figure 35.11 Girdling Blocks Translocation in the Phloem
35.4 How Are Substances Translocated in the Phloem?
Characteristics of translocation:
• Stops if phloem is killed.
• Proceeds in both directions
simultaneously.
• Inhibited by compounds that inhibit
respiration and limit ATP supply.
35.4 How Are Substances Translocated in the Phloem?
Plant physiologists needed to sample
pure phloem sap from individual sieve
tube elements.
Aphids feed on plants by drilling into
sieve tubes and inserting their stylet.
Pressure in the sieve tube forces sap
through stylet and into aphid’s digestive
tract.
Figure 35.12 Aphids Collect Sap
35.4 How Are Substances Translocated in the Phloem?
Plant physiologists use aphids by cutting
the body away from the stylet—phloem
sap continues to flow for hours and can
be collected and analyzed.
Using radioactive tracers, they can infer
how long it takes for translocation to
occur.
These and other experiments led to
development of the pressure flow
model.
35.4 How Are Substances Translocated in the Phloem?
Two steps in translocation require
energy:
• Transport of solutes from sources into
sieve tubes: loading.
• Removal of solutes at sinks: unloading.
35.4 How Are Substances Translocated in the Phloem?
Where loading is occurring, solute concentration in
those sieve tube elements is greater than in
surrounding cells. Water enters by osmosis, which
makes a greater pressure potential, and water plus
solutes is pushed toward the sink.
At the sink, the solutes are unloaded by active
transport, maintaining the pressure gradient.
Figure 35.13 The Pressure Flow Model
The Pressure Flow Model
Table 35.1 Mechanisms of Sap Flow in Plant Vascular Tissues
35.4 How Are Substances Translocated in the Phloem?
For the pressure flow model to be valid,
two requirements must be met:
• Sieve plates must be unobstructed so
that bulk flow is possible.
• There must be effective methods for
loading and unloading solutes.
35.4 How Are Substances Translocated in the Phloem?
Early electron microscope studies of sieve
tubes indicated the plates were blocked
by fibrous proteins.
These proteins were a response to
damage when phloem was prepared for
study.
When tissues were undamaged, sieve
plates were shown to be open.
35.4 How Are Substances Translocated in the Phloem?
Secondary active transport loads sucrose
into companion cells and sieve tubes by
a sucrose–proton symport.
The apoplast must have a high
concentration of protons—supplied by
the proton pump (active transport).
35.4 How Are Substances Translocated in the Phloem?
At sinks, sucrose is actively transported
out of sieve tubes and into the
surrounding tissues.
This maintains the pressure gradient, and
promotes buildup of sugars and
starches in storage areas, such as
developing fruits and seeds.
35.4 How Are Substances Translocated in the Phloem?
Many substances move through the
symplast via the plasmodesmata,
including at loading and unloading sites.
In sink tissues, plasmodesmata are
abundant and allow passage of large
molecules.
35.4 How Are Substances Translocated in the Phloem?
Plants (and viruses) produce “movement
proteins” that change the permeability of
plasmodesmata and allow large
molecules to pass.
Biologists study these proteins in hopes
of modifying plasmodesmata—for
example, to divert more photosynthetic
output to seeds to increase crop yields.
PHLOEM
Check out
35.4 Recap, page 777
35.4 Chapter summary, page 778, see WEB/CD activity 35.4
Self Quiz
Page 779: Chapter 35, Questions 9 and 10
For Discussion
Page 779: Chapter 35, Question 2
PHLOEM
Key terms:
aphids, companion cell, girdling, phloem, pressure
flow model, sieve plate, sieve tube member, sink,
source, stylet, translocation