Photoassimilate and
Phloem Transport
(Carbon Compounds
formed from Photosynthesis)
Goals and Objectives:
-
Be able to describe the types of carbon compounds
that are found in the phloem.
-
Be able to describe the structure of mature
phloem tissue.
-
Be able to describe the source-sink concept
and how it is involved in determining the direction and rate of translocation
in the phloem.
-
Be able to describe the Munch pressure-flow
hypothesis and show how it operates to drive translocation in the phloem.
-
Be able to describe how sugars are loaded
onto the phloem sieve tubes at the source and unloaded at the sink.
-
Be able to distinguish between allocation
and partitioning and give some examples for each.
Study Questions:
-
Why does a beaver only have to chew off the
bark of a tree in order to kill it?
Composition of Phloem Exudate
-
What kinds of substances are translocated
in the phloem?
-
Why is sucrose the major transport form of
carbohydrate in a plant?
Structure of Phloem Tissue
-
How do phloem cells differ from xylem cells?
-
What do phloem cells look like at maturity?
-
What are companion cells? How do they
function?
-
What are transfer cells? How do they
function?
P-Protein and Callose
-
For what does the term P-Protein stand?
-
When does P-Protein appear?
-
What are the two "interesting" properties
of P-Protein?
-
What is currently accepted about the function
of P-Protein?
-
What is callose?
-
How does callose formation function to aid
the plant?
Sources and Sinks
-
What is the definition of a source in plant
phloem transport terms?
-
What is the definition of a sink in plant
phloem transport terms?
Mechanism of Translocation in the Phloem
-
How does assimilate translocation begin?
-
Where is the hydrostatic pressure the greatest?
-
Where is the hydrostatic pressure the lowest?
-
What are the energy requirements for translocation
within the sieve elements?
-
Describe in your own words how the osmometer
pictured in Figure 11.7 works.
-
Are sieve tube elements under pressure?
-
Do the differences in sugar concentration
and turgor pressure in the sieve tube account for he measured rates of
transport?
-
Is there bidirectional flow within the phloem?
-
Do sieve elements offer a substantial resistance
to the flow of photosynthate in the phloem?
Phloem Loading and Unloading
Phloem Loading
-
Define Phloem Loading and Phloem
Unloading. Where do each of these occur?
-
Describe the symplastic and apoplastic pathways
of sucrose movement to the sieve element-companion cell complex.
-
What mechanism is used to load sucrose onto
the sieve element-companion cell complex in the apoplastic pathway?
-
What is the polymer trap model of symplastic
loading?
-
What are some of the arguments proposed for
explaining the two pathways for phloem loading? (page 226)
Phloem Unloading
-
Describe the symplastic pathway for phloem
unloading in young, developing leaves and root tips.
-
What are the two apoplastic routes for phloem
unloading?
Assimilate Distribution
Allocation
-
What is meant by allocation?
-
What are the three principal uses for photoassimilate
at the source?
-
How is photoassimilate used in leaf metabolism
and biomass?
-
What kind of carbohydrates are usually used
for storage?
-
Were are the two pools of storage carbohydrate
in plants?
-
How does the need for storage of carbohydrate
change over the life of a leaf?
-
About how much newly assimilated carbon is
transported out of the leaf? what happens to this carbon as it is
being translocated?
-
What dod the following enzymes do, and why/how
are they important in helping regulate the allocation of assimilated carbon?
-
Invertase
-
Sucrose Synthase
-
Sucrose Phosphate Synthase (SPS)
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Fructose-1,6-bisphosphatase (FBPase)
Partitioning of Assimilate Among Sinks
-
What does partition mean in relation to assimilated
carbon in plants?
-
What is the difference between primary and
secondary sinks in plants?
-
What are the three factors that determine
partitioning between competing sinks? Which of these three factors
is the most important in determining the strength of a sink?