Photoassimilate and Phloem Transport
(Carbon Compounds formed from Photosynthesis)

Goals and Objectives:

  1. Be able to describe the types of carbon compounds that are found in the phloem.
  2. Be able to describe the structure of mature phloem tissue.
  3. Be able to describe the source-sink concept and how it is involved in determining the direction and rate of translocation in the phloem.
  4. Be able to describe the Munch pressure-flow hypothesis and show how it operates to drive translocation in the phloem.
  5. Be able to describe how sugars are loaded onto the phloem sieve tubes at the source and unloaded at the sink.
  6. Be able to distinguish between allocation and partitioning and give some examples for each.

Study Questions:
  1. Why does a beaver only have to chew off the bark of a tree in order to kill it?
Composition of Phloem Exudate
  1. What kinds of substances are translocated in the phloem?
  2. Why is sucrose the major transport form of carbohydrate in a plant?
Structure of Phloem Tissue
  1. How do phloem cells differ from xylem cells?
  2. What do phloem cells look like at maturity?
  3. What are companion cells?  How do they function?
  4. What are transfer cells?  How do they function?
P-Protein and Callose
  1. For what does the term P-Protein stand?
  2. When does P-Protein appear?
  3. What are the two "interesting" properties of P-Protein?
  4. What is currently accepted about the function of P-Protein?
  5. What is callose?
  6. How does callose formation function to aid the plant?
Sources and Sinks
  1. What is the definition of a source in plant phloem transport terms?
  2. What is the definition of a sink in plant phloem transport terms?
Mechanism of Translocation in the Phloem
  1. How does assimilate translocation begin?
  2. Where is the hydrostatic pressure the greatest?
  3. Where is the hydrostatic pressure the lowest?
  4. What are the energy requirements for translocation within the sieve elements?
  5. Describe in your own words how the osmometer pictured in Figure 11.7 works.
Phloem Loading and Unloading
Phloem Loading
  1. Define Phloem Loading and Phloem Unloading.  Where do each of these occur?
  2. Describe the symplastic and apoplastic pathways of sucrose movement to the sieve element-companion cell complex.
  3. What mechanism is used to load sucrose onto the sieve element-companion cell complex in the apoplastic pathway?
  4. What is the polymer trap model of symplastic loading?
  5. What are some of the arguments proposed for explaining the two pathways for phloem loading? (page 226)
  6. Phloem Unloading

  7. Describe the symplastic pathway for phloem unloading in young, developing leaves and root tips.
  8. What are the two apoplastic routes for phloem unloading?
Assimilate Distribution
  1. What is meant by allocation?
  2. What are the three principal uses for photoassimilate at the source?
  3. How is photoassimilate used in leaf metabolism and biomass?
  4. What kind of carbohydrates are usually used for storage?
  5. Were are the two pools of storage carbohydrate in plants?
  6. How does the need for storage of carbohydrate change over the life of a leaf?
  7. About how much newly assimilated carbon is transported out of the leaf?  what happens to this carbon as it is being translocated?
  8. What dod the following enzymes do, and why/how are they important in helping regulate the allocation of assimilated carbon?
Partitioning of Assimilate Among Sinks
  1. What does partition mean in relation to assimilated carbon in plants?
  2. What is the difference between primary and secondary sinks in plants?
  3. 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?