Lecture 10- Phloem Flashcards
What name is given to the phloem transport cells?
Sieve tube elements
How are phloem transport cells different to that of the xylem?
They are living
They transport carbohydrates
How do sieve tube elements join?
Enlarged plasmodemata pores called sieve plates
How do sieve tube elements become hollow?
Membrane that encloses central vacuole (tonoplast) dissapears, nucleus and other components break down
What is the name given to movement of carbohydrates and other solutes in the phloem?
Translocation- from sources to sinks
What does it mean for a tree to be girdled?
A ring of bark containing phloem was removed
What happens after a tree is girdled?
Organic solutes collect in the phloem above the girdle- swelling occurs
Bark, then roots, the whole tree dies because sugar isn’t translocated downwards
What are the three characteristics of translocation?
- Stops if phloem is killed
- Proceeds in both directions simultaneously
- Inhibited by compounds that inhibit respiration and limit ATP supply
How do aphids feed?
Drilling into sieve tubes and inserting their stylet
Pressure in sieve tube forces sap through stylet into aphid
How do plant physiologists sample phloem?
Cut body of aphids away from stylet- phloem sap continues to flow and can be collected/analysed
How do plant physiologists find out how long translocation takes to occur?
Using radioactive tracers
What did all of these experiments and more lead to the theory of?
Pressure flow model
What are the two steps of translocation?
- Loading
- Unloading
What do the two steps of translocation require?
Energy
What is loading?
Transport of solutes from sources into sieve tubes
What is unloading?
Removal of solutes at sinks
What happens during loading?
- Solute concentration is sieve tubes is greater than surrounding cells
- Water enters by osmosis- greater pressure potential
- Water and solutes push towards sink
What happens at the sink?
Unloading
-Solutes are unloaded by active transport
-This maintains the pressure gradient
Water moves back into the xylem
What two conditions must be met for the pressure flow model to be valid?
- Sieve plates must be unobstructed so bulk flow is possible
- There must be an effective method for loading/unloading solutes
Why did early electron microscopes show that the sieve plates were blocked by fibrous proteins?
These proteins were a response to damage when phloem was prepared for study
What does secondary active transport do in sieve tubes?
Loading of sucrose into companion cells and sieve tubes by sucrose-proton symport
Explain how sucrose is carried from the apoplast to the symplast.
Sucrose-proton symplast (coupled), the apoplast has a high concentration of protons supplied by primary active transport (the proton pump)
How are solutes transported in sink regions?
Solutes are transported actively out of the sieve tube elements into surrounding tissues
How do many substances move through the symplast?
Via the plasmodesmata
Where are plasmodesmata more abundant?
Sink tissues- to allow passage of large molecules
How do plants and viruses allow more large molecules to pass?
Change the permeability of the plasmodesmata by producing movement proteins
Why are biologists interested in these movement proteins?
To be able to modify plasmodesmata
-For example: divert more photosynthetic output to seeds to increase crop yield
