4.4.4 - Translocation Flashcards
Features of the structure of the phloem.
Made up of sieve tube elements
End walls are perforated to form sieve plates
Companion cells are associated with sieve tube elements
What is the place where sucrose is made?
Source
What is the place where sucrose is used?
Sink
What are 3 things that the phloem transports?
Sucrose
Amino acids
Inorganic ions
What is the first stage of the mass flow theory?
Sucrose is actively transported into the phloem, companion cells are involved.
What happens after sucrose moves into phloem by active transport?
Water potential is lowered.
Water moves in by osmosis from xylem.
What happens after water moves into phloem by osmosis?
Pressure increases
Causes movement towards sink
What is sucrose used for in the sink?
Respiration
Stored as starch
Why is sucrose stored as starch?
Sucrose is soluble in water so affects water potential.
Starch isn’t.
Evidence for mass flow theory.
There is a pressure within sieve tubes, shown by sap being released when cut.
Concentration of sucrose is higher in source than sink.
Evidence against mass flow theory.
Function of sieve plates in unclear, they would hinder mass flow.
Not all solutes move at the same speed - they should do if movement is by mass flow.
Explain how the mass-flow hypothesis accounts for the movement of sugars from leaves to
roots.
(4)
Sugars loaded into phloem.
This lowers the water potential so water enters.
Pressure in the phloem increases which causes sugars to move.
Sugars converted to starch in root cells.
Increases water potential so water moves out.
Compare and contrast the structure of the xylem and phloem.
Both tissues contain tubes.
Both contain cellulose in cell walls.
Xylem vessels contain lignin but phloem does not.
Phloem contains companion cells but xylem does not.
