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Define translocation
Transport of assimilates throughout the plant in the phloem tissue.
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How are sugars transported in the phloem?
Transported in the form of sucrose.
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What is a source?
A part of a plant that releases sucrose into the phloem.
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What is a sink?
A part of the plant that removes sucrose from the phloem.
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How is sucrose loaded into the phloem?
By an active process.
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What do the companion cells do?
Use ATP to actively transport hydrogen ions out of the cytoplasm into surrounding tissue.
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What is set up?
A diffusion gradient.
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What do the hydrogen ions do?
Diffuse back into the companion cells using cotransporter proteins.
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What do the cotransporter proteins enable?
Hydrogen ions to bring sucrose into the companion cells.
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What happens to the concentration of sucrose?
The concentration of sucrose builds up in companion cells.
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Where does sucrose diffuse into?
Sucrose diffuses into sieve tube elements through plasmodesmata.
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At the source, what does sucrose enter and what effect does this have?
Enters the sieve tube elements which reduces the water potential inside the sieve tube.
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What does this reduced water potential mean?
Water molecules move into the sieve tube elements by osmosis from surrounding tissues.
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What does this movement increase?
Movement increases hydrostatic pressure in sieve tube at the source.
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At the sink, what can sucrose be converted into?
Starch for storage or can be used in a metabolic process e.g. respiration.
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What effect does this have on concentration of sucrose in these cells?
Reduces the sucrose concentration.
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How does sucrose move out of the sieve tube elements and where does it travel to?
Sucrose moves out of sieve tube elements by diffusion or active transport and moves into the surrounding cells.
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What does this movement into the surrounding cells cause?
Causes the water potential in the sieve tube element to increase, so water molecules move into the surrounding cells by osmosis.
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What does this movement by osmosis cause?
Hydrostatic pressure in the phloem at the sink to be reduced.
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Describe the flow of water along the phloem.
Water enters the phloem at the source, moves down the hydrostatic pressure gradient and leaves the phloem at the sink.
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What does this flow carry?
Carries sucrose and other assimilates along the phloem.
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What is this flow called?
Mass flow
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In what ways can mass flow occur?
In either direction, up or down the plant depending on where the sugars are needed.
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Can mass flow occur in the same phloem at different times?
Yes, mass flow can occur up or down the plant in the same phloem tube at different times.
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What can mass flow be moving up and down the plant?
May be moving assimilates up the plant in some tubes and down the plant in other tubes at the same time.
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Where are the sources located in a plant?
In the leaf and the roots.
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Where are the sinks located in a plant?
The leaves and the roots.
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When do the leaves act as a sink?
When the leaves are growing in early spring and they need energy so energy is supplied from stores in other parts of the plant.
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How do we know that the phloem is used?
If plants are given radioactively labelled CO2, then the labelled CO2 will appear in the phloem. Ringing trees to remove phloem results in sugars collecting. Aphid mouthparts when feeding on plant show them taking food from phloem.
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How do we known translocation needs ATP?
Companion cells have mitochondria, translocation can be stopped when formation of ATP is inhibited, must need ATP due to rate of flow of sugars in phloem.
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How do we known translocation uses this mechanism?
pH of companion cells is higher than that of surrounding cells, conc of sucrose is higher in source than sink.
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Is there evidence against this mechanism?
Not all solutes in phloem move at same rate, sucrose is moved to all parts of plant at the same rate, role of sieve plates is unclear.
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Other cards in this set

Card 2


How are sugars transported in the phloem?


Transported in the form of sucrose.

Card 3


What is a source?


Preview of the front of card 3

Card 4


What is a sink?


Preview of the front of card 4

Card 5


How is sucrose loaded into the phloem?


Preview of the front of card 5
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