7. Transport in Plants - Phloem Flashcards
What is the function of the phloem?
Its a tissue which transports organic solutes (sugars like sucrose) around the plant.
What is the structure of the phloem like?
It formed from cells arranged in tubes.
Sieve tube elements and companion phloem tissue.
What are sieve tube elements?
They are living cells that form the tube for transporting solutes.
They have no nucleus and few organelles so there is a companion cell for each sieve tube element.
What are companion cells?
They carry out living functions for sieve cells eg providing energy needed for active transport of solutes.
Define translocation?
Its the movement of solutes to where they’re needed in the plant.
Energy requiring process that occurs in the phloem.
It moves solutes from the sources to the sinks.
What are solutes sometimes called?
Assimilates.
What is the source?
Where assimilates are produced.
High concentration here.
What is the sink?
Where assimilates are used up
Lower concentration here.
EXAMPLE.
What is the source and sink of sucrose?
Source = leaves. Sink = other part of the plant, especially food storage organs and meristems in the roots, stems and leaves.
Meristems?
Areas of growth.
How are enzymes involved in translocation?
They maintain concentration gradient from the source to the sink by changing the solutes at the sink.
eg breaking them down or making them into something else.
This makes sure there is always a lower concentration at sink rather than at source.
EXAMPLE.
What happens in potatoes?
Sucrose is converted into starch in the sink, so there is always a lower concentration of sucrose at the sink than inside the phloem.
Makes sure there is a constant supply of new sucrose reaching the sink from the phloem.
What theory do scientists use to explain how solutes are transported?
The Mass Flow Hypothesis
- Source
- Sink
- Flow
The Mass Flow Hypothesis
1. Source
Active transport is used to actively load the solutes (sucrose) from companion cells into sieve tubes of phloem at the source (leaves)
This lowers water potential inside sieve tube so water enters the tubes by osmosis from xylem and companion cells - creates high pressure at the source end of phloem.
The Mass Flow Hypothesis
2. Sink
At sink end, solutes are removed from the phloem to be used up.
This increases water potential inside sieve tube, so water also leaves the tubes by osmosis.
This lowers the pressure inside sieve tubes.
The Mass Flow Hypothesis
3. Flow
The result is a pressure gradient from source end to the sink end.
Gradient pushes solutes along sieve tubes towards sink.
When they reach sink the solutes will be used (respiration) or stored (as starch).
Higher conc of sucrose at source=higher rate of translocation.
What organelles are present in companion cells?
Mitochondria.
Meaning they make lots of ATP.
ATP is needed to actively load solutes into the phloem at source.
What is the water potential like at the source and the sink?
Source - low water potential
Sink - High water potential
What is the pressure like at the source and the sink?
Source - High pressure
Sink - Low pressure.
Supporting Evidence 1
If a ring of bark is removed from a woody stem, a bulge forms above the ring. Fluid in bulge has higher concentration of sugars than the fluid from below the ring. Sugars can’t move past area where bark has been removed - evidence of downward flow of sugars.
Build up of sugar above ring causes a decrease in water potential so water moves into cells adding to bulge.
Supporting Evidence 2
Pressure in phloem can be investigated using aphids (they pierce the phloem, then their bodies are removed leaving mouthparts behind which allows sap to flow out).
Sap flows out quicker nearer the leaves than further down the stem - evidence of pressure gradient.
Supporting Evidence 3
Radioactive tracer such as radioactive carbon (14C) can be used to track the movement of organic substances in a plant.
Supporting Evidence 4
If a metabolic inhibitor (which stops ATP production) is put inot the phloem, then translocation stops - this is evidence that active transport is involved
Objection Evidence 1
Sugars travel to many different sinks, not just the one with the highest water potential as the model would suggest.
Objection Evidence 2
Sieve plates would create a barrier to mass flow.
A lot of pressure would be needed for the solutes to get through at a reasonable rate.
Where are the sugars made?
Leaves
thats why there is a donwward flow of sugars from source to sink
how do we use radioactive tracers?
Supplying a part of a plant (leaf) with an organic substance that has a radioactive label - then track its movement.
Movement tracked using autoradiography.
Plant is then killed (freezing using liquid nitrogen)and then whole plant is placed on photographic film.
Wherever the film turns black the radioactive substance is present.
What does the movement of radioactive tracers represent?
The translocation of the substances from source to sink over time
Autoradiography of plants killed at different times show an overall movement of solutes from the leaves towards roots.
