Phloem Flashcards
What is phloem?
Is a tissue which transports sugars to parts of the plant which they are needed and other assimilates
What are assimilates?
General term for organic molecules like carbohydrates and amino acids
What are sieve tube elements?what are companion cells?
-these are stacked/lined up end to end to allow the flow of sap having cytoplasm and no nucleus
-cells that control transport in the sieve tube elements full of mitochondria which phloem requires to move sap in translocation
What is active loading?
The process of loading sucrose into sieve tube elements
Tell me the process of active loading,
1-hydrogen you are actively transported from companion cells to general leaf tissue,creating a diffusion gradient of hydrogen ions
2-hydrogen ions diffuse back into companion cells through co-transporter proteins bringing sucrose with them
3-high concentration of sugar in companion cells causes sucrose to diffuse into the sieves tube element
What are sinks?what are sources?
Sinks:these are parts of the plant where materials are removed from a transport system like roots
Sources:parts of plants that load materials into the transport system
Like leaves
Describe the process of mass flow.
Stage 1: actively loaded sucrose into our companion cells,so there is a high concentration of sucrose in companion cell
Stage 2: sucrose moves into the sieve tube by diffusion causing the water potential to decrease
Stage 3: water moves in by osmosis from xylem(high water potential) to low in the sieve tube element
Stage 4:increase in hydrostatic pressure due water and assimilates:high hydrostatic pressure,causes mass movement towards the sink(roots)
Stage 5:sucrose unloaded at the sink as it moves by diffusion,this causes the water potential to decrease,so water also moves out by osmosis to the surrounding cells
The mass flow hypothesis is used to explain the movement of substances through phloem,use understanding of mass flow hypothesis to explain how pressure is generated inside of. Phloem tube.
-after active loading,the sucrose on companion cell is high and moves into the sieve tube elements by diffusion so water potential is decreased so bc water moves in by osmosis from sieve tube elements to xylem so all the water and assimilate make the hydrostatic pressure
What is the evidence for and against mass flow?
For:
- There is pressure in the sieve tube elements, as shown by sap being released when the stem of a plant is cut.
- The concentration of sucrose is higher in the leaves (source) of plants than in roots (sink).
- Increases in sucrose levels in the leaves are followed by a similar increase in sucrose concentration in the phloem.
- Metabolic poisons/a lack of oxygen inhibit translocation of sucrose in the phloem.
Against:
- The function of the sieve plates is unclear as they would appear to hinder mass flow (some suggest though they have a structural function to help prevent bursting under pressure).
- Not all solutes move at the same speed, they should do if it is mass flow.
- Sucrose is delivered at more or less the same rate to all regions, rather than going more quickly to the ones with the lowest sucrose concentration, which the mass flow theory would suggest
What the ringing experiment?
In this the bark and phloem of a tree are removed leaving just the xylem in the centre. Overtime the tissues above the missing ring swell with sucrose solution and the tissue below dies. This shows that sucrose is transported in the phloem.
What is the tracer experiment?
Plants are grown in a environment that contains radioactivity labelled carbon dioxide (14CO2). The presence of this means that they are incorporated into the sugar produced in photosynthesis.
The movement of these sugars can now be traced through the plant using autoradiography. Those areas that have been exposed to the radiation produced by the 14C in the sugars will appear black. It follows that these regions correspond to the area where the phloem is and therefore suggest that this is where the sugars are transported.
