Mass transport in plants Flashcards
How is water transported in plants?
Through xylem vessels which are long continuous columns that also provide structural support to the stem.
Explain the cohesion-tension theory?
Water molecules form hydrogen bonds with each other, causing them to stick together.
The surface tension of the water also creates this sticking effect.
Therefore as water is lost through transpiration, more can be drawn up the stem.
What are the three components of phloem vessels?
Sieve tube elements
Companion cells
Plasmodesmata
What are sieve tube components?
They form a tube to transport sucrose in the dissolved form of sap.
What are companion cells?
They are involved in ATP production for the active loading of sucrose into sieve tubes.
What are plasmodesmata?
Gaps between cell walls where the cytoplasm links, allowing substances to flow.
What is translocation?
The process whereby organic materials are transported around the plant.
How does sucrose in the leaf move into the phloem?
Sucrose enters companion cells of the phloem vessels by active loading, which uses ATP and a diffusion gradient of hydrogen ions.
Sucrose then diffuses from companion cells into the sieve tube elements through the plasmodesmata.
How do phloem vessels transport sucrose around the plant?
As sucrose moves into the tube elements, water potential inside the phloem is reduced.
This causes water to enter via osmosis from the xylem and increases hydrostatic pressure.
Water moves along the sieve tube towards areas of lower hydrostatic pressure.
Sucrose diffuses into surrounding cells where it is needed.
What evidence is there for the mass flow hypothesis of translocation?
Sap is released when a stem is cut, therefore there must be pressure in the phloem.
There is a higher sucrose concentration in the leaves than the roots.
Increasing sucrose levels in the leaves results in increased sucroses in the phloem.
What evidence is there against the mass flow hypothesis of translocation?
The structure of sieve tubes seems to hinder mass flow.
Not all solutes move at the same speed, but they would in mass flow.
Sucrose is delivered at the same rate throughout the plant, rather than to areas with the lowest sucrose concentration first.
How can ringing experiments be used to investigate transport in plants?
The bark and phloem of a tree are removed in a ring, leaving behind the xylem.
Eventually the tissues above the missing ring swells due to accumulation of sucrose as the tissue below begins to die.
Therefore sucrose must be transported in the phloem.
How can tracing experiments be used to investigate transport in plants?
Plants are grown in the presence of radioactive carbon dioxide, which will be incorporated into the plant’s sugars.
Using autoradiography, we can see that the areas exposed to radiation correspond to where phloem is.
