3.3.4 Mass transport in plants Flashcards
How is water transported in plants
Through xylem vessels; long, continuous columns that also provide structural support to the stem
Explain the cohesion-tension theory
Water evaporates from the leaves creating tension, water molecules form hydrogen bonds with eachother (cohesion) which moves the whole column of water up the xylem
What are the 3 components of phloem vessels
Sieve tube elements - form a tube to transport sucrose in the dissolved form of sap
Companion cells - involved in ATP production for active loading of sucrose into sieve tubes
Plasmodesmata - gaps between cell walls where the cytoplasm links, allowing substances to flow
Name the process whereby organic materials are transported around the plant
Translocation
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
Give evidence for the mass flow hypothesis of translocation
Sap is releases when a stem is cut, therefore 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
Give evidence 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, as 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 CO2 which will be incorporated into the plant’s sugars. Using autoradiography, we can see that the areas exposed to radiation correspond to where the phloem is
