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 molecules form hydrogen bonds with each other, causing them to stick together (cohesion)
-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 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 tube elements)
-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.
-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 increase hydrostatic pressure
-Water moves along the sieve tube towards areas of lower hydrostatic pressure.
-sucrose diffuses into surrounding cells where it is needed
Evidence 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 increase sucroses in the phloem
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 C02, which will be incorporated into the plants sugars. Using autoradiography, we can see that the areas exposed to radiation correspond to where the phloem is
