Translocation

• How is water transported in plants?

  Xylem - long, continuous columns that also provide structural support to the stem
• Explain the cohesion-tension theory?

  Water molecules from hydrogen bonds with each other causing them to stick together - cohesion

  The surface tension of the water also creates this sticking effects so as more water is lost through transpiration, more can be drawn up the stem
• Three components of phloem vessels?

  Sieve tube elements

  Companion cells

  Plasmodesmata
• 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 cel 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 phloem vessels by active transport which uses ATP and a diffusion gradient of H ions

  Sucrose then diffuses from companion cells into sieve tube elements through the plasmodesmata
• How do phloem vessels transport sucrose around the plant?

  As sucrose moves into the tube elements, the water potential inside the phloem is reduced

  Water enters by 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
• Evidence for the mass flow hypothesis of translocation?

  Sap is released when a stem is cut - pressure in the phloem

  Higher sucrose concentration in the leaves than the roots

  Increasing sucrose levels in the leaves results in increased sucroses in phloem
• Evidence AGAINST the mass flow hypothesis?

  Structure of sieve tubes hinders 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 than to areas with the lowest sucrose concentration first
• Ringing experiments?

  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
• Tracing experiments?

  Plants are grown in the presence of radioactive CO2 which is incorporated into the plant's sugars

  Using autoradiography we can see the areas exposed to radiation correspond to where the phloem is