Translocation Flashcards
Name the tissue that transports organic substances in plants.
Phloem
Name the process used to transport organic material through a plant.
Translocation
Name two substances usually transported in the phloem
Sucrose
Amino acids
Describe and explain the structure of phloem
Sieve tube elements form a tube of cells with perforations in end walls
Companion cells involved in ATP production for active loading of sucrose/amino acids into phloem
Plasmodesmata allow movement of organic substances between companion cell and phloem
Describe how sucrose is transported in phloem
At the source sucrose is actively transported into the phloem
By companion cells
Water potential in phloem decreases
Water enters phloem by osmosis
Produces high hydrostatic pressure
Mass flow /transport towards the sink
At sink sucrose removed /unloaded from phloem
Name two methods used to investigate the translocation of materiel in a plant
Ringing experiments
Use of tracers
Give evidence for the mass flow hypothesis of transloction
Sap is released when the stem is cut - must be pressure in the phloem
Higher concentration of sucrose in leaves than the roots
Increased sucrose levels in the leaves results in increased sucrose in the phloem
Give evidence for the mass flow hypothesis of translocation
Sap is released when the stem is cut - must be pressure in the phloem
Higher concentration of sucrose in leaves than the roots
Increased sucrose levels in the leaves results in increased sucrose in the phloem
Give evidence against the mass flow hypothesis of translocation
Structure of sieve tubes seems to hinder mass flow
NOT all solutes move at the same speed as they would in mas flow
Sucrose is delivered at the same rate throughout the plant, rather than to areas with the lowest sucrose concentration first
Organic materials move in opposite directions in phloem tissue
How can ringing experiments be used to investigate transport in plants?
Bark and phloem of tree removed in a ring, leaving the xylem intact
Tissues missing the ring will swell due to accumulation of sucrose
Sucrose must be transported in phloem
How can tracing experiments be used to investigate transport in plants?
Plants grown in presence of radioactive carbon dioxide which will be incorporated into sucrose
Use autoradiography to ‘see’ if areas exposed to radiation correspond with the position of phloem
How is sucrose loaded into the phloem at the source?
Hydrogen ions are actively transported into leaf cells using ATP generated by companion cell.
Hydrogen ions combine with sucrose molecules and move down H+ concentration gradient into companion cell (cotransport)
Sucrose diffuses through plasmodesmata down a concentration gradient into phloem
