Transport of Organic Substances in the Phloem

Question 1

What is the mass flow mechanism for the transport of organic substances in the phloem?

Answer 1

TRANSFER OF SUCROSE
-Sucrose is created within cells
-Sucrose diffuses by facilitated diffusion into companion cells
-Hydrogen ions are actively transported into spaces within cell walls using ATP
-Hydrogen ions diffuse through carrier proteins into sieve tube elements
-Sucrose molecules are transported along with hydrogen ions via co-transport

MASS FLOW OF SUCROSE
-Sieve tubes have a lower (More negative) water potential
-Xylem has a much higher water potential, therefore water moves from xylemk into sieve tubes by osmosis, increasing hydrostatic pressure
-Hydrostatic pressure gradient is created from source to sink
-Mass flow of sucrose solution into the sieve tubes

Question 2

What is the evidence for and against the mass flow hypothesis?

Answer 2

SUPPORT
-Sap being released when sieve tubes are cut = pressure
-Concentration of sap is higher in leaves than in roots
-Downward flow of sap in daylight but ceases at night

AGAINST
-Sieve plate function is unclear
-Not all solutes move at the same speed
-Sucrose is delivered at the same rate to all regions, rather than going more quickly to ones with lower sucrose conentration