Question

Using diagrams and words, show how the Pressure Flow Model for Phloem Transport works to load sucrose into the phloem at the sources, tranport through the stem to the sinks, and unload sucrose from the phloem at the sinks. Be sure to include the role of companion cells, plasmodesmata, and show at the membrane level how phloem loading occurs at sugar sources (leaves) and how phloem unloading works at sinks.

Answer 1

Mass flow hypothesis :- High concentration of organic substances , particularly sugar inside the source ( leaves ).That creat a diffusion gradient ( osmotic gradient ) in between source and xylem ,that draws water from xylem to sieve tube elements.This creat turgor or hydrostatic pressure in phloem. Movement of phloem sap occurs by mass flow from sugar source to sugar sink.

sugar is transported into sink via active transport.Now in phloem near sink areas their is a pressure difference between xylem and phloem.So,water moves from phloem back in xylem.

Xylem Phloem companion Source O Water sieve-Tube elements 1 Water Companion Sink cell