Loading from source
In the leaves, solutes are passed from photosynthesising cells into the companion cells of the phloem. The companion cells then load the solutes into the sieve tube elements via plasmodesmata. ATP hydrolysis is involved in this transfer
Pressure flow from source to sink
Entry of solutes into sieve tube elements lowers their water potential (makes the water potential more negative). As a result, water enters the sieve tube elements from the surrounding xylem vessels. This increases the pressure within the sieve tube elements, pushing their contents through the pores in the sieve plates
Unloading to a sink
As solutes are pushed down a pressure gradient, they are surrounded by cells with a lower solute concentration. Solutes leave the phloem into these cells, lowering their water potential •Water leaves the phloem by osmosis, returned to the surrounding xylem and is re-circulated •Cells in the sink use, or store, the solutes
Comments
No comments have yet been made