active transport is used to actively load the solutes (e.g. sucrose from photosynth) from companion cells into the sieve tubes of the phloem at the source (e.g. the leaves)
this lowers the water potential inside the sieve tubes
so water enters the tubes by osmosis from the xylem and companion cells
this creates a high pressure inside the sieve tubes at the source end of the phloem
2. sink
at the sink end, solutes are removed from the phloem to be used up
this increases the water potential inside the sieve tubes
so water also leaves the tubes by osmosis
this lowers the pressure inside the sieve tubes
3. flow
the result is a pressure gradient from the source end to the sink end
this gradient pushes solutes along the sieve tubes towards the sink
when they reach the sink the solutes will be used (e.g. in resp) or stored (e.g. in starch)
the higher the conc. of sucrose at the source, the higher the rate of translocation
experiments have shown that some sucrose is also transported through the cell walls of the phloem
companion cells contain many mitochondria, which means they can make lots of ATP. ATP is needed to actively load the solutes into the phloem at the source
