Phloem loading Flashcards
What is the symplast route for phloem loading?
diffuses through the mesophyll cells and into the sieve tubes through the plasmodesmata
this route is largely passive
sucrose end up in the sieve elements and water follows via osmosis
this creates a pressure of water that move the sucrose through the phloem by mass flow
What is the apoplast route for phloem loading?
sucrose travels through the cell walls and inter-cell spaces to the sieve elements
by a diffusion gradient
maintained by the removal of sucrose into the phloem vessels
How is sucrose actively loaded into companion cells?
H+ ions are actively pumped out of the companion cell and into the surrounding tissue using ATP
the H+ ions returns to the companion cell down a concentration gradient via a co-transport protein (for the sucrose)
increases the sucrose conc. in the companion cells and sieve elements through the many plasmodesmata between the linked cells
How are the companion cells adapted to transport sucrose?
have many infoldings in their cell membranes to give an increased SA for active transport of glucose
they also have many mitochondria to supply the ATP needed for the transport pumps
What happens after the sucrose builds up in the companion cells and sieve tube elements?
water moves in by osmosis
this leads to a build up of tugor pressure due to the rigid clel walls
the water carrying the assimilates moves into the tubes of the sieve elements
this reduces pressure in the companion cells and the water moves through the plant by mass flow to areas of low pressure
What is the effect of solute accumulation in the source phloem?
leads to an increase in turgor pressure that forces sap to regions of lower pressure in the sinks
the pressure differences in plants can transport solutes and water rapidly over many metres
solutes are transolcated either up or down the plant depending on the position of the source
