translocation

the main sources of assimilates are

• green leaves and green stems

• storage organs such as tubers and tap roots that are unloading their stores at the beginning of a growth period

• food stores in seeds when they germinate

main sinks in a plant

• roots that are growing and actively absorbing mineral ions

• meristems that are actively dividing

• any parts of the plant that are laying down food stores, such as developing seeds, fruits or storage organs

Mass flow hypothesis

• active transport is used to actively load the solutes into the sieve tubes of the phloem at the source

• 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

• 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

• the result is a pressure gradient from the source end to the sink end

• this gradient pushes solutes along the sieve tubes to where they’re needed

active loading

• in the companion cell, ATP is used to actively transport hydrogen ions out of the cell and into surrounding tissue cells

• this sets up a concentration gradient - there are more H+ ions in the surrounding tissue then in the companion cell

• A H+ ion binds to a co-transport protein in the companion cell membrane and re-enters the cell (down the concentration gradient)

• A sucrose molecule binds the the co-transport protein at the same time. The movement of the H+ ions is used to move the sucrose molecule into the cell, against its concentration gradient

• sucrose molecules are then transported out of the companion cells and into the sieve tubes by the same process

evidence for active loading

• advances in microscopy allow us to see the adaptations of the companion cells for active transport

• if the mitochondria of the companion cells are poisoned, translocation stops

• the flow of the sugars in the phloem is about 10,000 times faster than it would be by diffusion alone, suggesting an active process is driving the mass flow

• Aphids can be used to demonstrate the translocation of organic solutes in the phloem. Using evidence from aphid studies, it has been show that there is a positive pressure in the phloem that forces the sap out through the stylet. The pressure and therefore the flow rate in the phloem is lower closer to the sink than the source. The concentration of sucrose in the phloem sap is also higher nearer to the source than the sink